"Chemo brain" — chemotherapy-induced difficulties with focusing, thinking and remembering — may be caused by the cancer treatment's disruption to the brain's lymphatic system, an early-stage study suggests.

The study zoomed in on the meningeal lymphatics, the drainage network found in the protective tissue layer surrounding the brain. Dysfunction in this network has been linked to Alzheimer's disease, Parkinson's disease and traumatic brain injuries.

Now, the new research, published Oct. 13 in the journal Communications Biology, has linked damage to the meningeal lymphatics with the brain fog that patients commonly experience after receiving chemotherapy.

Using human and mouse cells, as well as living lab mice, researchers found evidence that a common type of chemotherapy drug that blocks cancer cells from dividing, called taxanes, damages the brain's lymphatic vessels and limits their drainage. Normally, the vessels would work together with the brain's glymphatic system to flush away metabolic waste.

"Lymphatic health really declined across all three models measured in different ways," study co-author Jennifer Munson, director of Virginia Tech's Cancer Research Center in Roanoke, Virginia, said in a statement. The vessels shrank and had fewer branches, which "are signs of reduced growth that indicate the lymphatics are changing, or not regenerating in beneficial ways," she said.

"Chemo brain" is a broad category of cognitive changes that follow chemotherapy and can last for years after treatment. "There's really a lot we don't know," Munson told Live Science, but these cognitive impairments have previously been linked to oxidative stress and inflammation, as well as impaired myelin production. (Myelin is fatty insulation that covers nerve fibers.)

"Others had looked on the neural side, so we wanted to focus on the meningeal side," Munson said.

To do this, Munson and her team used three models — human cells, mouse tissues and live mice — to assess whether chemotherapy drugs led to changes to the meningeal lymphatics at different scales.

First, they used cell lines to build a human-cell model of healthy meningeal lymphatics. This model paired cells from the lining of lymphatic vessels with meningeal cells. This enabled the team to tease apart the isolated effects of chemo on each cell's function. They also grew healthy mouse meningeal tissue in lab dishes to assess any structural changes triggered by the drug exposure.

They found that the drug docetaxel disrupted the cells in the human meningeal lymphatic model by reducing their coverage and length. The treatment also shrank the vessels within the mouse tissues and reduced the number of loops in the network structure.

Next, the researchers ran experiments with live mice, comparing mice treated with docetaxel to mice unexposed to the drug. Mice with cancerous tumors that were given the drug tended to have narrower meningeal lymphatic vessels, as well as fewer loops, compared to untreated mice.

The researchers wanted to see whether these docetaxel-induced structural changes led to impaired memory or changes in behavior. They found that healthy mice treated with docetaxel forgot objects they had previously seen, while the untreated mice showed clear signs of remembering them. MRI scans of the treated mice indicated that these cognitive issues correlated with the decreased flow of fluids through the lymphatic vessels, the authors wrote in the study.

Munson cautioned that this is an early-stage study and that there are many gaps left in our understanding of the link between "chemo brain" and meningeal lymphatics. She explained that one limitation of the research was that the chemotherapy drugs were administered over relatively short time periods, whereas chemotherapy courses for human cancer patients often last months.

Similarly, the memory issues the mice experienced were tested over a couple of days, whereas humans can sometimes experience chemo brain for years following treatment. "So it's possible that these lasting effects that we see in [human] patients may have different mechanisms that may not be captured fully here," Munson said.

It is important to replicate this research using samples from many individuals of different ages and to compare outcomes between tumor-bearing and tumor-free mice, to see if there's a difference in how the chemotherapy affects them, Munson said. She hopes that, eventually, this research will provide a new target for treating this side effect of chemotherapy.

"Ultimately, this work underscores the need to consider not only survival, but also the long-term, often overlooked neurological side effects of cancer treatment on cognitive well-being and quality of life," study co-author Monet Roberts, an assistant professor of biomedical engineering and mechanics at Virginia Tech, said in the statement.

This article is for informational purposes only and is not meant to offer medical advice.

We spend one-quarter to one-third of our lives sleeping, but while we rest, the brain remains hard at work, performing regular maintenance. Sleep is known to be important for learning and memory creation, and you may have heard that, during sleep, the brain rids itself of waste built up while we are awake. 

But how does the brain flush these toxins out of its system, and why does this happen only during sleep?

The brain's waste-disposal apparatus is known as the glymphatic system, which includes a network of tunnels that surround blood vessels in the brain. This system is an analog of the waste clearance system found in the rest of the body, called the lymphatic system.

The tunnels in the glymphatic network contain a clear, watery substance called cerebrospinal fluid (CSF), which contains nutrients and physically cushions the brain. CSF from these tunnels into the spaces of the brain, where it mixes with another fluid found between active brain cells.

Related: How does the brain store memories?

Here, the CSF picks up metabolic waste from those cells; this includes amyloid-beta proteins, which build up in the brain in Alzheimer's disease. The "dirty" fluid then gets flushed out of the space and exits the brain through lymphatic vessels that funnel the CSF into the lymphatic system, where it's cleared away.

Aside from clearing away this waste, the glymphatic system helps transport fats, sugars and chemical messengers in the nervous system and may play a role in distributing drugs within the brain.

