If you're seeing this message, it means we're having trouble loading external resources for Khan Academy.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Health and medicine

The medical world can be a confusing place. Patients and their families might feel overwhelmed by the large vocabularies and complicated explanations they get from their health care providers. Students entering health care also struggle to grasp the complexity of health sciences, and are forced to memorize huge amounts of information. We hope to make understanding the medical world a bit easier. Look around! These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay seeking it because of something you have read or seen in any Khan Academy video.
Community Questions

Circulatory system

Your heart sits in the middle of your chest and pumps blood from about 4 weeks after conception until the day that you die. It never stops, and over your lifetime it will pump ~175 million liters of blood. To visualize that, imagine the amount of water that falls over Niagara falls in a few minutes. Remarkable! This little pump is the size of your clenched fist and in an adult can weigh about 300 grams. Watch these videos to learn more about how the heart works, blood flow in arteries and veins, blood pressure, and lymphatics.

Circulatory system diseases

With the heart pumping 24 hours a day, 7 days a week, it’s absolutely vital to make sure things are flowing smoothly (pun intended!). Unfortunately, this isn’t always the case, and different parts of the circulatory system can cause problems: your heart, your blood vessels, and even the fluid in your tissues and blood itself can be the issue. To further complicate things, the underlying reasons for circulatory system problems vary from your genes (nature) to your lifestyle habits (nurture). An understanding of how different diseases can affect your circulatory system is important to combat this growing problem in the world.

Respiratory system

Place your hand on your ribs and inhale deeply. You’ll notice that your chest expands and your back straightens. As this occurs, air is rushing through your windpipe and branches off to either your left or right lung. After 20 to 30 more branch points, oxygen in the air ends up in the alveoli where it diffuses into the liquid that surrounds the alveoli, and slips into the blood. This microscopic gas exchange occurs rapidly, oxygen is taken into the body and carbon dioxide is removed from the body, and then you exhale. Learn more about the intricate and fascinating respiratory system in these videos!

Respiratory system diseases

Our lungs are composed of a bronchial tree (think of an upside down tree with millions of leaves), blood vessels bringing blood in and out, and a protein-rich fluid that forms a matrix holding everything together! If any part of this well-balanced organ isn’t working properly, a person can be left feeling short of breath. The lungs are also exposed to the outside environment, making them prone to infections. To counter infections, the lungs are lined with cells that have tiny protein bristles which wave back and forth and can literally sweep away dangerous bacteria. Learn more about diseases of the lungs and how modern medicine helps to keep them healthy!

Renal system

If you want to learn more about the renal system, then urine the right place! Take a minute and imagine a dirty pool filled with algae. Placing a filter in this pool will cause the algae to be flushed out and disposed of, and after a time you’ll have a nice, clean, crispy blue pool to enjoy. Just like the filter for a pool, our kidneys filter our blood and remove toxic wastes from it. Humans have developed a system of disposing of this waste, while retaining nutrients and substances important to our well-being. Our kidneys play a vital role in maintaining homeostasis, or the physiological balance of the body. Every thirty minutes, our kidneys filter the entire supply of blood in our body. Unfortunately, they are also prone to disease, which is illustrated by the fact that 1 in every 9 American adults currently has a kidney disease. Additionally, more than 82% of Americans waiting for a life-saving organ transplant is waiting for a kidney, but only 17% of these patients get one each year. In this tutorial, learn how the kidneys take blood and very selectively extract waste from it to expel from the body as urine.

Nervous system and sensory information

There are billions and billions of neurons in your brain (about 85 billion), and they’re all sending electrical signals throughout your body right now! They tell your eyes to move across this page, how to interpret the words that you read, how to maintain your posture, your heart rate, and your breathing...all of it in a fraction of a second. In this section, we’ll explore the nature of this vast, complex system, from the cellular level to how it operates at a sensory level. A common misconception is that we only have 5 senses (see, smell, taste, hear, and feel), but we have many more that are nuanced but equally important. Learn more about how our bodies are designed to interact with the world.

Executive systems of the brain

Aristotle asserted that what separates humankind from non-human animals is our ability to engage in high reasoning. This reasoning includes solving problems, making decisions, recalling and recording memories, and expressing complex emotions. We’ll explore different states of consciousness, and how our brain adapts and responds to stimuli. Learn all about the higher-order executive functions of the brain, which help you remember your friend’s name, learn a new language, and even fall asleep at night.

Hematologic system

It takes between 30 seconds to a minute for your blood to travel from your heart, to your body, and back to the heart again - perhaps a bit longer if the trip is out to your big toe! Our blood is incredibly important for transporting oxygen throughout the body. Hemoglobin, the protein that fills our blood cells, has wonderful mechanisms to allow it to bind to both oxygen and carbon dioxide. This is important for effective and quick transport of the gases around our body. Our blood is about 45% cells and 55% plasma, so the old adage “blood is thicker than water” quite literally holds true in scientific terms! Learn more about how this amazing system works in the following videos.

