The circulation of blood in mammals is made possible with the aid of blood vessels and the heart.

Blood vessels

The blood vessels are the tubes within the body through which the blood flows away or to the heart. There are three major types of blood vessels. These are arteries, veins and capillaries.


Arteries have thick muscular walls with small lumen or canal. The walls are elastic to withstand the high pressure, i.e. high blood pressure coming from the heart.
1. They carry blood away from the heart.
2. They transport oxygenated blood with the exemption of pulmonary artery.


Veins are less elastic than arteries with large lumen or canal. They have valves to prevent back flow of blood because of reduced pressure.

1. They carry blood to the heart
2. They transport deoxygenated blood except pulmonary vein


Capillaries are the tiny blood vessels joining the arteries to the veins. They are found in the junction between arteries and veins around tissues and organs of the body. Capillaries have thin walls and tiny in other to penetrate to all parts of the internal organs.

The thin wall of the capillaries makes for easy diffusion of oxygen, nutrients and waste products between the cells and the blood.

1. Capillaries permit the exchange of materials between blood and tissues.
2. They link the arteries and veins.
It should be known that arteries divide and become smaller as they leave the heart to form arterioles. Meanwhile, the smaller veins from capillaries which join to form the bigger veins are called venules. The direction of circulation of blood in the body is demonstrated in the diagram below.

Differences between Artery and Vein


1. It has thick/muscular wall.
2. It has elastic wall.
3. It carries blood away from the heart.
4. It carries oxygenated blood, except the pulmonary artery

5. Blood in its pink or bright red in colour.
6. It is situated deep in the muscles.
7. It has small lumen

8. Pressure is high.
9. Pulse is readily detectable.
10. It has no valve except semilunar valves.
11. It starts as large vessels and subsequently sub-divide.


1. It has thin/less muscular wall.
2. Its wall is non-elastic
3. It carries blood to the heart.
4. It carries deoxygenated blood, except the pulmonary vein.
5. Blood in its dark – red in colour
6. It is superficially located or situated.
7. It has large lumen.
8. Pressure is low.
9. Pulse is not readily detectable.
10. It has valves.

11. It starts as tiny vessels and subsequently unite or merge into large vessels.


The human heart is a muscular and the most powerful organ responsible for the pumping of blood round the body. It works continuously throughout the life of a person. Living organisms

cease to exist when their hearts stop functioning.
The heart is located within the chest or thoracic cavity and behind the breastbone (sternum) and between the two lobes of lungs. Each pumping action of the heart is known as heartbeat. Each animal has a certain number of heartbeat per minute. This can be counted by feeling the pulse at the wrist. Man has an average of 72 heartbeats per minute when at rest. This can be increased to 100 or more during activity or excitement. Small animals like rats may have 200 heartbeats per minute, whereas big animals like elephant may have only about 12.

The heart is made up of special muscles called the cardiac muscles which enables it to go on pumping continuously. It is reddish in colour and is enveloped in a two layered tough protective membrane called the pericardium. The space is filled with pericardial fluid. This fluid reduces the friction caused by the pumping movements of the heart between the heart wall and the surrounding tissues.