The system was first discovered in mice, but brain scans have shown that this system also exists in the human brain.

"Glymphatic connection is the plumbing of the brain," Jonathan Kipnis, an immunologist at Washington University in St. Louis, told Live Science in an email. 

The system depends on star-shaped cells called astrocytes, which form connections with blood vessels. Water channels in the astrocytes help the CSF flow from one place to another.

Dr. Maiken Nedergaard, whose team at the University of Rochester Medical Center discovered the glymphatic system in 2012, said the involvement of astrocytes in this important process surprised her. 

"Neuroscience has always been very neuron-centric. Astrocytes were regarded as the housekeeping cells," she told Live Science. "And here, we are saying that astrocytes can tell the brain what to do."

The team uncovered the glymphatic system by putting fluorescent molecules in the CSF of mice and tracing its flow. In similar experiments, they found that this flow was tied to sleep — it decreased by nearly 95% in awake mice compared with sleeping mice. 

Nedergaard theorizes that, because our brain focuses on integrating all the information we get when we are awake, it can't really clean itself at the same time, she said. "This definitely explains our biological need for sleep," she said. 

A person's state of sleep also seems to affect how CSF navigates the maze of billions of neurons to remove waste. Sleep synchronizes neurons, sending high-energy waves of charged particles through the fluid between cells. This helps move CSF into these spaces.

That said, recently, there have been some contradictory results about whether sleep drives the flow of waste-carrying CSF out of the brain. Some scientists found that the process can happen independent of whether an animal is asleep or awake. 

However, Kipnis told Live Science that an experimental method used in that study may itself affect glymphatic flow, muddying the results. In addition, he said many scientists agree that this waste clearance is most efficient during sleep. 

Nedergaard added that there isn't enough data in the study about how waste is actually removed from the brain. "[The data looks at] moving the garbage from one room to another, not getting rid of it."

The glymphatic system is thought to accumulate wear and tear with age, and this could contribute to a buildup of waste and abnormal proteins, potentially leading to diseases like Alzheimer's. "I think that every disease that is affected by accumulation of debris would be dependent on glymphatic function," Kipnis said.

Nedergaard agreed. In theory, therapies aimed at improving sleep quality could benefit people with slow glymphatic flow and potentially slow the progression of neurodegenerative diseases, she noted.

Editor's note: This story was updated on June 24, 2024, to correct the description of the findings of the cited 2024 Nature Neuroscience study. The story was first posted June 19.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

The lymphatic system is a network of tissues and organs that help rid the body of toxins, waste and other unwanted materials. The primary function of the lymphatic system is to transport lymph, a fluid containing infection-fighting white blood cells, throughout the body, according to the journal Lymphatic Research Biology. 

The lymphatic system primarily consists of lymphatic vessels, which are similar to the veins and capillaries of the circulatory system. The vessels are connected to lymph nodes, where the lymph is filtered. The tonsils, adenoids, spleen and thymus are all part of the lymphatic system.

• Related: How to improve your circulation

Description of the lymphatic system

There are hundreds of lymph nodes in the human body. They are located deep inside the body, such as around the lungs and heart, or closer to the surface, such as under the arm or groin, according to the American Cancer Society. The lymph nodes are found from the head to around the knee area. 

The spleen, which is located on the left side of the body just above the kidney, is the largest lymphatic organ, according to the U.S. National Library of Medicine (NLM). "The spleen . . . acts as a blood filter; it controls the amount of red blood cells and blood storage in the body, and helps to fight infection," said Jordan Knowlton, an advanced registered nurse practitioner at the University of Florida Health Shands Hospital. 

If the spleen detects potentially dangerous bacteria, viruses, or other microorganisms in the blood, it — along with the lymph nodes — creates white blood cells called lymphocytes, which act as defenders against invaders. The lymphocytes produce antibodies to kill the foreign microorganisms and stop infections from spreading. Humans can live without a spleen, although people who have lost their spleen to disease or injury are more prone to infections.

The thymus is located in the chest just above the heart, according to Merck Manual. This small organ stores immature lymphocytes (specialized white blood cells) and prepares them to become active T cells, which help destroy infected or cancerous cells. 

Tonsils are large clusters of lymphatic cells found in the pharynx. According to the American Academy of Otolaryngology, they are the body's "first line of defense as part of the immune system. They sample bacteria and viruses that enter the body through the mouth or nose." They sometimes become infected, and although tonsillectomies occur much less frequently today than they did in the 1950s, it is still among the most common operations performed and typically follows frequent throat infections.

Lymph is a clear and colorless fluid; the word "lymph" comes from the Latin word lympha, which means "connected to water," according to the National Lymphadema Network. 

Plasma leaves the body's cells once it has delivered its nutrients and removed debris. Most of this fluid returns to the venous circulation through tiny blood vessels called venules and continues as venous blood. The remainder becomes lymph, according to the National Cancer Institute.

Unlike blood, which flows throughout the body in a continuous loop, lymph flows in only one direction — upward toward the neck. Lymphatic vessels connect to two subclavian veins, which are located on either sides of the neck near the collarbones, and the fluid re-enters the circulatory system, according to the National Cancer Institute.

