The Circulatory System for Middle School Science

May 7

7 min read

Have you ever wondered how oxygen gets from your lungs to your toes? Or how a tiny cut on your finger stops bleeding and starts to heal? These amazing processes happen thanks to your circulatory system!

The circulatory system is made up of your heart, blood, and blood vessels. Together, they work 24/7 to deliver oxygen and nutrients to every cell in your body and remove waste products like carbon dioxide. Your heart acts like a powerful pump, beating over 100,000 times a day to keep blood moving. The blood vessels—arteries, veins, and capillaries—are the roads that carry blood throughout your body.

Without the circulatory system, your organs wouldn’t get the energy they need, and your body couldn’t fight off disease or heal injuries.

You can check out the pages of my body system unit from Teachers Pay Teachers below. You can also continue on to read more about the circulatory system.

The Circulatory System

The circulatory system is the body’s transportation network, responsible for delivering oxygen and nutrients to every cell and carrying away waste products like carbon dioxide. It consists of the heart, which acts as a pump; the blood, which carries essential substances; and the blood vessels—arteries, veins, and capillaries—that serve as pathways throughout the body.

Blood moves in a continuous loop, first picking up oxygen from the lungs, delivering it to the body, picking up carbon dioxide and other waste, and finally returning to the heart to start the process again. This system also plays a key role in fighting disease, regulating body temperature, and healing injuries. Together, the parts of the circulatory system keep the body alive, energized, and in balance.

Functions of the Circulatory System

The circulatory system does much more than just move blood—it plays a key role in keeping your entire body healthy. In this section, you’ll learn how it transports oxygen and nutrients, removes waste, and helps protect and regulate the body. Each of these functions is essential for survival and helps keep all your other body systems working properly.

1. Transporting Oxygen and Nutrients

One of the circulatory system’s most important jobs is transporting oxygen and nutrients to every cell in the body. When you breathe in, oxygen enters your lungs and is picked up by your blood. The heart then pumps this oxygen-rich blood through arteries to all parts of your body. Along the way, blood also delivers nutrients from the food you eat, like glucose and vitamins, which your cells need for energy, growth, and repair.

2. Removing Waste Products

As your cells use oxygen and nutrients, they create waste products such as carbon dioxide and other chemicals. The circulatory system helps remove these wastes by carrying them away in the blood. Carbon dioxide is transported back to the lungs, where it is exhaled. Other waste products are carried to the kidneys or liver for processing and removal. This cleanup system keeps your body from becoming overloaded with harmful substances.

3. Protecting and Regulating the Body

The circulatory system also helps protect your body and keep it in balance. White blood cells in your blood fight off infections, while platelets help stop bleeding when you get a cut. The system also helps regulate body temperature by moving heat around the body. For example, when you’re hot, blood vessels near your skin widen to release heat. This constant regulation helps maintain homeostasis, keeping your internal environment stable and healthy.

Parts of the Circulatory System

The circulatory system is made up of three main parts: the heart, blood, and blood vessels. Each plays an important role in keeping blood moving and supporting the body.

Heart

The heart is the powerhouse of the circulatory system. It sits in the middle of your chest, slightly to the left, and is protected by your rib cage. The heart is a muscular organ divided into four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Each side of the heart has a different job.

The right side receives blood from the body that is low in oxygen and pumps it to the lungs, where it picks up fresh oxygen and gets rid of carbon dioxide.
The left side takes this oxygen-rich blood from the lungs and pumps it out to the rest of the body.

Valves between the chambers make sure blood flows in only one direction. The heart beats about 60 to 100 times per minute, depending on your activity, and never stops—pumping around 2,000 gallons of blood every day!

Blood

Blood is a special fluid that delivers oxygen and nutrients to every part of your body and removes waste products. An average person has about 5 liters (or 1.3 gallons) of blood. Blood is made up of several important parts:

Red blood cells (RBCs): These cells carry oxygen from the lungs to the body using a protein called hemoglobin, which gives blood its red color.
White blood cells (WBCs): These cells are part of the immune system. They fight bacteria, viruses, and other harmful invaders.
Platelets: These tiny cell fragments help your blood clot when you get a cut or injury, preventing too much bleeding.
Plasma: This yellowish liquid makes up over half of your blood. It carries nutrients, hormones, and waste products, and helps keep your body temperature steady.

