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Part XXI: The Heart and Blood System

The Heart and Heart Disease

Topic Chapters Index


Blood Vessels

L.S. of part of an artery and part of a vein  © Shirley Burchill

Longitudinal sections (L.S.) of part of an artery (left) and part of a vein (right).

The three types of blood vessels have different functions and so you would expect them to have different structures.



T.S. through an artery © Shirley Burchill

Transverse section through an artery showing the elastic tissue in the wall.

The arteries need to have elastic tissue in their walls. They receive the blood which has just been pumped from the heart. As this blood moves into them under pressure the arteries stretch. The elastic tissue makes sure that the arteries return to their original size. This also helps to smooth out the flow of blood. The main arteries branch into smaller arteries in the body. These smaller arteries still have elastic tissue in their walls. The smallest of these branches are called arterioles and they connect to the capillary system.


A small capillary bed © Shirley Burchill

A small capillary bed. Every living cell is in direct contact with a capillary.


TS through a capillary with living cells surrounding it © Shirley Burchill

TS through a capillary with living cells surrounding it and a red blood cell passing through it.

The capillary system consists of very tiny vessels whose walls are very thin, often just one cell layer. The capillary network is so immense that every living cells of the body is in contact with a capillary.

LS through a capillary showing the red blood cells © Shirley Burchill

LS through a capillary showing the red blood cells travelling through one by one.

Exchange between the blood and the living cells

TS through a capillary © Shirley Burchill

TS through a capillary showing the exchange between the blood and the surrounding cells.

Oxygen and sugar, for example, move from the blood into the cells surrounding the capillary. Carbon dioxide and other waste products move from the living cells into the blood.



T.S. of a vein © Shirley Burchill

Transverse section (TS) of a vein showing thin wall and large lumen (space for the blood to move through).

The capillaries link up to form small veins, called venules, which in turn link up to form the larger veins. Veins have thin walls with no elastic tissue in them. Because the walls of these blood vessels are thin there is more space in the vessels for the blood to move through. The blood is not pumped through the veins. Any effect from the pumping action of the heart is lost as the blood moves into the capillary system. Blood moves through the veins because of local muscle action only and it moves slowly. Veins from the legs have valves in them which stop the blood reversing its direction as it moves towards the heart.



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Section through a vein © Shirley Burchill

Section through a vein (x40)


Blood moves around the body in special tubes called blood vessels. It is kept moving by the pumping action of an organ called the heart.

There are three different kinds of blood vessels. The arteries (singular: artery) always carry blood away from the heart and towards the capillaries.

The veins always carry blood which has come from the capillaries and is moving back to the heart.

The capillaries (singular: capillary) are extremely thin blood vessels. They form networks which reach every living cell of the body.


Blood sample © Shirley Burchill

Blood sample (x680). Some red cells and a white cell have been highlighted.


Blood is made of a liquid plasma which has red and white blood cells in it. The plasma is mostly water with many different chemicals dissolved in it. Since the blood moves around the body it is used to transport chemicals to the parts of the body that need them. Plasma contains chemicals such as proteins, hormones, soluble food molecules and urea.

Plasma also has special cells suspended it. These cells are of two types:

Red Blood Cells

Drawing of red blood cells © Shirley Burchill

Red blood cells are made in the bone marrow which is found in the centre of the long bones in the limbs. Each red blood cell has a biconcave shape. These cells do not contain a nucleus, in fact they are the only cells in the body without nuclei. The shape of each red blood cell is designed to give the cell maximum efficiency for picking up and releasing oxygen. The presence of a nucleus would change this shape.

Each red blood cell is packed with an iron-containing protein called haemoglobin. It is the haemoglobin which carries the oxygen. There are five million red blood cells in every millimetre cubed of blood! Each of these cells have a life span of around 120 days, after which it is broken down by the liver. So red blood cells are constantly produced by the bone marrow to replace those which have been destroyed.

It is in the capillary system of the lungs where oxygen moves into the red blood cells. The oxygen remains attached to the haemoglobin until the blood reaches a capillary network in the body. Once in a capillary the oxygen moves from the red blood cells into the living cells which surround the capillaries.


White Blood Cells

Drawing of white blood cells © Shirley Burchill

The lymphocytes produce anti-bodies and the phagocytes 'eat' the broken up bacteria cells.

There are many different types of white blood cells. All of them help the body to fight disease caused by organisms such as bacteria. Some types of white blood cell produce chemicals called antibodies which surround the invading bacteria and cause them to break up. Other types literally ‘eat’ the bits of bacteria after they have been destroyed. There are around 7,000 white blood cells in one millimetre cubed of blood. Of course this number will be higher if a person is suffering from an infection.


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