• Related: Vitamin E: sources, benefits and risks
• Related: Vitamin A: sources, benefits and deficiency

Diseases and disorders

Diseases and disorders of the lymphatic system are typically treated by immunologists. Vascular surgeons, dermatologists, oncologists and physiatrists also get involved in treatment of various lymphatic ailments. There are also lymphedema therapists who specialize in the manual drainage of the lymphatic system.

The most common diseases of the lymphatic system are enlargement of the lymph nodes (also known as lymphadenopathy), swelling due to lymph node blockage (also known as lymphedema) and cancers involving the lymphatic system, according to Dr. James Hamrick, chief of medical oncology and hematology at Kaiser Permanente in Atlanta.

When bacteria are recognized in the lymph fluid, the lymph nodes make more infection-fighting white blood cells, which can cause swelling. The swollen nodes can sometimes be felt in the neck, underarms and groin, according to the NLM.

Lymphadenopathy is usually caused by infection, inflammation, or cancer. Infections that cause lymphadenopathy include bacterial infections such as strep throat, locally infected skin wounds, or viral infections such as mononucleosis or HIV infection, Hamrick stated. "The enlargement of the lymph nodes may be localized to the area of infection, as in strep throat, or more generalized as in HIV infection. In some areas of the body the enlarged lymph nodes are palpable, while others are to deep to feel and can be seen on CT scan or MRI."

Inflammatory or autoimmune conditions occur when a person's immune system is active, and can result in enlargement of lymph nodes. This can happen in lupus, according to Hamrick. 

Lymphoma:

This refers to cancer of the lymph nodes. It occurs when lymphocytes grow and multiply uncontrollably. There are a number of different types of lymphoma, according to Dr. Jeffrey P. Sharman, director of research at Willamette Valley Cancer Institute and medical director of hematology research for the U.S. Oncology Network.

"The first 'branch point' is the difference between Hodgkin lymphoma and non-Hodgkin lymphoma (NHL)," Sharman said. Non-Hodgkin lymphoma is one of the most common cancers in the United States, according to the American Cancer Society. It’s estimated that around 80,000 people in the US will be diagnosed with the NFL in 2022.

The most common types of NHL are follicular, which accounts for about 30% of all NHL cases; diffuse large B-cell lymphoma (DLBCL), which comprises 40 to 50% of NHL cases; and Burkitt's lymphoma, which accounts for 5% of NHL cases, according to the Lymphoma Research Foundation. "The remainder of cases makes up the bewildering complexity of NHL," Sharman said.

"Though there can be a significant range within an individual category, the clinical approach to each category is unique and the expectations of patient outcome varies by category," Sharman said.

When a person has had surgery and/or radiation to remove a cancer, the lymphatic flow back to the heart and can result in swelling or lymphedema, Hamrick noted. This most commonly occurs in women who have had surgery to remove a breast cancer. Part of the operation to remove the breast cancer involves removing lymph nodes in the armpit. 

The more lymph nodes removed the higher the risk of chronic bothersome swelling and pain due to lymphedema in the arm, Hamrick explained. "Fortunately, modern surgical techniques are allowing for fewer lymph nodes to be removed, and thus fewer cases of severe lymphedema for breast cancer survivors."

Some interesting research has been done on why people possibly get lymphoma. For example, VU University Medical Center in Amsterdam researched a nationwide Dutch pathology registry between 1990 and 2016. From the research, they estimated that the risk of developing anaplastic large cell lymphoma in the breast after getting implants is 1 in 35,000 at age 50, 1 in 12,000 at age 70, and 1 in 7,000 at age 75. The study was published in the Jan. 4, 2018 issue of the journal JAMA Oncology.

Diagram of the lymphatic system

Castleman disease:

This disease refers to a group of inflammatory disorders that cause lymph node enlargement and can result in multiple-organ dysfunction, according to the Castleman Disease Cooperative Network. While not specifically a cancer, it is a similar to a lymphoma and is often treated with chemotherapy. It can be unicentric (one lymph node) or multicentric, involving multiple lymph nodes. 

Lymphangiomatosis:

This disease involves multiple cysts or lesions formed from lymphatic vessels, according to the Lymphangiomatosis & Gorham's Disease Alliance. It is thought to be the result of a genetic mutation. 

Lymphatic filariasis:

Considered a neglected tropical disease (NTD), this parasitic disease is caused by the infiltration of a thread-like worm that infects the lymphatic system, according to the Centers for Disease Control and Prevention (CDC). Most cases worldwide of lymphatic filariasis are caused by a parasite called Wuchereria bancrofti. Millions of these microscopic worms can enter a person’s bloodstream through mosquito bites, before travelling to a lymph node where they will grow and reproduce. The majority of cases of lymphatic filariasis are asymptomatic and show no external signs of the disease, according to the World Health Organisation (WHO). In chronic cases, it can cause lymphoedema and skin and tissue thickening called elephantiasis. A blood test is required to diagnose the disease. “People living for a long time in tropical or sub-tropical areas where the disease is common are at the greatest risk for infection. Short-term tourists have a very low risk,” according to the CDC’s FAQ page on Lymphatic filariasis. 

