Exploring Biology Topics can reveal the incredible complexity and interconnectedness of living systems.
Blood – Definition, Composition and its Functions
Blood is a red coloured liquid which circulates in our body. Blood is red because it contains a red pigment called haemoglobin in its red cells. Blood is a connective tissue. Blood consists of four things: plasma, red blood corpuscles, white blood corpuscles, and platelets. Thus, the main components of blood are Plasma, Red Blood Corpuscles (or Red Blood Cells), White Blood Corpuscles (or White Blood Cells), and Platelets.
This plastic bag contains human blood. The blood comes from donors. It is sent to hospitals in plastic packs.
There are about 5 million red cells in one drop of blood. This micrograph shows red blood cells moving through a capillary.
Plasma is a liquid (which is also called the fluid matrix) and the three types of cells, red blood cells, white blood cells, and platelets keep floating in it. We can now define blood as follows: Blood is a liquid (or fluid matrix) called plasma with red cells, white cells, and platelets floating in it. We will now describe all four components of blood in a little more detail.
The liquid part (or fluid part) of blood is called plasma. Plasma is a colourless liquid that consists mainly of water with many substances dissolved in it. Plasma contains about 90 percent water. Plasma also contains dissolved substances such as proteins, digested food, common salt, waste products (like carbon dioxide and urea), and hormones. Plasma carries all these dissolved substances from one part to another part in the body. Red blood cells, white blood cells, and platelets are immersed in this liquid called plasma.
Red Blood Cells
Red blood cells are red in colour due to the presence of a red pigment called haemoglobin inside them (see Figure 81). Red blood cells (RBC) are carriers of oxygen. Red blood cells carry oxygen from the lungs to all the cells of the body. It is actually the haemoglobin present in red blood cells which carries oxygen in the body. Haemoglobin performs a very important function of carrying oxygen from the lungs to body tissues. Haemoglobin also carries some of the carbon dioxide from body tissues to the lungs (most of the carbon dioxide is carried by plasma of blood in the dissolved form). Red blood cells are circular in shape. Red blood cells do not have nuclei.
This micrograph shows the different types of cells in blood: red cells, white cells, and platelets (small pink cells).
This person is donating blood. Our body can make up for the loss of this blood very quickly. Blood donation saves other people’s lives who have met with an accident or are seriously ill. We should make it a habit to donate blood periodically. Remember ‘Rakt duan’ is ‘jeevan daan’.
Red blood cells have to be made quickly because they do not live for very long. Each red blood cell lives for about four months. One reason for the short life of red blood cells is that they do not have nuclei. It has been estimated that about three million red blood cells of the human blood die every day but four times that number are made in the bone marrow every day. So, when we donate blood to save the life of a person, then the loss of blood from our body can be made up very quickly, within a day. This is because red blood cells are made very fast in our bone marrow. Please note that most of the cells in the blood are red blood cells.
White Blood Cells
White blood cells fight infection and protect us from diseases. This is because white blood cells help’ to fight against germs and other foreign bodies which cause diseases. Some white blood cells can eat up the germs (like bacteria) which cause diseases. Other white blood cells make chemicals known as ‘antibodies’ to fight against infection. In other words, white blood cells manufacture antibodies that are responsible for providing immunity in our body (due to which we are protected from disease and infection).
In fact, white blood cells are called soldiers of the body. This is because they protect the body from the attack of disease-causing germs (pathogens) and other harmful foreign materials. White blood cells are either spherical in shape or irregular in shape. All white blood cells have a nucleus though the shape of the nucleus is different in different types of white blood cells. White blood cells (WBC) in the blood are much smaller in number than red blood cells.
Platelets are tiny fragments of special cells formed in the bone marrow. Platelets do not have nuclei. Platelets help in the coagulation of blood (or clotting of blood) in a cut or wound. For example, when a cut or wound starts bleeding, then platelets help clot the blood (make the blood semi-solid) due to which further bleeding stops. All the blood cells are made in the bone marrow from the cells called stem cells.
When we get a cut anywhere on our body, then blood starts coming out. Platelets present in the blood produce a mesh of fine threads which trap the red blood cells forming a clot that blocks the cut and stops bleeding.
Functions of Blood
Blood has three main functions in the human body. These are the Transport of substances (like respiratory gases, oxygen, and carbon dioxide; digested food or nutrients; waste products; hormones; enzymes and ions) from one part of the body to the other, Protection against disease, and Regulation of body temperature. We can now say that: The important functions of blood in our body are as follows:
- Blood carries oxygen from the lungs to different parts of the body.
- Blood carries carbon dioxide from the body cells to the lungs for breathing out.
- Blood carries digested food from the small intestine to all the parts of the body.
- Blood carries hormones from the endocrine glands to different organs of the body (where they are needed).
- Blood carries a waste product called urea from the liver to the kidneys for excretion in urine.
- Blood protects the body from diseases. This is because white blood cells kill the bacteria and other germs which cause diseases.
- Blood regulates the body temperature. This is because the blood capillaries in our skin help to keep our body temperature constant at about 37°C.
Blood Transport in Humans
The main transport system in human beings (or man) is the ‘blood circulatory system’ (which is sometimes called just the ‘circulatory system’ for the sake of convenience). In the human circulatory system, blood carries oxygen, digested food, and other chemicals like hormones and enzymes to all the parts of the body. It also takes away the waste products (or excretory products) like carbon dioxide and urea produced in the body cells. The human blood circulatory system consists of the heart (the organ which pumps and receives the blood) and the blood vessels (or tubes) through which the blood flows in the body. In the blood circulatory system, the blood flows through three types of blood vessels arteries, veins, and capillaries.
Blood circulatory system in humans (Arteries are shown in red colour and veins in blue colour. Capillaries which join arteries to veins are not shown).
The blood vessels of the circulatory system are present in each and every part of the human body due to which the blood reaches all the parts of the body. In addition to the blood circulatory system for transport in human beings, there is another system called the lymphatic system which also helps in the transport of materials in the human body. The liquid which circulates and carries materials in the lymphatic system is called lymph. Thus, in human beings, various substances are transported through two liquids called ‘blood’ and ‘lymph’. We will first describe the blood circulatory system which is the main transport system in humans.