ICSE Class 8 Biology Chapter 01 Blood The Circulatory Fluid in Our Body

Unit 1: Life Processes - Transport of Food and Minerals in Animals & Plants

Blood - The Circulatory Fluid in Our Body

Syllabus

1. The circulatory system in human beings.

Counting of pulse - finding an average - noting changes after exercise and rest.

2. Different types of blood cells - blood groups - transfusion of blood.

Finding out more about blood groups and blood transfusion (E).

Viewing slides of RBC/WBC - if available (E).

Functions of the blood.

Extension activity: Talking to a pathologist to find out how blood tests can reveal the presence of infection and other diseases like diabetes, HIV.

Teacher's Note

The circulatory system works 24/7 to keep us alive, similar to how a water supply system constantly delivers water throughout a building.

Human Circulatory System

You have learnt that both human beings and the animals need food and oxygen to keep themselves alive. Simultaneously, they give out certain harmful substances like carbon dioxide and nitrogenous wastes. The intake of food and oxygen, and the removal of these wastes are carried out by a transport system. This transportation is carried out mainly by the blood. The blood is transported to all parts of the body by a pumping organ, i.e. heart.

The to and fro movement of the blood carried out by the heart is called the blood circulatory system.

Our circulatory system consists of blood, blood vessels and heart. Together, they transport substances to various parts of the body. The blood flows through blood vessels. Its flow is regulated by the heart. The heart acting like a pump pushes and receives the blood to and from the whole body through the vessels.

The Blood

The blood is a red coloured fluid which consists of two parts:

(1) Plasma, the liquid part; and

(2) Corpuscles, the cellular part.

The plasma is yellowish in colour. Most of it is water (about 90 per cent), and the remaining (10 per cent) consists of the following nutrients:

Food, vitamins and minerals.

Blood proteins which include antibodies that protect the body from the diseases and fibrinogen that helps the blood to clot.

Waste products that include urea and carbon dioxide.

Hormones; The chemical messengers which help to coordinate different body functions.

Blood tastes saltish due to the dissolved minerals, like sodium chloride.

Teacher's Note

Just as a delivery truck carries packages and waste materials through city streets, blood carries nutrients and removes waste products throughout our body.

The Corpuscles or the Blood Cells

The corpuscles or the blood cells are of three types:

Red blood cells (RBCs), White blood cells (WBCs) and Platelets.

1. The red blood cells (RBCs) are circular or disc-shaped. These cells initially had a nucleus to start with, but they loose it during maturation.

The RBCs are about 4.5 to 5.0 million in one cubic millimetre of blood. They are produced by our bone marrow (central hollow part of the bone) and have a lifespan of 120 days. Their red colour is due to the iron and protein compound called haemoglobin which acts as the "oxygen-carrier".

In human blood, more than two million red blood cells get destroyed every second but they are replaced immediately by the new ones.

2. The white blood cells (WBCs) are also called leucocytes. They are larger than the RBCs and have a distinct nucleus. They are much less in number, about 5000 to 8000 per cubic millimetre of blood. The WBCs capture and destroy the germs that try to enter the human body. Thus, they protect us from diseases as far as possible i.e., they provide us immunity.

The white blood cells are basically of two types depending upon the presence or absence of granules in their cytoplasm.

Granulocytes (granular) and agranulocytes (non-granular).

Granulocytes are further distinguished into neutrophils, eosinophils and basophils, while the agranulocytes are classified into monocytes and lymphocytes.

3. The platelets are somewhat round in shape and the smallest in size. They are about 2-3 lacs per cubic millimetre (cu mm) of blood. The platelets help in the clotting of blood which prevents excessive blood loss.

You must have sometimes experienced a prick in your skin by a thorn, a needle or a cut by a razor blade. The blood flows out from the pricked spot for a short time and then it stops. Do you know that the blood stops flowing after sometime by itself? Why? It happens because our body has its own mechanism of preventing this loss by forming a blood clot. This clot plugs the injury to stop bleeding. Blood clotting is a complex process and its mechanism is given below:

The damaged blood vessels and platelets release an enzyme.

This enzyme helps in the production of a protein called thrombin.

This, in turn, helps in the conversion of fibrinogen (present in the blood plasma) into fibrin.

Fibrin forms a fine mesh work into which the red blood cells get trapped, and a watery liquid (serum) oozes out of the wound. Serum is actually the blood plasma without its fibrinogen.

The fibrin network together with the blood cells contract. This makes a solid clot, which plugs the cut.

In certain diseases, the number of platelets get reduced to a great extent. Dengue fever is one such disease in which the number of platelets get reduced to as low as 25-30 thousand per cu mm of blood and the body shows bleeding symptoms. Delhi and many other parts of the country lose a number of lives due to dengue fever every year.

Activity 1

To examine the red and white blood cells with the help of prepared slides.

Place the prepared slide of human blood under the microscope. First, try to focus the slide under low power magnification. Observe the red blood cells and the white blood cells.

Request your teacher to help you to focus the slide under high power magnification. Examine the slide again. The red and white blood cells will now be more clearly visible.

The RBCs are comparatively small, disc-shaped and without a nucleus.

The WBCs are larger with a distinct nucleus.

Conclusion: Apparently, the blood appears to be a thick "homogenous" fluid, but when observed under the microscope, its two main cellular components, RBCs and WBCs, are distinctly seen. [Note: The platelets are too small to be seen at this magnification].

Teacher's Note

Looking at blood under a microscope is like zooming into a city map - what looks like a simple red fluid reveals a complex community of different cell types working together.

Functions of the Blood

1. The blood transports the digested food (nutrients) from the intestines to those organs where they are either stored or utilised (e.g. the liver).

2. It carries oxygen from the lungs to the tissues and carbon dioxide produced in the tissues to the lungs from where it is breathed out.

3. It carries the cellular waste products to the kidneys from where they are removed from the body.

4. It maintains water balance in the tissues. It takes in excess water, if any, from our tissues and also provides water when they need it.

5. It regulates body temperature by distributing the heat (produced in our tissues) in different parts of the body.

6. It protects against diseases by destroying the disease-causing germs.

7. It prevents excessive bleeding by blood clotting.

The Blood Vessels

The blood is carried to different parts of the body through tubular blood vessels.

The blood vessels are of three kinds - arteries, veins and capillaries.

An artery is a vessel which carries the blood away from the heart to other parts of the body. It has thick, elastic and muscular walls, and the blood in it flows with jerks (as felt in the pulse).

A vein is a vessel which takes the blood from an organ towards the heart. It has thin muscular walls and the blood in it flows smoothly.

Differences between arteries and veins:

Capillaries are the terminal branches of an artery, which rejoin to form a vein. A capillary is a very narrow tube whose walls have a single layer of cells with no muscles. Although the wall of a capillary is very thin, yet an exchange of nutrients, waste products and gases can take place between the blood and the body fluids.

Teacher's Note

Arteries are like major highways carrying traffic away from a city center, while veins are like return routes bringing traffic back, and capillaries are the small streets where actual delivery happens.

Heart - The Pumping Organ

The heart is a muscular organ. It beats non-stop throughout one's life. It has four chambers - the two upper auricles and the two lower ventricles (Fig. 1.3). It receives the blood from different parts of the body and then pumps it into the lungs for gaseous purification [pulmonary (lung) circulation]. The oxygenated blood from the lungs returns to the heart, which it then pumps again into different parts of the body [systemic (body) circulation]. In this way, blood passes twice through the heart, making one complete round through the body. It is called double circulation.

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