Tonsilloliths:

Tonsil stones are another problem that can happen to the lymphatic system. Small bits of debris catch on the tonsils and white blood cells attack the debris and leave behind a hard biofilm that breathes oxygen. They are not smooth like regular stones, though. "Instead, they look like prunes, with crevices where bacteria can accumulate," said Chetan Kaher, a dentist in London. Usually, tonsil stones fall away and get swallowed, but sometimes they need to be manually removed.

Diagnosis and treatment

Diseases of the lymphatic system are usually diagnosed when lymph nodes are enlarged, Hamrick noted. This may be discovered when the lymph nodes become enlarged enough to be felt ("palpable lymphadenopathy") or are seen on imaging studies such as CT scans or MRIs.

The majority of enlarged lymph nodes are not dangerous; they are the body's way of fighting off an infection, such as a viral upper respiratory infection. If the lymph nodes become significantly enlarged and persist longer than the infection, then they are more worrisome. There is no specific size cutoff, but typically nodes that persist at larger than a centimeter are more worrisome and warrant examination by a doctor.

Common symptoms of any lymphatic disorder include swelling of the arm or groin, weight loss, fever and night sweats, according to Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York. "A PET or CAT scan is usually ordered to further investigate." 

The diagnosis of lymphadenopathy depends on the location of the abnormal lymph nodes and other things that are going on with the patient. If the patient has a known infection, then the lymph nodes can simply be followed to await resolution with treatment of the infection. If the nodes are growing quickly and there is no obvious explanation then typically a biopsy is warranted to look for a cancer or an infection. If the node can be felt then this can be done at the bedside with a needle, according to Hamrick. 

If the lymph node is deeper, such as in the abdomen or pelvis, Hamrick said the biopsy might need to be done by an interventional radiologist using image guidance to place the needle into the node. Sometimes the biopsy needs to be done by a surgeon in the operating room. This is often where the most tissue can be obtained to make a diagnosis, he said.

With many types of lymphoma and leukemia, there are unique treatment options for each type, according to Sharman. "There is no one 'summary' of treatment options. Treatment options can include traditional chemotherapy, immunotherapy (such as using antibodies or immune modulating drugs), and even radiation."

Treatment of lymphatic diseases depends on treating the underlying cause. Infections are treated with antibiotics, supportive care (while the immune system does its job, as in a viral infection) or antivirals. Lymphedema can be treated by elevation, compression and physical therapy. Cancers of the lymphatic system are treated by chemotherapy, radiotherapy, surgery, or a combination of those modalities, Hamrick noted. 

In last several years, Sharman noted that there has been explosion of new treatment options. "There are a handful of newly approved drugs that target the actual disease causing processes within cells. Ibrutinib, idelalisib, obinutuzumab, lenalidomide have been approved in various indications and it is likely that we will see multiple more in coming years. 

Additional resources

If you're looking for more information about lymphoma visit the charity Lymphoma Out Loud or The Sidney Kimmel Comprehensive Cancer Center. For a deeper dive into the body's immune system check out "Immune: A Journey into the Mysterious System That Keeps You Alive" by Philipp Dettmer.

Bibliography

• Melody Swartz, "The physiology of the lymphatic system," Advanced Drug Delivery Reviews, Volume 50, August 2001, https://doi.org/10.1016/S0169-409X(01)00150-8
• Hiroo Suami & Mario F. Scaglioni, "Anatomy of the Lymphatic System and the Lymphosome Concept with Reference to Lymphedema," sEMINARS IN pLASTIC sURGERY, vOLUME 32, 2018, https://doi.org/10.1055/s-0038-1635118 
• Antonino Carbone et al, "Follicular lymphoma," Nature Reviews Disease Primers, Volume 5, December 2019, https://doi.org/10.1038/s41572-019-0132-x
• Naritee Sukswai et al, "Diffuse large B-cell lymphoma variants: an update," Pathology, Volume 52, January 2020, https://doi.org/10.1016/j.pathol.2019.08.013 
• Guillermo Oliver et al, "The Lymphatic Vasculature in the 21st Century: Novel Functional Roles in Homeostasis and Disease," Cell, Volume 182, July 2020, https://doi.org/10.1016/j.cell.2020.06.039

The human body is a complex network of systems that work together to keep life-sustaining processes running smoothly. These systems break down food for fuel, clear away waste, repair damaged tissues and DNA, fight infectious germs and monitor the outside world so we can move through it safely. 

Many scientists spend their days working to understand how each bodily system performs its jobs, how the systems interact, and what can happen when one or more of them falter. Such malfunctions can stem from aging or disease, for instance, and through medical care, doctors aim to get derailed systems back on track. 

Here's a quick rundown of the systems of the human body, its vital organs and its "vestigial" organs, as well as a few fascinating facts about how the body works.

What are the different systems of the human body? 

Our bodies consist of a number of biological systems that carry out specific functions necessary for everyday living. Some organs and tissues play roles in multiple systems at once.

Related: Strange, two-faced brain cells confirmed to exist, and they may play a role in schizophrenia 

Circulatory: The job of the circulatory system is to move blood, nutrients, oxygen, carbon dioxide and hormones around the body. It consists of the heart, blood, blood vessels, arteries and veins. According to the Cleveland Clinic, the adult human body's network of blood vessels is more than 60,000 miles (around 100,000 kilometers) long. 

