Blood Vessels: Types of Blood Vessels and Functions – Science Class 7

There are different types of blood vessels in the body. You know that during inhalation a fresh supply of oxygen fills the lungs. Oxygen has to be transported to the rest of the body. Also, the blood picks up the waste materials including carbon dioxide from the cells.

This blood has to go back to the heart for transport to the lungs for removal of carbon dioxide as you have learned in previous chapters. So, two types of blood vessels, arteries and veins are present in the body.

Arteries carry oxygen-rich blood from the heart to all parts of the body. Since the blood flow is rapid and at high pressure, the arteries have thick elastic walls.

Veins are the vessels that carry carbon dioxide-rich blood from all parts of the body back to the heart. The veins have thin walls. There are valves present in veins that allow blood to flow only towards the heart

It is important to remember that the pulmonary artery carries blood from the heart, so it is called an
artery and not a vein. It carries carbon dioxide-rich blood to the lungs. The pulmonary vein carries oxygen-rich blood from the lungs to the heart.

Do you see the arteries divide into smaller vessels? On reaching the tissues, they divide further into extremely thin tubes called capillaries. The capillaries join up to form veins that empty into the heart.


The walls of the chambers of the heart are made up of muscles. These muscles contract and relax rhythmically. This rhythmic contraction followed by its relaxation constitutes a heartbeat. Remember that heartbeats continue every moment of our life. If you place your hand on the left side of your chest, you can feel your heartbeat.

The doctorfeels your heartbeats with the help ofan instrument called a stethoscope.

A doctor uses the stethoscope as a device to amplify the sound of the heart. It consists of a chest piece that carries a sensitive diaphragm, two earpieces, and a tube joining the parts. Doctors can get clues about the condition of your heart by listening through a stethoscope.

Components of Blood

In humans, mature red blood cells are also known as Erythrocytes and are an important part of blood. The main function of Red Blood Cells is to transport oxygen from the lungs to the rest of the body.

RBCs contain a red pigment called Hemoglobin which is also called iron pigment. This red pigment binds with oxygen and transports it to all the parts of the body and ultimately to all the cells. The blood appears red because of the presence of Hemoglobin. 2.4 million new erythrocytes are produced per second in our body.

On average, we have 5 million RBC’s per microlitre of blood. Another important type of blood cell is WBC’s. These are also called Leucocytes. There are normally 4000- 11,000 white blood cells present per microlitre of blood. Their main function is to fight against infectious diseases and foreign materials.

You would have noticed that blood coming out from a cut stops after some time and a dark red clot plugs the cut. The clot is formed because of the presence of another type of cells in the blood called Platelets or Thrombocytes. The physiological range for platelets is 200,000-500,000 per microlitre of blood.

Functioning of Circulatory System- Blood

While playing or riding a bicycle you often fall down and get injured. You might get hurt and start bleeding. Have you ever thought about what blood is and what is its composition? Blood is the fluid that flows in blood vessels.

Its main functions are:

  1. To transport substances like digested food from the small intestine to the other parts of the body.
  2. To carry oxygen from the lungs to the cells of the body.
  3. To transport waste to the excretory organs like kidneys.
  4. Blood is a specialized bodily fluid that delivers necessary substances to the body’s cells.
  5. Invertebrates, it is composed of blood cells suspended in a liquid called Blood Plasma.
  6. Blood consists of 3 types of cells: RBC’s i.e. Red Blood Cells, WBC’s i.e. White Blood Cells, and Platelets.

Read More- Human Digestive System- Parts and Functions | Process of Digestion