Body Systems in Humans

In this lesson we cover the five major human body systems: digestive, respiratory, circulatory, nervous, and excretory.

The Digestive System

The goal of this system is to break down food for digestion and absorption of materials. Food first enters the body through the mouth, then passes to the stomach and the intestine. It is broken down along the way into small, soluble particles that can be used by cells. Unused food is expelled from the body as waste. The organs involved in these processes form the digestive system. See the figure below for more information.

The Respiratory System

The goal of this system is to absorb oxygen in exchange for carbon dioxide. Humans breathe in through inhalation filling their lungs with air containing oxygen. Once oxygen is absorbed the excess air is exhaled or breathed out, which rids our bodies of waste carbon dioxide. You can see the organs involved in the respiratory system in the figure below.

The Circulatory System

The goal of this system is to transport absorbed oxygen and food particles and transport them throughout the body, it also removes any waste byproducts from cells. The circulatory system consists of the heart, blood, and blood vessels. You can see the organs involved in the circulatory system in the figure below.

How the Respiratory and Circulatory Systems Connect

To connect and transport air through the body, the circulatory system and respiratory systems work together. When air enters the nose it is passed through a series of smaller and smaller tubes. First the trachea or windpipe, then into a left and right bronchus, then each bronchus branches into thousands of narrow bronchioles, finally, each bronchiole ends in millions of tiny air sacs called alveoli (singular alveolus). In the alveolus is where the respiratory and circulatory intersect as oxygen is absorbed into the bloodstream here. See the figure below for more information.

The circulatory system has three main types of blood vessels: arteries, veins, and the smallest capillaries. Diffusion allows the oxygen to pass through the alveoli into the capillaries, which is disbursed into the bloodstream.

Blood – The Body’s Transportation System

Unlike unicellular organisms, multicellular organisms individual cells do not have direct access to the external environment. This means substances that are required for the function of the cell must be brought to the cell. In humans, the substance we use to transport around these substances is blood. See the figure below to find the composition of blood.

Blood Pressure

Doctors measure blood pressure as a simple first step to assess the health of the circulatory system. The device used to measure blood pressure is called a sphygmomanometer. It consists of an inflatable cuff that is wrapped around the arm. Air is pumped into the cuff, squeezing it against the artery in the arm and restricting the blood flow. Air is then slowly let out of the cuff to the point where the blood pressure matches the cuff pressure, letting blood force its way back through the artery. A doctor can listen for the sound of the blood using a stethoscope.

Blood pressure can indicate serval things about a person’s health:

  • The volume of blood: If a person has lost a lot of blood through injury, the blood pressure will be low.
  • Heart rate: A fast-beating heart pushes blood rapidly through the arteries, building up blood pressure.
  • Artery size: Large, open arteries conduct larger volumes of blood, producing low blood pressure.
  • Small, narrow, or partly clogged arteries produce high blood pressure.
  • Artery elasticity: Flexible arteries can easily expand, letting more blood through. Loss of elasticity results in “hardening” of the arteries, producing higher blood pressure.
  • Blood viscosity: Viscosity refers to the thickness of the blood. Thick fluids flow less easily than thin, watery fluids. Blood viscosity is a measure of the balance between red blood cells and plasma.

How the Circulatory and Digestive Systems Connect

The circulatory and respiratory systems work together to transport oxygen throughout your body, likewise, the digestive and circulatory system work together to transport food and energy throughout your body. The transfer of food from the digestive system to the circulatory system takes place at the inner lining of the small intestine, see the figure below. Covering this lining are millions of tiny, fingerlike projections called villi (singular: villus). Each villus contains a network of capillaries. Food is absorbed by the intestine into the capillaries through absorption, they are then transported through the arteries.

Like alveoli, villi have thin walls that allow particles to pass through into the circulatory system. Both have small tiny projections that allow the alveoli and villi to take up large amounts of area that are fed back into the capillaries.

The Excretory System

Up until this point we have discussed how we obtain resources, but that is only half the equation, we also must have a way of expelling waste byproducts. Filtering waste materials from the blood is the main function of the excretory system. While we will not be going into great detail of this process in grade 8, your kidneys are key organs in this process.

Sensory Awareness Systems

Your body is constantly struggling to keep equilibrium in spite of its surrounding environment. For example, when your body cools down, it will attempt to warm itself back up through shivering, likewise, when your body overheats you will begin to sweat in an attempt to cool it down. To keep your body temperature stable, your nerves, muscles and blood all function together. Your nervous system monitors conditions outside the body through the skin. Information from the temperature receptors goes to the heat regulating centre of your brain (the hypothalamus). Responding to this information, the brain sends nerve signals to your muscles, skin, and blood vessels. Working together, your muscles, skin, and blood vessels adjust your blood flow and muscle activity. In response, your body increases its heat production or reduces its heat loss. Your body’s responses to stimuli are coordinated by the nervous system (the brain, spinal cord, and nerves) and the endocrine system (glands that produce chemical messengers called hormones). In the body, a number of factors can affect the smooth working of the body systems: digestive, respiratory, circulatory, nervous, and excretory systems.

The information on this page and it associated figures are adapted from the Science Focus 8.
Science Focus 8, pp. 154-162
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