Digestive: The digestive system consists of a series of connected organs that together allow the body to break down and absorb nutrients from food and remove waste. It includes the mouth, esophagus, stomach, small intestine, large intestine, rectum and anus. The large intestine is home to microorganisms that are collectively called the gut microbiome and influence our health in various ways. The liver and pancreas also have roles in the digestive system because they produce digestive juices filled with enzymes to break down the components of food, such as carbohydrates, fats and proteins, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

Endocrine: The endocrine system consists of a network of glands that secrete hormones — long-range chemical messengers that regulate how cells and tissue function — into the blood. These hormones, in turn, travel to different tissues and regulate many bodily functions, such as metabolism, growth and sexual function, according to Johns Hopkins Medicine. For example, the pancreas releases the hormones insulin and glucagon to regulate blood sugar. Conditions like diabetes and insulin resistance arise from the body having too little insulin or not responding to it adequately. 

Related: Meet the 'exclusome': A mini-organ just discovered in cells that defends the genome from attack

Immune: The immune system is the body's defense against bacteria, viruses and other pathogens that may be harmful. Components of the system include the lymph nodes, which contain infection-fighting cells called lymphocytes. These lymphocytes are one of many types of leukocyte, or white blood cell. The immune system also includes the spleen, the bone marrow and a gland called the thymus. The immune system can learn to recognize antigens — proteins on the surface of bacteria, fungi and viruses — and alert the body to their presence. Some immune cells make proteins called antibodies that attach to these antigens and mark invaders for destruction. 

Lymphatic: The lymphatic system includes the lymph nodes, lymph ducts and lymph vessels and is considered part of the immune system. Its main job is to make and move lymph, a clear fluid that contains white blood cells. The lymphatic system also removes excess lymph fluid from the body's tissues and returns it to the blood.

Nervous: The nervous system controls both voluntary actions, such as conscious movements, and involuntary actions,like breathing, and it sends signals to and detects signals from different parts of the body. Conscious actions are controlled by the somatic nervous system, while involuntary actions are controlled by the autonomic nervous system. The autonomic nervous system dictates whether we're in "rest and digest" or "fight or flight" mode. The nervous system can further be split up into the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system, or the nerves connecting the CNS to every other part of the body.

Muscular: The body's muscular system consists of hundreds of muscles that aid movement, blood flow and other bodily functions, according to the Library of Congress. There are three types of muscle: skeletal, which is connected to bone and helps with voluntary movement; smooth, which is found inside organs and helps to move substances through them; and cardiac, which is found in the heart. The body's largest muscle by mass is the gluteus maximus, but the two latissimus dorsi are the largest in terms of surface area.

Related: Why is it harder for some people to build muscle than others?

Reproductive: The reproductive system allows humans to produce offspring. The male reproductive system includes the penis and the testes, which produce sperm. The female reproductive system includes the vagina, uterus and ovaries, which produce eggs. During fertilization, a sperm cell will fuse with an egg cell that, in a successful pregnancy, will then implant in the uterus. The fertilized egg will then mature into what's called a blastocyst, then an embryo and, finally, a fetus. A placenta forms to support this process. 

Skeletal: Our bodies are supported by the skeletal system, which contains between 206 and 213 bones in an adult human body, due to slight variations in people's anatomy, according to the medical resource StatPearls. These bones are connected by tissues called tendons, ligaments and cartilage. As infants, humans have about 300 bones, but some fuse together as the child grows. The skeleton not only helps us move but is  also involved in the production of blood cells and the storage of calcium. The teeth are also part of the skeletal system, but they aren't considered bones. The smallest bones in the body are found in the ear, and the largest is the femur, or thigh bone, which is also one of the heaviest body parts.

Respiratory: The respiratory system allows us to take in oxygen and expel carbon dioxide through breathing. It includes the lungs; trachea, or windpipe; and the diaphragm, a muscle that pulls air into and pushes air out of the lungs.

Urinary: The urinary system helps eliminate a waste product called urea, which is produced when certain foods are broken down. The system includes the two kidneys; two ureters, or tubes leaving the kidneys; the bladder; two sphincter muscles; and the urethra. The kidneys filter blood in the body to make urine that then travels down the ureters to the bladder and exits the body through the urethra.

Integumentary: The skin, hair and nails make up the integumentary system. Skin is the body's largest organ. It protects our innards from the outside world, serving as our first defense against bacteria, viruses and other pathogens, for instance. Our skin also helps regulate body temperature and eliminate waste through perspiration, or sweat. 

Related: Scientists discover new way humans feel touch 

What are the body's vital organs?

Click the purple circles to learn about the body's vital organs, including the brain, lungs, heart, liver and kidneys. They're considered vital because you need a functioning brain, heart, liver, at least one kidney and at least one lung to survive. That said, there are medical devices and treatments that can make up for a loss of function in these organs, at least temporarily — for example, ECMO machines can do the work of the heart and lungs, and dialysis can filter the blood of people with kidney failure.

Fast facts

• The average adult male body contains about 36 trillion cells, the average adult female body contains 28 trillion cells and a 10-year-old has about 17 trillion. 
• It's often said that there are 78 organs in the human body, but the number actually differs depending on whom you ask. 
• There's a popular idea that the body replaces itself every seven years. But that's not really true, because tissues renew themselves at different rates. 
• Oxygen is the most common element in the human body, followed by carbon. 
• The average adult body contains about 1.2 to 1.5 gallons (4.5 to 5.5 liters) of blood. 
• Humans' average body temperature has fallen slightly over time, so it's no longer 98.6 degrees Fahrenheit (37 degrees Celsius). 
• The most detailed map of the human brain to date contains more than 3,300 types of brain cells. 

