Human Body Systems — Organ Systems and Homeostasis

1.Organ systems are one team

The countless cells that make up the body need nutrients and oxygen to live, and they produce carbon dioxide and waste as they work. The digestive system absorbs nutrients, the respiratory system takes in oxygen and expels carbon dioxide, the circulatory system carries all of these throughout the body, and the excretory system filters waste out of the blood and removes it.

This lesson is a general explanation to help you understand the structure and workings of the body. It cannot replace judgments about an individual's symptoms or treatment, so if something is wrong with your body, please consult a medical professional.

2.The digestive system and the circulatory system

Digestion is the process of breaking the large molecules in food down into small molecules the body can absorb. In the mouth, food is chewed and an enzyme in saliva starts breaking down starch; in the stomach, gastric juice begins breaking down protein. In the small intestine, digestion is completed with the help of enzymes in pancreatic juice and intestinal juice and of bile, which is made in the liver and stored in the gallbladder, and most nutrients are absorbed through the villi lining the inner wall of the small intestine. The large intestine mainly absorbs water.

The circulatory system consists of the heart, blood vessels, and blood. The heart is a pump with 2 atria and 2 ventricles. The route on which blood travels from the heart through the lungs to pick up oxygen and back is called pulmonary circulation, and the route on which it travels from the heart around the whole body, delivering oxygen and nutrients to cells, and back is called systemic circulation. Arteries carry blood away from the heart and veins carry blood back to it, while the actual exchange of substances takes place in the very fine capillaries.

3.How fast does blood circulate?

An adult's body holds roughly 4.5–5.5 L of blood. The amount of blood the left ventricle pumps out in one contraction of the heart is called the stroke volume, and in a resting adult it is usually around 70 mL. Multiplying this by the number of heartbeats in 1 minute gives the amount of blood the heart pumps out per minute (cardiac output).

If you do the calculation, even at rest an amount similar to all the blood in the body passes through the heart about every minute. During exercise, heart rate and stroke volume both increase, so cardiac output grows several times over, carrying more oxygen to the muscles. That is why your heart beats fast when you exercise.

One common misconception is that “blood in the veins is blue.” Venous blood, having given up its oxygen, is just a darker red than arterial blood, not blue. Veins look bluish through the skin because of the way light is absorbed and scattered as it passes through skin and blood vessels; it is closer to an optical illusion.

4.The respiratory system and the excretory system

When you breathe in, air passes through the nose, trachea, and bronchi to reach the alveoli, tiny air sacs in the lungs. The alveoli are surrounded by capillaries, so oxygen diffuses into the blood and carbon dioxide diffuses into the alveoli. The lungs have no muscles of their own to move them; air flows in and out as the diaphragm and the muscles between the ribs expand and shrink the chest cavity.

Excretion is the removal from the body of waste produced by cell activity. Nitrogen waste produced when proteins are broken down is converted in the liver into urea, which is relatively less toxic, and the kidneys filter urea, excess water, and salt from the blood to make urine. The kidneys are also responsible for regulating the amounts of water and salt in the body. Carbon dioxide leaves through the lungs, and some waste also leaves in sweat. Feces are leftover material that was never digested, so strictly speaking they count as elimination, not excretion.

5.The nervous system and the endocrine system — the body's two control systems

The nervous system consists of the central nervous system, made up of the brain and spinal cord, and the peripheral nervous system, which branches out through the whole body. Stimuli picked up by the sense organs travel quickly along nerves as electrical signals, and the central nervous system makes a judgment and sends commands to the muscles.

When your hand touches a hot pot, you pull it away before you even think. Unconditioned reflexes like this are very fast because the command is issued directly by the spinal cord without going through the brain (a spinal reflex). There are also unconditioned reflexes centered in the medulla or the midbrain, such as sneezing and the pupillary reflex. The sensation of heat reaches the brain only afterward.

The endocrine system sends signals by releasing hormones into the blood. It is slower than the nerves, but its effects are wider and longer-lasting. Insulin from the pancreas lowers blood sugar, and glucagon raises it.

The well-known “tongue map” — the picture in which sweetness is tasted only at the tip of the tongue and bitterness only at the back — is a widespread misconception. The taste buds that detect flavor are spread all over the tongue, and each area can sense several tastes.

6.The immune system — the body's lines of defense

Skin and mucous membranes, nasal mucus, and stomach acid are the first line of defense, keeping pathogens from getting into the body. When a pathogen does get in, a defense that doesn't distinguish between kinds of invaders comes first: some white blood cells, for example, engulf and destroy it. The swelling and heat at the site of inflammation are part of this process.

Next comes a defense that targets only a specific pathogen. Lymphocytes recognize the pathogen's distinctive features (antigens) and make antibodies that fit them exactly, or remove infected cells. Memory cells remember a pathogen the body has fought once, so if it enters again, the response is faster and stronger. A vaccine works by showing the body, in advance, an antigen that has been treated so it can't cause disease, or the information for one, so that this memory forms.

Most colds are caused by viruses. Antibiotics, which target bacteria, have no effect on viruses. Medical professionals decide which medicine is needed, and if symptoms are severe or last a long time, it is a good idea to see a doctor.

7.Homeostasis — keeping the inside steady

Whether it's below freezing outside or over 30 °C, a person's body temperature doesn't stray far from roughly 36.5–37 °C. This tendency to keep conditions inside the body — such as body temperature, blood sugar, and the concentration of body fluids — steady even when the outside environment changes is called homeostasis.

Homeostasis works mainly through feedback. When it's hot, blood vessels in the skin widen and you sweat, releasing heat; when it's cold, blood vessels in the skin narrow and your body shivers, producing heat. When blood sugar rises after a meal, insulin is released to lower it, and when blood sugar falls, insulin release decreases. It works much like a thermostat on a home heating system, which switches off when the temperature rises and on when it falls.