Circulatory System – JSS2 Intermediate Science Lesson Note (Second Term)

The circulatory system is the system of heart, blood vessels and blood that moves oxygen, food and water to all body cells and carries away waste.

Behavioural objectives

1. Define the circulatory system and name its three main parts.
2. Name the four chambers of the heart and state the job of each side.
3. Distinguish between arteries, veins and capillaries.
4. List the components of blood and state the function of each.
5. State the main functions of blood.

What is the circulatory system?

The circulatory system is the body’s transport network that carries materials to and from every cell.

• Heart: the pump that keeps blood moving (like the engine of a transport system).
• Blood vessels: the tubes that carry blood (like roads in Lagos or Abuja).
• Blood: the liquid that carries materials (like the trucks on the road).

What it moves:

• To the cells: oxygen, digested food and water.
• Away from the cells: waste such as carbon dioxide.

What is the heart?

The heart is a muscular organ that pumps blood round the body.

• Size: about the size of your clenched fist.
• Position: in the chest, slightly to the left.
• Beating: it beats about 100,000 times a day and never rests.

What are the chambers of the heart?

The heart has four chambers: two atria on top and two ventricles below.

• Atria (upper chambers): receive blood coming into the heart.
• Ventricles (lower chambers): have thick, muscular walls and pump blood out of the heart.
• Right side: collects blood low in oxygen from the body and sends it to the lungs.
• Left side: receives oxygen-rich blood from the lungs and pumps it to the whole body.

How does blood flow round the body?

Blood flows in a loop: heart, lungs, heart, body, and back to the heart.

1. Blood low in oxygen returns from the body to the right side of the heart.
2. The right side pumps it to the lungs.
3. In the lungs, the blood picks up oxygen and gives out carbon dioxide.
4. Oxygen-rich blood returns to the left side of the heart.
5. The left side pumps it to all parts of the body.

What are the types of blood vessels?

There are three types of blood vessels: arteries, veins and capillaries.

What are arteries?

Arteries are blood vessels that carry blood away from the heart.

• Memory aid: A for Artery, A for Away.
• Blood carried: usually rich in oxygen and bright red.
• Walls: thick and stretchy because blood flows at high pressure.
• Exception: the artery to the lungs carries blood low in oxygen.

What are veins?

Veins are blood vessels that carry blood back to the heart.

• Blood carried: usually low in oxygen and carrying waste.
• Valves: work like one-way gates to stop blood flowing backwards.
• Exception: the vein from the lungs carries oxygen-rich blood.

What are capillaries?

Capillaries are the tiniest blood vessels, joining arteries to veins.

• Size: too small to see without a microscope.
• Walls: very thin, so oxygen and food pass easily into cells.
• Waste: carbon dioxide and other waste pass from cells into them.

How do the blood vessels compare?

What is blood?

Blood is the red liquid tissue that flows through the blood vessels and carries materials round the body.

What are the components of blood?

Blood has four components: red blood cells, white blood cells, platelets and plasma.

• Red blood cells: carry oxygen using haemoglobin, a red protein that contains iron.
• White blood cells: fight germs and protect the body from disease.
• Platelets: help blood to clot at a cut, forming a scab that stops bleeding.
• Plasma: the yellowish liquid part that carries digested food, hormones and waste.

What are the functions of blood?

Blood transports materials, protects the body, helps regulate temperature and removes waste.

• Transport: carries oxygen and food to the cells.
• Protection: fights infections and clots at wounds.
• Regulation: helps keep body temperature steady at about 37°C.
• Waste removal: carries carbon dioxide to the lungs to be breathed out.

Evaluation

1. Which side of the heart pumps blood to the whole body, the left or the right?
2. State two differences between an artery and a vein.
3. Which part of the blood helps to stop bleeding after an injury?
4. Why are capillary walls very thin?
5. Name the substance in red blood cells that carries oxygen.

Assignment

Draw a simple flow diagram showing how blood moves from the heart to the lungs, back to the heart and round the body. Label the right side of the heart, the left side of the heart, the lungs and the body.