What are vestigial organs?

There are arguably some parts of the human body that don't serve any useful purpose, such as the male nipple. That said, the usefulness of some organs is still up for debate, as scientists have often judged the worth of body parts before discovering their purposes. 

Broadly speaking, vestigial body parts are defined as those that have lost their original physiological significance to humans over the course of evolutionary history. The idea is that, while we inherited them from an ancient ancestor, we could really do without them in the modern day. 

Wisdom teeth are held up as one example of a vestigial body part, as the modern human jaw is often too small to accommodate a third set of molars. Some people also carry remnants of a vomeronasal organ that is largely thought to be nonfunctional in humans; animals use equivalent organs to detect each other's pheromones. 

Some scientists consider the human tailbone, or coccyx, vestigial because it's no longer a full-blown tail. But it's far from useless, as it still anchors many muscles, ligaments and tendons. And the appendix has gotten a bad rap for supposedly being both vestigial and useless, but more recently, scientists have uncovered possible functions for the long-maligned body part. 

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

Editor's note: This page was last updated on April 5, 2024.

With all that's known about human anatomy, you wouldn't expect doctors to discover a new body part in this day and age. But now, researchers say they've done just that: They've found a network of fluid-filled spaces in tissue that hadn't been seen before.

These fluid-filled spaces were discovered in connective tissues all over the body, including below the skin's surface; lining the digestive tract, lungs and urinary systems; and surrounding muscles, according to a new study detailing the findings, published today (March 27) in the journal Scientific Reports.

Previously, researchers had thought these tissue layers were a dense "wall" of collagen — a strong structural protein found in connective tissue. But the new finding reveals that, rather than a "wall," this tissue is more like an "open, fluid-filled highway," said co-senior study author Dr. Neil Theise, a professor of pathology at New York University Langone School of Medicine. The tissue contains interconnected, fluid-filled spaces that are supported by a lattice of thick collagen "bundles," Theise said. [11 Body Parts Grown in the Lab]

The researchers said these fluid-filled spaces had been missed for decades because they don't show up on the standard microscopic slides that researchers use to peer into the cellular world. When scientists prepare tissue samples for these slides, they treat the samples with chemicals, cut them into thin slices and dye them to highlight key features. But this fixing process drains away fluid and causes the newfound fluid-filled spaces to collapse.

Rather than using such slides, the researchers discovered these fluid-filled spaces by using a new imaging technique that allows them to examine living tissues on a microscopic level.

The researchers are calling this network of fluid-filled spaces an organ — the interstitium. However, this is an unofficial distinction; for a body part to officially become an organ, a consensus would need to develop around the idea as more researchers study it, Theise told Live Science. The presence of these fluid-filled spaces should also be confirmed by other groups, he added.

Official designation aside, the findings may have implications for a variety of fields of medicine, including cancer research, Theise said. For example, the findings appear to explain why cancer tumors that invade this layer of tissue can spread to the lymph nodes. According to the researchers, this occurs because these fluid-filled spaces are a source of a fluid called lymph and drain into the lymphatic system. (Lymph is a fluid that contains infection-fighting white blood cells.)

A new organ?

The human body is about 60 percent water. About two-thirds of that water is found inside cells, but the other third is outside cells and is known as "interstitial" fluid. Although researchers already knew that there is fluid between individual cells, the idea of a larger, connected interstitium — in which there are fluid-filled spaces within tissues — had been described only vaguely in the literature, Theise said. The new study, he said, expands the concept of the interstitium by showing these structured, fluid-filled spaces within tissues, and is the first to define the interstitium as an organ in and of itself.

The new work is based on the use of a relatively new technology called a "probe-based confocal laser endomicroscopy" or pCLE. This tool combines an endoscope with a laser and sensors that analyze reflected fluorescent patterns and gives researchers a microscopic view of living tissues.

Back in 2015, two of the study authors — Dr. David Carr-Locke and Dr. Petros Benias, both of whom were at Mount Sinai-Beth Israel Medical Center in New York City at the time — were using this technology when they saw something unusual while examining a patient's bile duct for cancer spread. They spotted a series of interconnected cavities in the tissue layer that didn't match any known anatomy, according to the report. When a pathologist made slides out of this tissue, the cavities disappeared — a mystery that was later found to be a consequence of the slide-making process.

In the new study, the researchers first used pCLE on cancer patients who were undergoing surgery to remove the pancreas and the bile duct. The imaging technique indeed showed the fluid-filled spaces in the connective tissue. When the tissue samples were removed from the body, they were quickly frozen, which allowed the fluid-filled spaces to stay open so the researchers could see them under a microscope.

Later, the researchers saw these same fluid-filled spaces in other samples of connective tissue taken from other parts of the body, in people without cancer, Theise said. "The more tissues I saw, the more I realized it's everywhere," he said.

The researchers think that the fluid-filled spaces may act as shock absorbers to protect tissues during daily functions, the researchers said.