Blood Vessels

Blood vessels are the tubes that carry blood throughout your body. There are three main types:

Arteries carry oxygen-rich blood away from the heart to the body.
Veins bring oxygen-poor blood back to the heart.
Capillaries are tiny vessels where oxygen and nutrients pass into cells and waste is picked up.

Together, these parts form a powerful system that keeps every part of your body alive and functioning.

Oxygen and Carbon Dioxide Transfer in the Capillaries

Your body needs oxygen to make energy, and it must get rid of carbon dioxide, a waste product. This gas exchange happens in the capillaries, the tiniest blood vessels in your body, both around your body’s cells and inside your lungs.

At the Body’s Cells

As oxygen-rich blood reaches your body’s cells, it passes through capillaries that wrap around each cell. The oxygen in the blood moves out of the capillaries and into the cells, where it is used to produce energy. At the same time, the cells release carbon dioxide, which is a waste product from making energy. This carbon dioxide moves into the capillaries and is carried away in the blood to be removed.

Cells need oxygen to perform a process called cellular respiration, which turns food into energy. Without oxygen, cells can’t function properly. Getting rid of carbon dioxide is also important because too much of it can make the blood too acidic and harm the body.

At the Lungs

When the blood returns to the lungs, it goes through another set of capillaries around tiny air sacs called alveoli. Here, carbon dioxide moves out of the blood and into the alveoli, so you can breathe it out. At the same time, oxygen from the air you just inhaled moves from the alveoli into the blood.

This process refreshes the blood with new oxygen and removes harmful carbon dioxide. It’s how your blood gets ready to go back to the heart and start the cycle all over again.

The Intestines and the Circulatory System

After you eat and drink, your digestive system breaks food down into tiny parts your body can use. These nutrients and water must then enter the circulatory system so they can be delivered to every cell. This important job happens in the capillaries located in the walls of your intestines.

In the Small Intestine: Nutrient Absorption

The small intestine is where most digestion and nutrient absorption takes place. The inside of the small intestine is lined with millions of tiny finger-like structures called villi, and each villus is covered in even smaller microvilli. These structures increase the surface area for absorption.

As digested nutrients like glucose, amino acids, vitamins, and minerals pass through the wall of the small intestine, they enter the capillaries inside the villi.
From there, the blood carries these nutrients to the liver and then to the rest of the body, where they are used for energy, growth, and repair.

In the Large Intestine: Water Absorption

After nutrients are absorbed in the small intestine, the leftover material moves into the large intestine (also called the colon). This is where water and some minerals are absorbed.

The walls of the large intestine also have capillaries that absorb water and send it into the bloodstream.
This process helps your body stay hydrated and keeps the balance of fluids just right.

Without the circulatory system collecting nutrients and water from the digestive system, your body wouldn’t get the energy and hydration it needs to survive. This teamwork keeps all your organs and cells functioning properly!

How the Circulatory System Interacts with Other Body Systems

The circulatory system doesn't work alone—it teams up with many other body systems to keep you alive and healthy. Here's how it interacts with some of the major systems:

Respiratory System: The circulatory and respiratory systems work closely together to exchange gases. The respiratory system brings oxygen into your lungs when you breathe in. The circulatory system picks up this oxygen and delivers it to the rest of your body. At the same time, it collects carbon dioxide from your cells and brings it back to the lungs so you can breathe it out.

Digestive System: After you eat, the digestive system breaks food down into nutrients like glucose (sugar), vitamins, and minerals. These nutrients are absorbed into the blood in the small intestine. The circulatory system then carries them to cells all over your body to provide energy and materials for growth and repair.

Nervous System: The nervous system controls the heart and blood vessels. For example, when you're scared or excited, your brain tells your heart to beat faster. The circulatory system also delivers oxygen and nutrients to the brain so it can function properly.

Muscular and Skeletal Systems: Muscles need a steady supply of oxygen and nutrients to move, and bones need these materials to grow and stay strong. The circulatory system delivers what both systems need and removes waste products like carbon dioxide and lactic acid.

Immune System: Your blood contains white blood cells, which are part of the immune system. They travel through the blood to fight off infections and keep you healthy. The circulatory system also helps carry signals that trigger immune responses when something harmful is detected.

The circulatory system is like a delivery service and waste removal crew for your whole body. It works with every other system to make sure cells get what they need and can do their jobs.

Flashcards on the Circulatory System

Flashcards are the best way to practice remembering new information. You can get digital and printable flashcards at Teachers Pay Teachers, or you can use the flashcards here for free!