Blood Circulation

BLOOD CIRCULATION

Blood circulation refers to the continuous movement of blood through the heart, blood vessels, and organs of the body. It ensures the supply of oxygen and nutrients to tissues and removes waste products such as carbon dioxide. Depending on the course followed by blood, circulation is classified into the following types:

1.     Systemic circulation (Greater circulation)

2.     Pulmonary circulation (Lesser circulation)

3.     Portal circulation

1. Systemic Circulation

Systemic circulation is the part of the cardiovascular system that carries oxygenated blood from the heart to all parts of the body and returns deoxygenated blood back to the heart.

The oxygenated blood is pumped out from the left ventricle into the aorta, the largest artery of the body. The aorta gives rise to many branches that carry blood to different organs and tissues.

From the aorta, blood divides into smaller systemic arteries, which further branch into arterioles. These arterioles lead to an extensive network of systemic capillaries present in all tissues of the body, except the air sacs (alveoli) of the lungs, which are supplied by pulmonary circulation.

Across the thin walls of capillaries, exchange of substances occurs. Oxygen and nutrients are delivered to the tissues, while carbon dioxide and metabolic wastes are collected from the cells.

During this exchange:

• Blood unloads oxygen (O₂)
• Blood picks up carbon dioxide (CO₂)

In most tissues, blood flows through one capillary bed and then enters a systemic venule. Venules merge to form larger systemic veins, which carry deoxygenated blood away from the tissues.

Finally, the deoxygenated blood returns to the right atrium of the heart through the superior vena cava (from upper body) and inferior vena cava (from lower body).

Thus, the circulation of blood from the left ventricle to the right atrium is called systemic circulation.

Flow Sequence of Systemic Circulation:

2. Pulmonary Circulation

Pulmonary circulation is concerned with the movement of blood between the heart and the lungs. The right side of the heart acts as the pump for pulmonary circulation.

Deoxygenated blood from the right ventricle is pumped into the pulmonary trunk, which divides into right and left pulmonary arteries. These arteries carry blood to the respective lungs.

Inside the lungs, the pulmonary arteries divide and subdivide into smaller vessels, finally forming capillaries around the alveoli (air sacs). Here, gaseous exchange takes place:

• Carbon dioxide (CO₂) moves from blood into the alveoli and is exhaled
• Oxygen (O₂) moves from the alveoli into the blood

The oxygenated blood from pulmonary capillaries then collects into venules and forms pulmonary veins. These veins leave the lungs and carry oxygenated blood to the left atrium of the heart.

There are two pulmonary veins from each lung, and pulmonary veins are unique because they are the only veins that carry oxygenated blood.

The contraction of the left ventricle then pumps this oxygenated blood into the systemic circulation.

Thus, the circulation of blood from the right ventricle to the left atrium is called pulmonary circulation.

Flow Sequence of Pulmonary Circulation:

3. Portal Circulation

Portal circulation is a special type of circulation in which venous blood passes from one capillary bed to another before returning to the heart.

In portal circulation, blood from the digestive organs, spleen, and pancreas is collected and transported to the liver through the portal vein.

A vein that carries blood from one capillary network to another is called a portal vein.

The hepatic portal vein is formed by the joining of several veins, including:

• Splenic vein (from spleen)
• Superior mesenteric vein (from small intestine)
• Inferior mesenteric vein (from rectum and colon)
• Gastric veins (from stomach)
• Cystic vein (from gall bladder)

Blood reaching the liver through portal circulation is rich in nutrients absorbed from the stomach and intestines. This allows the liver to:

• Store glucose as glycogen
• Detoxify harmful substances
• Regulate nutrient levels

The liver receives oxygenated blood separately through the hepatic artery. After processing, blood leaves the liver through hepatic veins, which drain into the inferior vena cava and return blood to the heart.

Key Features of Portal Circulation: