Lymphatic system is a system consisting of lymph, lymphatic capillaries, lymph vessels and lymph nodes.

Lymph is another medium of transportation of substances in human body. Lymph is a colourless or light yellow fluid present in tiny tubes called lymph capillaries, which join to form large lymph vessels. Lymph does not contain red blood cells and blood platelets. So, it is not red like blood. Lymph contains special types of white blood cells called lymphocytes. The lymphocytes are produced from lymph nodes. The lymph nodes are present on lymph vessels at intervals. The function of lymphocytes is to fight infection and to clean lymph. Thus, it protects our body from diseases.

 A large number of substances such as water, proteins, enzymes, hormones, carbohydrates and fats are also present in lymph. In the body, lymph flows only in one direction i.e. from body tissues to heart. Lymph is also called extracellular fluid because it lies outside body cells.

Functions of lymph

1. Lymph transports oxygen, food materials, hormones etc. to the body cells.
2. Lymph transports carbon dioxide and waste products from body cells to blood.
3. Lymph destroys bacteria and other foreign substances and thus protects us from diseases.

Test Your Understanding and Answer These Questions:

1. What is lymph?
2. What are the major functions of lymph?
3. What is the other term for ‘extracellular fluid’?
4. Write a note on lymphatic system in human beings.
5. What are the differences between lymph and blood?

An electrocardiogram is a graphic record of changes in electrical voltage when the heart beats. The machine by which the electrocardiogram is recorded is known as electrocardiograph.

The importance of ECG is that it gives us accurate information about the working of heart. So, it is very important in diagnosis of a number of heart diseases.

Test Your Understanding and Answer These Questions:

1. Expand ECG.
2. What is ECG? What is its importance?

In human beings the same blood passes twice through the heart for completing one cycle. Since the blood flows twice through the heart so it is called double circulation. The double circulation has two components. These are:

Pulmonary Circulation
The circulation of blood between the heart and the lungs is called pulmonary circulation.

Systemic circulation
The circulation of blood between the heart and all the parts of the body except lungs is called systemic circulation.

Test Your Understanding and Answer These Questions:

1. Write names of two components of double circulation.
2. What is the meaning of the term “double circulation”. Explain its components.

 The human heart is a conical, hollow and muscular organ which helps in pumping blood around all the parts of body. It is covered by a two layered membrane called pericardium. The function of pericardium is to protect heart from shock and mechanical injuries.

Internally, the human heart is divided into four chambers. The upper two chambers of heart are called atria (singular atrium) or auricles,. The lower two chambers of heart are called ventricles. The atria (or auricles) are further classified as left atrium (left auricle) and right atrium (right auricle). In the same way, ventricles are also classified as left ventricles and right ventricles.

Atria open into ventricles of their sides. The openings between atria and ventricles are guarded by valves. The valve present between left atria and left ventricle contains two flaps and is called bicuspid valve. The valve present between right atria and right ventricles contains three flaps and is known as tricuspid valve. The function of these valves is to prevent the backflow of blood from ventricles into atria when the ventricles contract. The right atrium and ventricle are separated from left atrium and ventricle by a partition called septum.

Working of Heart

 The function of heart is to pump the blood around all the parts of body. This is done by regular contraction and relaxation of the atria and ventricles. The contraction and relaxation of the atria and ventricles take place alternately. One complete contraction and relaxation of the heart is called heart beat. The heart of a normal healthy person beats about 70 to 72 times per minute under normal conditions. But, the rate of heart beat can increase during special circumstances. For example, during hard physic exercise the rate of heart beat can be more than 100 per minute.

Now, we shall discuss how the heart circulates blood in our body.

1. The muscles of right atrium relax and it receives deoxygenated blood from all the parts of the body. The deoxygenated blood is forced into right ventricle, by the contraction of right atrium.

2. When the right ventricle contracts, the de-oxygenated blood is forced into lungs. During the contraction of right ventricle the backward flow of blood into right atrium is prevented by the bicuspid valve. In the lungs, the deoxygenated gives up carbon dioxide and picks up oxygen. So, in lungs the blood becomes oxygenated.

3. Then the muscles of left atrium relax and it receives oxygenated blood from the lungs. The oxygenated blood is then forced into left ventricle, by the contraction of left atrium.

4. When the left ventricle contracts, the oxygenated blood is pumped to all the parts of the body except lungs. During the contraction of left ventricle the backward flow of blood into left atrium is prevented by the tricuspid valve. In the body, the oxygenated blood gives up oxygen to the cells and takes back carbon dioxide from them. So, in the body the blood becomes deoxygenated.This deoxygenated blood then flows back to the heart and enters by the right atrium. In this way, the whole process is repeated again to ensure continuous flow of blood in the body.

Test Your Understanding and Answer These Questions:

1. What is heart beat?
2. Explain working of human heart.
3. Explain structure of human heart.
4. What is the function of valves in heart?
5. What is the affect of exercise on heart beat?
6. Draw human heart and label its various parts.
7. Name the membrane that covers human heart

In human beings blood transports various substances to all the organs of the body. Blood reaches all the parts of the body through circulatory system, which consists of blood vessels and heart.

Blood vessels
Blood vessels are the tubes through which blood flows in the body. Blood vessels are of three types:

1. Veins
2. Arteries
3. Capillaries

VeinsVeins are thin walled vessels which carry blood from all the parts of the body to heart. They have valves to ensure that the blood flows only towards the heart. Veins are present superficially under the skin. Veins carry deoxygenated blood from all the parts of the body to heart. In veins blood flows smoothly.

Arteries

Arteries are thick walled vessels which carry blood from heart to all the parts of the body. In arteries valves are not present. Arteries are present deep under the skin. Except pulmonary artery, all the arteries carry oxygenated blood from heart to all the parts of the body. In arteries blood flows with jerks.

Capillaries 

Capillaries are narrow and thin walled vessels. These are present between arteries and veins. Their function is to exchange food materials, gases, hormones and waster products between blood and body cells.

Test Your Understanding and Answer These Questions:

1. What are differences between arteries and veins?
2. Name the various blood vessels found in human beings.
3. Explain different types of blood vessels found in human beings.
4. Point out the differences between arteries, veins and capillaries.

Rh factor is an antigen present on the red blood cells. This antigen was first discovered in Rhesus monkey. The persons having Rh factor are called Rh positive (Rh+). And the persons not having Rh factor are called Rh negative (Rh-). About 85% humans are Rh positive. An Rh negative mother may lose her baby if the mother has Rh positive baby.

Test Your Understanding and Answer These Questions:

1. What do you mean by Rh factor?

Depending upon the compatibility of antigen A and B with antibodies a and b, human blood can be classified into four groups: A, B, AB and O. Every human being has one of these blood groups. Blood transfusion is successful between the persons having some blood groups or certain special groups as shown in the table.

If you carefully observe the table you will find that a person having blood group O can give blood to all the groups. Hence, a person having blood group O is called universal donor. On the other hand, a person having blood group AB can receive blood from all the groups and hence, a person having blood group AB is called universal recipient.

Test Your Understanding and Answer These Questions:

1. What is universal donor?
2. What is universal recipient?
3. Name the four blood groups in human beings.
4. Name the blood groups called as universal donor and universal recipient.
5. Hema’s blood group is A, Vivek’s blood group is O and Shikha’s blood group is AB. Who can receive blood from whom and donate blood to whom?

The transfer of blood from a healthy person to another person is called blood transfusion. Blood transfusion is done, whenever there is loss of a lot of blood from the body due to an accident or during surgery. The healthy person who gives blood is called donor while the other person who receives blood is called recipient.

Care should be taken before blood transfusion that the bloods of both donor and recipient must match with each other. If the bloods of two persons do not match, the blood of the patient will agglutinate which may cause death of the patient. Agglutination may be defined as the sticking together of red blood cells to form large clumps. The cause of agglutination was discovered by a scientist named Landsteiner. Landsteiner discovered that the cause for mismatching of blood was the reaction between antigens present on the red blood cells of donor and antibodies present in the plasma of recipient. Two different antigens were discovered and named ‘A’ and ‘B’. Their corresponding antibodies which react and clump the blood were termed ‘a’ and ‘b’ respectively.

Test Your Understanding and Answer These Questions:

1. What is agglutination?
2. What are antigens and antibodies?
3. What do you mean by blood transfusion?
4. Who discovered the cause of agglutination of blood?
5. Name the antigens and antibodies found in human beings.
6. How many types of antigens and antibodies are present in human blood?

 Clotting of blood is also called coagulation of blood. When a person gets injured, blood starts to flow out of the injury. But, after sometime blood solidifies at the site of injury and thus prevents the excessive loss of blood from the body. This is called clotting of blood. Clotting of blood is a complex process which takes place as described below :

When blood starts to flow out of injured tissue, a substance called thromboplastin is released by the blood platelets. The thromboplastin in the presence of calcium acts on a protein present in blood plasma called prothrombin and coverts it into thrombin. The thrombin in turn acts on a soluble protein present in plasma called fibrinogen and converts it to fibrin. Fibrin forms a network of elongated thread like fibres in which red blood corpuscles get entangled and give it a solid appearance called blood clot.

 It should be noted that blood does not solidify inside the non-injured blood vessels because blood has a substance heparin which prevents it from clotting.

Test Your Understanding and Answer These Questions:

1. What is clotting of blood?
2. Write a note on mechanism of blood clotting.
3. Why blood does not clot inside a blood vessel without injury?

In human body, various substances such as oxygen, digested food, hormones and enzymes are transported from one part of the body to other parts through blood and lymph. Blood is red in colour due to the presence of red coloured pigment called haemoglobin in it. basically, blood is a connective tissue. In a healthy person about 5.5 litre of blood is present in the body.

The main components of blood are :

1. Red Blood Corpuscles or Red Blood Cells (RBC)
2. White Blood Corpuscles or White Blood Cells (WBC)
3. Platelets, and
4. plasma

let’s now discuss the various components of blood more detail.

Red Blood Cells

Red blood cells are circular, non-nucleated cells which are red in colour due to the presence of a red coloured pigment called haemoglobin in them. The function of red blood cells is to transport oxygen from lungs to different tissues of the body and carbon dioxide from tissues to lungs. The life span of red blood cells is about 120 days.

White Blood Cells

White blood cells are spherical or irregular in shape, nucleated and larger than red blood cells. There number is much less than red blood cells. The main job of white blood cells is to eat up bacteria and fight infection. White blood cells also produce antibodies which help us in fighting infection and protect us from diseases. Thus, white blood cells provide immunity to out body. Due to these reasons, white blood cells are also called soldiers of the body. The average life span of white blood cells is 3 – 4 days.

Platelets

Platelets are tiny fragments of cells, which help the blood to clot when you cut yourself. The average life span of platelets is 7 days.

Plasma
 Blood is made up of a pale yellow liquid called plasma. It contains red blood cells, white blood cells and blood platelets. Blood plasma also contains dissolved particles of food (like proteins, glucose, amino acids, fatty acids and minerals), waste materials (like carbon dioxide and urea) and hormones.

Functions of Blood

Following are the functions of blood in our body:

1. Oxygen is transported from the lungs to all the parts of body through blood.
2. Blood also transports carbon dioxide from the tissues to lungs.
3. Blood helps us in fighting infection by eating up bacteria and protects us from diseases.
4. Blood transports the food absorbed in the intestine to all the parts of the body.
5. Blood also transports various types of hormones and enzymes from one part of the body to other parts.
6. It plays a role in regulating the temperature of body.
7. Blood also helps in excretion of waste materials such as urea out of the body.
8. Blood solidifies (clots) at the site of an injury or cut and thus prevents its loss from the body.

Test Your Understanding and Answer These Questions:

1. What is blood plasma?
2. What are blood platelets?
3. List the components of blood.
4. What makes red blood cells red?
5. What is the life span of blood platelets?
6. What are different functions of blood?
7. What is blood? Describe its composition.
8. What is the life span of white blood cells?
9. Give the structure and function of red blood cells.
10. How much blood is present in a healthy human being?
11. What are the medium of transportation in human body?
12. Which component of blood kills bacteria in human body?
13. Why white blood corpuscles are are called ‘soldiers of the body’?
14. Give differences between red blood cells and white blood cells.