THE CIRCULATION OF BLOOD

There are three plans or types of blood circulation in human beings, other mammals and birds. They are the pulmonary circulation, systemic circulation and the coronary circulation.

 

The pulmonary circulation

(Heart ->lung ->heart)

The pulmonary circulation transports blood between the heart and the lungs. The superior vena cava returns blood from the head and upper limbs into the right artrium. Blood from the lower limbs and the abdomen is brought to the right atrium by the inferior vena cava. The right artrium contracts and the blood is forced through the tricuspid valves into the right ventricle. The right ventricle contracts forcing the blood up into the pulmonary artery. The pulmonary artery divides into right and pulmonary arteries each leading to right and left lungs respectively. When the right ventricle relaxes the semi-lunar valves close up to prevent the blood from rushing back into the ventricle. The pulmonary arteries divide and sub-divide into numerous arterioles which in turn subdivide into microscopic capillaries in the lungs. With the thin walls of the capillaries and the moist individual cells of the lungs exchange of gases, nutrients and wastes takes place between the blood and the cells of the lungs. The capillaries unite and reunite to form venules. The venules in turn form the pulmonary vein that brings back the oxygenated blood into the left atrium of the heart. When the left atrium contracts the blood is forced through the bicuspid (mitral) valves into the left ventricle.

 

The importance of pulmonary circulation

1. Pulmonary circulation in the first place prevents the mixing of deoxygenated blood with oxygenated blood. This is found only in the birds and mammals which account for their being very active animals and also being warm-blooded (homeothermic). The mixing of de-oxygenated blood with oxygenated blood in the three-chambered hearts of the Amphibians and Reptiles makes them sluggish and poikilothermic.
2. Pulmonary circulation makes oxygenated blood available in the left ventricle for systemic circulation. Deoxygenated blood gives out carbon (IV) oxide, urea and other wastes and picks oxygen and other nutrients to enter the systemic circulation.
3. It checks accumulation of wastes and enhances renewal of the blood.
4. It prevents deoxygenated blood from entering the systemic and coronary circulation and ensures that only oxygenated blood does.

 

Systemic circulation

(Heart -> rest of body -> heart)

Systemic circulation transports blood between the heart and all parts of the body. The oxygenated blood arrives the left ventricle. The left atrium relaxes and the left ventricle contracts. Blood is forced up the aorta to all parts of the body. Blood rich in oxygen and other nutrients is taken to all living cells in the body. Exchange of materials takes place and the living cells are provided with means for releasing energy for use. By this all energy dependent vital processes of living organism is maintained. The aorta is the main artery and on leaving the heart, it forms and aortic arch where it gives out three branches before running down the trunk. The arterial supplies in the systemic circulation are as follows:

1. At the curve or bend, the first two branches the right and left carotid arteries pass through the neck into the head to supply the head, brain and face.
2. The first branch which is the right carotid artery gives a branch to the right arm as the right subclavian artery.
3. The third branch is the left subclavian artery which supplies the left arm.
4. The main artery continues down the trunk as the aorta and in the thorax gives out a pair of small branches to a pair of ribs as the phrenic arteries.
5. A major branch, the hepatic artery supplies the liver.
6. Next is the mesenteric artery which branches into the stomach as gastric artery, the spleen as splenic artery and the intestine as the intestinal artery.
7. The right and left renal arteries supplying the respective kidneys.
8. The testicular and ovarian arteries are the genital arteries that supply the testes and ovaries in the male and female respectively.
9. The abdominal artery now gives rise to two major branches which go into the lower or hind limbs as common iliac arteries and later the femural arteries into the legs.

 

The major veins that return the blood back to the heart in the systemic circulation are as follows:

1) The right and left jugular veins from the head, brain and face join the right and left subclavian vein and enter the right auricle as the superior vena cava.

 

2) The inferior vena cava drains the blood from the rest parts of the body into the same right atrium after receiving blood from:

• Phrenic veins from the ribs and intercoastal muscles.
• Hepatic vein from the liver.
• Hepatic portal vein from the gut.
• Renal veins from kidneys.
• Genital veins from the ovaries and testes.
• Common iliac veins and femural veins from the thigh and legs.

 

Coronary circulation

The coronary circulation supplies the heart. It starts at the aorta where a branch of the coronary artery takes oxygenated blood to all the contractile cells of the heart through a complex network of capillaries. The coronary vein drains the spent blood into the right atrium of the heart. It is a very short circulation but very vital. The heart muscles are very strong and they beat rhythmically without stop throughout the life of the organism. They are not easily fatigued. There is a high and continuous energy demand and utilization in the heart hence its own special circulation.

 

Open and closed blood circulatory systems

There are two systems of blood circulation throughout the animal world. These are open and closed blood circulatory systems.

 

1) Open blood circulatory system

This is found in most invertebrates. The heart which is dorsally placed is tubular in structure.

This tubular heart extends most of the body’s length and ends up in a single dorsal blood vessel called aorta.Theheart is provided with paired holes called ostia. The heart pumps blood forward through the single dorsal blood vessel (aorta) into the haemocoel (body cavity). Blood is collected from the haemocoel through the paired ostia and returned to the heart.

 

2) Closed blood circulatory system

In this system, blood leaves the heart in closed pipes (blood vessels and capillaries) and circulates round the body and returns to the heart (a closed pumping organ) without being exposed. This is the system in the earthworms and the vertebrates. A closed system is more efficient and better for a large and complex body. The blood is always on the move and moves rapidly through the various vessels all the time. It only requires the assistance of a lymphatic system for a more effective exchange of materials between the cells and the body fluid. In the lymphatic system, fluid flow is slow but bathes the cells directly with nutritive fluid. A closed system is better for very active animals as a complete circulation of blood occurs in a short time.