Hematologic system diseases

Blood is incredibly important in combating disease and the healing process after an injury. It acts as a highway for medicine, stops bleeding, fights infections, controls cells from multiplying too fast, and so much more. But things can go wrong with blood too! What if your blood couldn’t clot and stop you from bleeding, or started to clot uncontrollably? What if your red blood cells or white blood cells suddenly disappeared? Blood contains many different types of tissues doing very different jobs, making diseases of the blood produce a variety of symptoms, including continuously feeling tired and bone pain. Learn about the different blood diseases, how they are diagnosed, and the cool ways health professionals treat these conditions.

Immune system

Chances are, you’ve had a fever or a cough at least once in your life (unless you live in a bubble, in which case go outside more!). Have you ever wondered why your body is reacting this way? It turns out humans have an entire arsenal of weapons against invaders, like bacteria and viruses. We have specialized cells that destroy various foreign bodies in a number of ways, including consumption, expulsion, and degradation. Learn how these little fighters are keeping you alive and healthy in this tutorial.

Musculoskeletal system

Our muscles and bones keep us moving and form the basic physical structure for the rest of our organ systems. They also act as a sort of protective armor against physical damage. Muscles are connected to bones, and bones are connected (via ligaments) to other bones. In these videos, we’ll go into how and why we have conscious control of our muscles, how our bones fit in with this control, how each of these components are connected, and much more. A fun fact: The largest muscle in your body is your gluteus maximus (your buttock!), and the bone most often broken is the collar bone!

Endocrine system

When you’re nervous before an important speech, or asking someone out on a date, you might feel butterflies in your stomach. This is actually the result of your endocrine system releasing hormones! You can’t really point to any single organ as “the endocrine system”, because it’s actually a family of glands that secrete hormones into the body. Hormones seep into the blood (imagine putting a tea bag into hot water), and as the blood flows around the body, it carries with it these important hormone molecules that interact with specific target cells and organs. This signaling system helps to keep the entire body well-balanced and on the same page.

Lab values and concentrations

Ever wonder about your lab values and what they mean? Lab values measure amounts of electrolytes or cells in your blood and occasionally tell you about how hormones and enzymes are working! Dive deeper and get a good understanding of concentrations as well!

Endocrinology and diabetes

In this section, we’ll revisit the endocrine system. After a review, we’ll explore how our hormones can cause different kinds of symptoms and behaviors, including normal childhood growth and precocious puberty (puberty kicking in at an earlier age than normal). After that, we’ll take a closer look at diabetes, which is a growing endemic in the world as we see a greater availability of cheap, low quality foods. This will include a focus on glucose concentration and other blood sugar levels, and what your body (and modern medicine) can do to maintain a healthy balance in your body.

Colon disease

The colon, otherwise known as the “large intestine,” is a tube that’s about 5 feet long (1.5 meters). This is where the majority of fluid reabsorption occurs in your GI tract (the tract extending from your mouth to your anal sphincter). The colon is susceptible to multiple diseases, including hyperplasia, dysplasia, and cancer. Learn more about healthy colon tissue and these three diseases in the following videos. Join Sal and Dr. Andy Connolly as they (and you) take a microscopic look at colon tissue!

Cervical spine

Your cervical spine is the uppermost part of your spine, the part that makes up your neck. Take a look at the 7 vertebra that compose the cervical spine, and see different views of a real person’s spine! Join Sal and Dr. Mahadevan as they inspect these X-rays and discuss its alignment and protection in airway management.

Healthy lifestyle

If you looked at our “Circulatory system diseases” section, you already know that diseases related to an unhealthy lifestyle are on a critical rise. We hope that the following set of videos will allow you to develop a healthier lifestyle, and help you improve the lives of others as well. This is important for parents, children, students, and anyone who wants to take better care of their body. Learn some of the fundamentals behind staying healthy: Reducing your salt, keeping your weight in a healthy range, and exercising regularly.

Health care system

The health care system in the United States is rapidly changing. To better understand these changes, we review the health care insurance, drug pricing, physician compensation, and much more! join us as we explore the basics about the Health Care system in the US, including a comparison with European healthcare.


Circulatory system
Your heart sits in the middle of your chest and pumps blood from about 4 weeks after conception until the day that you die. It never stops, and over your lifetime it will pump ~175 million liters of blood. To visualize that, imagine the amount of water that falls over Niagara falls in a few minutes. Remarkable! This little pump is the size of your clenched fist and in an adult can weigh about 300 grams. Watch these videos to learn more about how the heart works, blood flow in arteries and veins, blood pressure, and lymphatics.
All content in “Circulatory system”

Heart introduction

No organ quite symbolizes love like the heart. One reason may be that your heart helps you live, by moving ~5 liters (1.3 gallons) of blood through almost 100,000 kilometers (62,000 miles) of blood vessels every single minute! It has to do this all day, everyday, without ever taking a vacation! Now that is true love. Learn about how the heart works, how blood flows through the heart, where the blood goes after it leaves the heart, and what your heart is doing when it makes the sound “Lub Dub”.