Structure of the heart

The mammalian heart is divided into four chambers: the left and right auricles or atria (singular atrium) and the right and left ventricles. The walls of the ventricles are normally thicker than those of the auricles. Again, the wall of the left ventricle is thicker and more muscular than that of the right ventricle. This is because it is through the left ventricle that blood is pumped or forced out of the heart to all parts of the body. In other words, it requires more pressure to force blood out of the heart to all parts of the body. There is a central wall in the heart called the septum which divides it into a right and left half, the two halve having no connection with each other.
Between the left auricle and left ventricle is an aperture guarded by a valve known as biscupid or mitral valve. The valve is made up of two cusps which move in one direction only, i.e. into the ventricle, thereby enabling the valve to channel the blood in one direction which is from the auricle into the ventricle. When the ventricle is filled with blood, the cusps close up the aperture between the left auricle and ventricle. This ensures that blood does not flow back into the auricle.
Similarly, a tricuspid valve consisting of three cusps is situated at the aperture between the right auricle and right ventricle. It works in the same manner as the biscupid valve, i.e. it allows the flow of blood from the right auricle to the right ventricle and not vice versa.
The cusps of both the biscupid and tricupid valves are attached to the inner walls of the ventricles by a special fibrous, non-elastic cords known as the chordae tendineae. These cords are themselves attached to the conical projections on the inner walls of the papillary muscles. The anchorage provided by the chordae tendineae prevents the valves from being forced upwards into the auricles when the ventricles contract. The pulmonary artery and the largest artery called aorta are equipped with semi-lunar valves. These valves allow the blood to flow out of the heart and into the arteries.


The heartbeat is caused by alternate contraction and relaxation of the four muscular chambers of the heart. The heartbeat occurs in two stages which are the diastole and systole.


Diastole is the first stage of the heartbeat during which the two auricles contract, creating a high pressure in the blood contained in them. The pressure causes the biscupid and triscupid valves to fold downwards into the ventricles, allowing blood to flow from the auricles into the ventricles, hence “impure” or deoxygenated blood enters the right ventricle from the right

auricle while “pure” or oxygenated blood enters the left ventricle from the left auricle. When the ventricles are full, the cuspid valves close up the apertures between the upper and lower chambers of the heart. This happens by means of blood forcing the cusps upwards so that they meet one another at the center of the apertures.


Systole is the second stage of heartbeat which involves the contraction of the two ventricles. This results in the blood being forced into the two trunks of the main arteries and out of the heart. Deoxygenated blood from the right ventricle passes into the pulmonary artery while oxygenated blood from the left ventricle passes into the aorta. While this is happening, blood from the body fills up the auricles again. The first stage of the next heartbeat then follows after a short lapse of time, and the cycle repeats itself.

The Main Blood Vessels of the Body

It has been said earlier that all arteries carry blood away from the heart while all veins carry blood to the heart. With the exception of pulmonary artery, all arteries contain oxygenated blood while all veins except pulmonary vein contain deoxygenated blood.

The main blood vessels (arteries and veins) of the body and the organs involved are shown below.

It should also be noted that the

hepatic portal vein

is the only major vessel or vein which starts and ends with capillaries.

Examples of major arteries in the body and the organs they supply are:
Blood vessel (artery) Organ supplied
Carotid artery Head
Pulmonary artery Lungs
Subclavian artery Forelimbs
Hepatic artery Liver
Mesenteric artery Stomach and intestine
Renal artery Kidney
Gonadial artery Gonads
Intercostal arteries Wall of thorax
Iliac artery Hind limbs

Continue reading here


Please share if you find our article good and useful

Important topics related to the above article

1. Recognizing living things
2. Biology as an enquiry in science
3. Branches of biology
4. Processes of methods of science
5. Usefulness of science
6. Living and non-living things
7. Characteristics of living things
8. Differences between plants and animals

20. Diffusion
21. Osmosis
22. Plasmolysis
23. Haemolysis
24. Turgidity
25. Faccidity
26. Nutrition
27. Feeding
28. Cellular respiration
29. Excretion
30. Growth
31. Cell reaction to its environment
types vertebrae and the vertebral column
32. Movement and responses

33. Reproduction
34. Skeleton
35. Type of skeleton
36. Bones of axial and appendicular skeleton
37. Joint
38. Functions of skeleton in man
39. Supporting tissues in plants
40. Mechanisms of supports in plants
41. Uses of fibres to plants
42. Functions of supporting tissues in plants
43. test for Food substances

53. Population studies
54. Ecological factors

Related posts

let us know what you think

This site uses Akismet to reduce spam. Learn how your comment data is processed.