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

Human anatomy and physiology

Get introduced to all of the major organ systems of the body. You'll learn some general anatomy (roadmap for your body), and how the organs work to keep you alive! Watch some videos, read some articles, try some flashcards, and then quiz yourself!

Advanced circulatory system physiology

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.

Advanced respiratory system physiology

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!

Advanced hematologic system physiology

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.

Advanced endocrine system physiology

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.

Endocrine system diseases

Advanced nervous system physiology

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.

Nervous system diseases

Advanced gastrointestinal physiology

Advanced muscular-skeletal system physiology

Muscular-skeletal diseases

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.

Infectious diseases

There's an intricate dance between humans and viruses, bacteria, molds, and even tiny worms! They want to survive and replicate just like humans do... and this is why they cause infections.

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!

Current events in health and medicine

Learn more about some of the current issues in health and medicine. Dr. Rishi Desai is a pediatric infectious disease physician and former epidemiologist with the Centers for Disease Control and Prevention (CDC)

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.



Advanced circulatory system physiology

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.
Community Questions
All content in “Advanced circulatory system physiology”

Circulatory system 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 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!

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.

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 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.

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.

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.

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!