Blood vessels

Where does your blood go after it leaves the heart? Your body has a fantastic pipeline system that moves your blood around to drop off oxygen and food to those hungry cells, and removes cell waste. Learn how arteries carry blood away from the heart, how veins bring blood back to the heart, and about the different layers of cells that make up these blood vessels.

Blood pressure

Using the stethoscope to check blood pressure is a technique that’s been used for >100 years! Blood pressure is one of the major vital signs frequently measured by health care workers, and it tells us a lot about our blood circulation. Learn what blood pressure is, how it relates to resistance in a tube, why it is necessary to get oxygen to your cells, and how it can change as you age. We’ll finally put it all together by relating pressure, flow, and resistance in one awesome equation!


Welcome to the lymphatic system! Learn about how it is a critical part of the circulatory system. Find out how it comes to the rescue of the cardiovascular system and the immune system. Also discover how it moves fluid in one direction, like blood, but without a heart!

Fetal circulation

At one stage or another in development, every friend you know had gill slits and a tail. Pretty crazy thought, huh? Fetal development is incredible, and it’s important to understand exactly how it happens. The structure and function of the circulatory system is incredibly complex, and fetuses are no exception. Find out how the heart and circulatory system work in the fetus!

Blood pressure control

The human body enjoys stability. For example, if your blood pressure changes, the body puts a couple of brilliant systems into motion in order to respond and bring your blood pressure back to normal. There are some quick responses using nerves and some slower responses using hormones. The system using hormones is sometimes called the renin-angiotensin-aldosterone-system (RAAS), which is the main system in the body for controlling blood pressure. When your blood pressure drops too low or gets too high, your kidneys, liver, and pituitary gland (part of your brain) talk to each other to solve the problem. They do this without you even noticing! Learn how the body knows when the blood pressure has changed, and how hormones like angiotensin 2, aldosterone, and ADH help return blood pressure to back to normal.

Arterial stiffness

Believe it or not, the arteries are elastic and when they recoil they actually push blood along when the heart is relaxing (diastole). This is known as the windkessel effect and is the same basic principle used by some water guns. Unfortunately, with all the work that the circulatory system has to do, our arteries can become rigid with age. When the arteries get stiff like lead pipes, the problem is quite different then when the arteries actually get clogged up, but just as important.

Heart muscle contraction

Your heart is made of a special type of muscle, found nowhere else in the body! This unique muscle is specialized to perform the repetitive task of pumping your blood throughout your body, from the day you’re born to the day you die. We’ll take an in-depth look of how the heart accomplishes this on a cellular level, and learn about the proteins actin and myosin that are the workhorses that tug and pull on one another to create every single muscle contraction. You’ll appreciate the fact that your heart beat is a fairly sophisticated process!

Heart depolarization

Your heart relies on the flow of electricity to maintain a steady, consistent beat - like an automatic pump that maintains a regular rate and rhythm throughout your life! There are specialized heart cells that allow positive current to travel quickly throughout the heart muscle. In these videos, we’ll check out the flow of this positive charge on a macroscopic and microscopic level.

Nerve regulation of the heart

Although your heart can beat independently, your nervous system is important as an external regulator. Your brain can tell your heart to speed up or slow down, depending on the scenario. For example, when you’re falling asleep, your nervous system will cause your heart to slow down, and 8 hours later when your phone alarm goes off, your nervous system will speed up your heartbeat! So even though your heart muscle beats by itself, the nerves can ramp up or down the speed. Check out the videos to learn more about how the nerves help to regulate the heart.

Preload and afterload

After using your jeans for a while, you’ll begin to notice small tears and rips developing in the fabric. Why doesn’t this happen to your heart as well? Well, your heart manages to stay healthy despite all of the “wall stress” that pulls on the heart walls. During different parts of the heart cycle (afterload vs. preload) the mechanics of “wall stress” change dramatically. Learn exactly what preload and afterload mean, and how we can use pressure-volume loops to estimate their values.

Changing the PV loop

Once you’ve learned about the PV loop, a natural question arises - Does it ever change shape? It turns out that there are precisely three things that can change the shape of the loop: 1. Preload, 2. Afterload, and 3. Contractility. That’s it! The tricky part comes when you try to change one and you realize that the body begins to change the other two as well as a natural consequence. In order to simplify, you’ll find that PV loops are sometimes even described as PV boxes. You’ll get to learn about PV loops, PV boxes, and even play around with them yourself in this tutorial!

Pressure volume loops

The pressure volume loop is one of the classic figures that helps us to conceptualize and understand the mechanics of the left ventricle of the heart. In addition to a filling up with blood and squeezing out blood there is a (very short) period of time when the heart muscle is contracting and relaxing with no volume change! As the left ventricle moves around the PV loop with each lub dub you get a sense for the amazing amount of work it does as pressures and volumes go up and down, all day, every day. This is a fascinating area where physics and biology meet to produce something miraculous.