Our bodies are fascinating. Human biology is amazingly complex and it can be mind-boggling when you consider how many complicated processes are carried out by our bodies every second.

Whether you are a bio expert or know absolutely nothing about the human body, it’s useful to know at least the basics about human biology so you can make informed decisions that will help keep your body working smoothly. Plus, it's interesting!

Read on to learn about the 15 human biology basics that everyone should know. (These are also all covered in Brainscape's online Biology flashcards).

15 basic human biology facts

human biology plastic model of lungs

1. The human body has 12 systems

The 12 human body systems are the cardiovascular system, the digestive system, the endocrine system, the immune system, the integumentary system, the lymphatic system, the muscular system, the nervous system, the reproductive system, the respiratory system, the skeletal system, and the urinary system.

All of these systems work together to ensure that our bodies work correctly:

All of these are only some of the main functions of each system, but each system performs many others.

2. There are four blood groups: A, B, AB, and O

Your lettered blood type is determined by which antibodies are in your plasma and which antigens are found on your red blood cells. Antibodies are blood proteins, while antigens are substances that activate an immune response and control what enters and exits a cell. Each blood group can be either positive or negative, resulting in eight possible blood types. The +/- part of a person’s blood type is determined by the presence (or absence) of a third antigen called the Rh factor.

Our bodies can handle blood without the presence of an antigen that we usually have, but cannot handle the introduction of a new antigen into the circulatory system. That’s why people with O- blood are known as universal donors; anyone can use O- blood. People with AB+ blood, on the other hand, are universal recipients; they have every antigen in their blood already.

3. Our DNA is stored in 23 pairs of chromosomes within the nucleus of every cell in our body

DNA human biology
DNA has 23 pairs of chromosomes and exists in the nucleus of every cell in our body.

Each cell has a full set of chromosomes which contain all the genetic material needed to determine the makeup of our entire bodies. That’s why cloning of animals can be done with just one cell. All the genetic material that defines us is inside each and every cell of our body, from our hair follicles to toenails.

4. Our immune system fights off infection mostly through the use of antibodies and microphages

Antibodies actually fight infection by killing the virus or foreign bacteria, while microphages are white blood cells that surround and contain the foreign cells (or other objects) to prevent the spread of disease.

5. There are more non-human cells in our body than human ones

There are ten times more bacteria cells in our bodies than our own human cells. These bacteria are harmless or even help us perform key bodily functions, such as digestion. Even our DNA itself isn’t all from human evolution. Human DNA includes the genes from at least eight retroviruses that were absorbed into our own genetic code at some point. The viral genes in our DNA now perform important functions, especially related to reproduction.

6. We have more than five senses (and each has its own sensory organ or specialized receptors)

While we typically think of the traditional five senses of touch, taste, hearing, vision, and smell, our bodies can actually sense many other things. Some of the most important senses include:

  1. Vision
  2. Hearing
  3. Smell
  4. Taste
  5. Touch
  6. Balance
  7. Temperature
  8. Proprioception (spatial body awareness, aka why you can touch your nose with your eyes shut)
  9. Pain

Each is associated with its own organ (taste with the tongue, smell with the nose) or sensory receptor (the skin contains separate touch, temperature, and pain receptors).

7. Our appendix does actually still have a purpose

We have long thought that the appendix was simply the result of evolution—a body part that once had a purpose, but that no longer does anything other than occasionally get infected.

Research has revealed, though, that the appendix actually serves as an important place for the bacteria in the digestive system to rest and reproduce. You can definitely live without it (so don’t worry if yours has been removed!), but when it’s still a part of your system, the appendix can be a real help.

8. Nearsightedness and farsightedness are caused by defects in the shape of our eyeballs

Nearsightedness, or myopia, is caused by a greater curve in the cornea of the eye or by an elongation of the eyeball. Farsightedness, or hyperopia, is caused by a corneal curve that is too small or by a having a short eyeball. Some evidence indicates that nearsightedness is genetic.

glasses showing the words clearly in a blurry book, bad vision fixed with glasses

9. Vaccines safely help the body to recognize and fight off infections later in life

By injecting the body with dead virus cells, we activate an immune system response to the virus without actually catching the disease. This allows our bodies to create antibodies to fight off the infection if we are ever exposed to it. That’s why sometimes people get a fever as a side effect of a vaccine. They’re not catching the disease, however: the body is simply practicing how it would kill that virus if it ever came into the body (and fever is an important part of that).

10. We still aren’t 100% sure why people yawn

Many scientists today think that yawning is a way to keep our brains alert in times of stress, but exactly why that happens or what the yawn does to help our body isn’t 100% clear. That could be why they are contagious; we are alerted to a potential stressor by another person. Others believe that yawns are a reaction to being tired, as a way to reengage. Yawning may help us get more oxygen to help our brains perform better, or it may cool down the brain, which gets hotter in times of stress. We still aren’t exactly sure what role yawning serves in human biology.

11. The red color of our blood is caused by the shape of the structure created when iron and oxygen bond with hemoglobin

Many people assume that blood is red simply because of all the iron in it, much like the reason rust is red. That’s actually not entirely accurate. The red color is created because the iron is bound in a ring of atoms in hemoglobin called porphyrin. This structure has a shape that makes the blood appear red. When oxygen is bound to the porphyrin ring, it changes the shape, making our red blood cells appear as an even more vivid shade of red.

12. The brain works harder while we are asleep than during the day when we are awake

Many people assume that sleep helps the brain rest, but our brains are actually busier during slumber. When we sleep and dream, our brains carry out important functions that they cannot perform while focusing on movement and conscious thought. During sleep, our brains process things we learned and emotions we felt during waking hours and saves them in our memory. That’s why sleep is so important to learning.

woman sleeping in bed for rest
Reminder: Be sure you're getting enough sleep daily!

13. The liver has over 500 functions.

Our liver doesn’t just filter toxins from the blood. It does much more to keep our bodies healthy. Some of its other important functions include creating bile that breaks down fat and carries away waste, producing cholesterol, regulating blood clotting, processing hemoglobin, and so much more. As you can see, the liver is vital to our health, so treat it well.

14. Sunburn doesn’t just increase your risk of skin cancer; it also damages blood vessels

A moderate sunburn can do long-term damage to the blood vessels in your skin, making it more difficult for the affected skin to heal and stay healthy. It can actually take four to fifteen months for these capillaries and small arteries and veins to return to a normal condition.

15. All body parts can repair themselves (except teeth)

Innate human biology allows us to repair ourselves pretty easily for the most part. While any serious damage to the body can take a long time to heal, all of our body parts have the ability to start healing and regenerating on their own—except teeth. Since the enamel of teeth is not a living tissue, it cannot regenerate, even if the injury goes deep enough to damage the living part of the tooth. That’s why a chipped tooth always takes a visit to the dentist to be entirely fixed.

Dig deeper into human biology

Human biology is one of the most fascinating sciences. Understanding it will not just give you the academic foundation you need to enter careers in medicine or science, but it will also help you take care of your own health.

And always remember that Brainscape's biology flashcards are an essential biology study tool. They're designed by top students and educators around the world, and they use a study system based on the most current cognitive science to be the most effective way to learn biology at any level. Use them to ace your biology test, study for the MCAT, or even just deepen your own knowledge. And feel free to make your own flashcards in Brainscape as well!

Human biology scientist with tubes and gloves