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Transport System In Humans | Grade 7 Science Lesson

In humans, the transport system carries nutrients and oxygen to the cells. It removes waste products like carbon dioxide and excess water from the cells in the body. This system is also known as the circulatory system.

The heart pumps blood throughout the body. Blood vessels are tubes that carry blood within the body. Blood is a fluid that transports substances throughout the body. All three make up the transport system.

The heart

The heart is a muscular organ and is about the size of a clenched fist. It lies between the lungs. Th heart acts as a ‘pump’ that transports blood to all parts of the body.

The size of the heart is closely related to body size. Generally, the heart weighs 0.59% of the total body mass.

The heard has four chambers. They are two upper chambers called right atrium and the left atrium, andtwo lower chambers called the right ventricle and the left ventricle.

On the left side of the heart, the chambers have blood rich in oxygen (oxygenated blood) and on the right side, the chambers have blood with less oxygen and more carbon dioxide (deoxygenated blood).

The wall of the left ventricle is thick enough to exert a strong force in order to push blood to all parts of the body.

The valves in the chambers prevent the blood from flowing backwards. So, the blood flows in one direction only.

Blood circulation

Blood flowing through the transport system is known as blood circulation. The heart pumps the blood to the lungs from where it is pumped back to the heart. The blood from the heart is then pumped to the rest of the body and returns to the heart. The heart keeps the blood circulating.

As the blood circulates around the body, it passes through the heart twice. This is called double circulation.


The body contains about 4 to 5 litres of blood that flows constantly in our transport system.

Blood transports food (nutrients) and oxygen to all parts of the body and removes waste products such as carbon dioxide. As blood flows to the different parts of the body, nutrients and oxygen pass from the blood into the cells. Waste products such as carbon dioxide pass from the cells into the blood.

Blood and its components

Blood can be separated into its components – red blood cells, white blood cells, plasma and platelets.

Red blood cells are biconcave (concave on both sides) and have no nucleus.

White blood cells are fewer in number. The white blood cells protect the body against infections and diseases.

Plasma is a light-yellow liquid containing mainly water. Many substances such as nutrients are dissolved in it.

Less than 1% of blood is made up of platelets. When there is a cut in the skin, platelets group together at the cut and this causes blood to clot. This prevents further bleeding and allows the skin to start healing.

When a person loses too much blood, a blood transfusion is carried out. The blood being transfused must be matched with the blood type of the person.

Blood vessels

Blood is transported by blood vessels.

Arteries carry blood away under high pressure from the heart. Therefore, arteries have thick walls to withstand the pressure of blood flowing through them.

Veins transport blood at low pressure back to the heart. Therefore, they have thinner walls compared to the arteries.

Capillaries are tiny blood vessels that are found between the cells of almost all the tissues of the body. They are only one cell thick. This allows a quick exchange of substances between the blood and the tissues.

Pulse and blood pressure

Our heart keeps on pumping blood throughout our entire lifetime at about 72 beats per minute (bpm). Each time the heart beats, a pulse is produced.

Using a stethoscope, we can hear our heart making rhythmic ‘lub-dub’ sounds. These sounds are produced when the valves open and close.

During exercise, cells require more energy. As such, the heart beats faster to allow nutrients and oxygen to reach the cells faster.

Blood pressure is the force that blood exerts on the walls of arteries as the left ventricle pumps the blood to the rest of the body. It can be measured using an instrument called sphygmomanometer. When we engage in physical activities such as running, climbing a flight of stairs and hiking, our heart beats faster. This results in increased blood pressure.