Theise noted that there may be quite a bit of information already known about this fluid-filled space; it's just that researchers "didn't know what they were looking at." Indeed, the researchers plan to conduct a review of the scientific literature "for all the things we know about this [body part] but didn't know we knew it," Theise said.

New Questions

The idea presented in the study appears to be "a completely new concept," said Dr. Michael Nathanson, chief of the digestive diseases section at Yale University School of Medicine, who was not involved with the study. "From the evidence they presented it's quite possible they're correct," Nathanson told Live Science.

Previously, physicians had a somewhat nebulous understanding of the interstitial space, Nathanson said. They knew it was a space with fluid found outside the cells, but no one had ever entirely explained what this means. The new study "did a nice job" of trying to define it, he said.

The findings are consistent with what Nathanson and colleagues observed in a study published in 2011. At that time, Nathanson and colleagues observed a network of dark fibers, but they weren't able to figure out exactly what it was. "I was pleased that they substantiated our impression that this network exists" and were able to define it, Nathanson said.

The new finding "allows us ask all kinds of questions we didn't even know to ask beforehand," Nathanson said. For example, could this area become altered in disease, or play a role in driving disease, he said.

Editor's note: This article was updated at 11:20 am E.T. to include comments from Dr. Michael Nathanson.

Original article on Live Science.

Fins are not really the most noticeable thing about the tuna. Watch these marine wonders at an aquarium, and you're more likely to notice their large, gaping mouths or their silver, streamlined bodies.

But fins, it turns out, are one of the weirdest things about these fast-swimming fish. In a trick never seen before in any animal with a backbone, tuna use their lymphatic system to control their dorsal and anal fins.

Yes — the lymphatic system. This network of fluid-filled vessels and nodes, best known for producing those swollen bumps you get on your neck when you have a fever, acts like a hydraulic system to stiffen the fins and increase the tuna's mobility when they're chasing prey, said Barbara Block, a biologist at Stanford University's Hopkins Marine Station. The discovery, Block said, was completely unexpected.

"It's the first example of lymphatic fluids or the lymphatic system taking part in motion control in vertebrates," Block told Live Science.

Speed and control

Tuna are adapted for two things: speed and long, ocean journeys. Yellowfin tuna, for example, have been clocked cruising at 47 mph (75 km/h), and tuna species regularly migrate huge distances across the Pacific Ocean. [Gallery: See Photos of the Freakiest-Looking Fish]

Tuna have sleek, teardrop-shaped bodies and small fins to help them cut through the water. These fins move only subtly, Block said. In fact, she and her colleagues never would have noticed the tiny adjustments the fish make with their fins if they hadn't been able to watch them in the Monterey Bay Aquarium's massive Open Sea tank. Such detailed observations never would have been possible in the wild, Block said.

"People like me just spent hours watching these tunas," she said.

The plot deepened when Vadim Pavlov, a postdoctoral researcher in biomechanics at Stanford, dissected some bluefin fins for a separate study and discovered a strange fluid-filled cavity under the second dorsal and anal fins (the vertical fins on the tuna's back and belly). The researchers called in Benyamin Rosental, a postdoctoral researcher in regenerative medicine at the Hopkins Marine Station, who helped analyze the cells within the fluid and the tissue that made up the cavity and its attached vessels. The fluid, he found, was lymphatic fluid. The lymphatic system plays a role in draining the body of excess fluid and is a main highway for the transport of immune cells; no one had ever seen it integrated into a fin structure like this.

Repurposed system

At first the team thought perhaps the fish was using the chamber to regulate its temperature. The reality turned out to be much more surprising. The researchers found that small muscles at the base of the fins contract to push lymph fluid into that chamber under the fin, and, from there, into channels in the fin itself. The fluid then forces the fin into a more erect position. The stiffened fins form a pivot point for the speedy fish, giving them a way to make sharp, quick turns in the water: Imagine the difference between trying to turn a canoe with a pool noodle versus a sturdy wooden oar.

High-speed video taken at the Monterey Bay Aquarium and at Stanford's Tuna Research and Conservation Center shows tuna using this ability to snatch squid snacks from the water. The lymphatic hydraulic system is found in both Pacific bluefin tuna (Thunnus orientalis) and yellowfin tuna (Thunnus albacares), the researchers reported today (July 20) in the journal Science. Mackerel, the evolutionary lineage that gave rise to tuna, don't universally have this type of control, Block said, but the researchers did find a similar structure in Pacific bonito and the Spanish mackerel, both close cousins of tuna.

"It's clear that in the evolution of these cruise specialists, there are steps along the way and some of these fish have the hydraulic fin mechanism and some of them don't," Block said.

The researchers are now using sophisticated instruments to study tuna swimming abilities in more detail, measuring things like acceleration and kinematic movement in the water. Beyond just understanding the "amazing specializations" of tuna, Block said, the goal is to help inspire new innovations in robotics.

"There are really going to be some interesting opportunities in the world of autonomous vehicles to learn from what the tunas are doing," she said. 

Original article on Live Science. 

A rare condition called elephantiasis, which tends to strike people in tropical parts of the world, was long thought to occur due to a parasitic infection. But a new study shows that the condition can have another cause: sharp crystals found in certain soils.

