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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 in seeking it because of something you have read or seen in any Khan Academy video.
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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.


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.
All content in “Endocrine system”

Introduction to hormones

You might hear the word “hormone” thrown around a lot. But what exactly is a hormone? And, more importantly, why should you care what a hormone is? Hormones are built from single amino acids, chains of amino acids (peptides), or cholesterol, and they are a form of chemical communication between cells. Sometimes the signal is being sent a long distance, for example from the brain down to the bones in the toe, and other times the signal is being sent to cells that are just a few millimeters away. Either way, hormones allow important conversations to take place within the body.

Biological basis of behavior: Endocrine system

Consider any behavior, sleeping for instance, and think about all of the organs that have to work together to have it go smoothly. The heart and lungs need to slow down, the brain needs to stop taking in the cues from the environment, and the bladder needs to wait until morning to empty. This coordinated effort is achieved by a number of unique hormones acting on different tissues. Learn more about how this process works and why it’s so critical to our everyday lives!