In elephantiasis, a person's limbs become discolored and swollen with fluid. They may swell to enormous sizes, resembling the limbs of an elephant. The most common cause of the condition is a mosquito-borne parasitic infection called lymphatic filariasis, in which microscopic worms invade the body's lymph nodes and vessels, causing the swelling.

But in 2014 and 2015, there were reports of two intense outbreaks of elephantiasis in western Uganda, an area that is not known to harbor these worms. [The 10 Most Diabolical and Disgusting Parasites]

Indeed, when the Uganda Ministry of Health investigated the outbreak, the agency identified another cause of elephantiasis: a disease called podoconiosis. And what was thought to be an outbreak of a parasitic disease was far from it; rather, the symptoms of podoconiosis slowly build up over decades, gradually developing into elephantiasis, the study said.

"People can be suffering from podoconiosis, a noninfectious disease, for decades before it becomes obvious that they are developing elephantiasis," lead study author Dr. Christine Kihembo, a senior field epidemiologist at the Uganda Ministry of Health, said in a statement. "Many of the people affected in western Uganda probably had been suffering silently without help for more than 30 years."

In September 2015, Kihembo and her team visited the region to try to determine why people there were getting the disease. Their findings were published published Monday (April 10) in the American Journal of Tropical Medicine and Hygiene.

In the study, the researchers looked at 52 people who had hard, asymmetrical swelling of both legs that lasted for at least one month, along with other relevant symptoms, such as itching or burning in their legs,  rigid toes and skin with a "mossy" appearance, the researchers wrote in the study.

They ran blood tests and ruled out the usual case of this swelling, the parasitic infection called lymphatic filariasis. In 40 of the 52 people with symptoms, the parasite infection was not found. And so the researchers concluded that these 40 people probably had the other cause of the swelling, podoconiosis.  

But what causes podoconiosis? To answer this question, the researchers looked to the soil.

Prolonged exposure to volcanic soil was thought to cause podoconiosis, the study said. When a person spends a lot of time walking barefoot on volcanic soil, mineral crystals penetrate the soles of his or her feet. These crystals make their way into the lymphatic system, and damage it. The lymphatic system, which includes the lymph nodes, is a network of tissues and organs that help rid the body of waste. When the system is damaged, a clear fluid called lymph, which contains white blood cells, can build up, causing the characteristic swelling of elephantiasis.

From interviews with the participants, the researchers learned that about half were farmers. Of these farmers, about two-thirds never wore shoes while farming and two-thirds never washed the soil off their feet during the day, the researchers found. In contrast, most of the people in the control group wore shoes while farming. However, not wearing shoes at home was also associated with developing the disease.

Farming while barefoot was strongly associated with developing the disease, the researchers wrote.

Indeed, the researchers recommended that in the future, the villagers be provided with shoes to help reduce their risk of developing the disabling disease.

Originally published on Live Science.

A Brazilian man with elephantiasis, a rare condition in which people's limbs become discolored and swell to enormous sizes, was recently featured on the popular Animal Planet show "River Monsters," which often films in tropical, heavily forested locales.

As the name "elephantiasis" implies, the condition causes a person's limb to resemble that of an elephant.

Elephantiasis is actually a complication of a parasitic infection called lymphatic filariasis. More than 120 million people in 73 countries have this infection, according to the Centers for Disease Control and Prevention (CDC).

The worm-like parasites that cause the infection are spread by mosquito bites, said Dr. Amesh Adalja, an infectious-disease specialist and a senior associate at the University of Pittsburgh Medical Center's Center for Health Security.

However, most people who become infected with the parasites do not have any symptoms, Adalja told Live Science. And only some develop long-term symptoms, such as elephantiasis, he said. It's unclear why some people go on to develop elephantiasis and others do not, he added.

Once in the body, the parasite lives in the lymphatic system, according to the CDC.  The lymphatic system, which includes the lymph nodes, is a network of tissues and organs in the body that help rid the body of waste. When it is not working properly, lymph (a clear fluid containing white blood cells) can build up. [The 10 Most Diabolical and Disgusting Parasites]

Symptoms develop when the parasites start to alter the lymphatic system, Adalja said. A single mosquito bite is not enough to cause these changes, however. Rather, a person needs to be infected repeatedly with the parasites for symptoms to develop, and each time a person is infected, the damage to the lymphatic system worsens, he said. 

The infection can eventually cause the lymphatic vessels to become so dilated that the lymph fluid pools in a region of the person's body — usually, the limbs, breasts or genitalia, he said. This pooling is what causes the massive swelling associated with elephantiasis.

The swelling causes the skin to stretch beyond its normal capabilities, Adalja said. This can severely damage the skin, and ultimately lead to the change in color and texture that occurs in elephantiasis, he said.

Although the infection can be treated in its earlier stages with anti-parasitic medications, it's unclear whether treatment is beneficial once a person has developed elephantiasis, Adalja said.

Rather, the goal is to treat people early, before they have such symptoms, he said. Indeed, campaigns to eradicate the disease from affected areas generally target the entire population, he added.

Though the disease is widespread in tropical areas, it's not something that people in the United States need to worry about because the parasite is not found here, Adalja said.

Follow Sara G. Miller on Twitter @saragmiller. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.