Circulatory System
What is the circulatory system?
The circulatory system can be also called as the vascular system of the body. It is the system of distribution of nutrients, to all parts of the body, via the blood. It also includes the collection of waste material from the organs of the body. This distribution and collection takes place through the blood vessels called as arteries and veins.
Where is the circulatory system situated?
The circulatory system is present throughout the body in the form of a vast network of arteries, veins and capillaries.
How can we describe the circulatory system?
Arteries
Arteries are thick-walled, elastic, tubular structures carrying oxygenated blood from the heart to the rest of the body. The largest artery in the body is as thick as a garden hose. The smallest arteries that are less than 0.1 mm in diameter are called as arterioles. The union of the branches of arteries is known as anastomosis.
The artery is made of thick, elastic tissue having a thick muscular coat. The coat also contains tiny receptors that detect the amount of stretch in the artery. This helps it to withstand and regulate blood pressure. When the body is at rest the arteries contain approximately 17% of the total blood in the body. The arteries lie deeper in the muscles than veins.
Veins
Veins are thin walled structures that carry blood towards the heart. They are much thinner than the arteries. The largest of the veins can admit two fingers at the most. The smallest veins are called as venules. The veins have a thinner muscular wall than arteries. Veins act as a reservoir for blood, when the body is at rest, as they hold up to 70% of the total blood in the body. Veins are more superficial than arteries and are seen as bluish structures on the external surface of the body.
Capillaries
These are the smallest vessels forming a very extensive network in the body. They are delicate structures with a very narrow bore. Capillaries ensure that blood is supplied to each and every cell in the human body. They carry out the transfer of oxygen directly to the cell.
Flow of Blood
Blood flows in two types of circuits:
- The lung or pulmonary circuit
- The body or systemic circuit
The pulmonary circuit
In this circuit the blood travels from the heart to the lungs for oxygenation and then back to the heart via the pulmonary vessels. The aorta, which is the largest artery, then carries it to the rest of the body.
The systemic circuit
Here the blood is transported to the various parts of the body from the heart and is then carried back to the heart by the large veins in the body. The veins arising from the gastro-intestinal system do not go directly to the heart but enter a network called the portal circulation in the liver. This purifies blood of all its toxins and is thus essential. From the liver circulation the blood is then carried to the inferior vena cava and then to the heart.
Blood flows to various parts of the body due to the pressure of the ventricular contractions and the thickness of the arterial system. But the veins are thin and their pressure is also one-tenth of the arterial blood pressure. How does blood flow through them? There are certain mechanisms that allow adequate venous return. These are the contraction of muscles, which squeeze the blood into the veins. One example of this is the calf muscle which pumps blood while walking into the systemic circulation. One other mechanism is that of inspiration. The air we inspire causes expansion of the chest cavity. This causes decrease pressure in the chest called as negative pressure. The blood, which is in the rest of the body, is pulled towards the heart due to this negative pressure.
Function of the circulatory system?
As we saw, the main function of the circulatory system is the transport of blood to and from the heart. The veins act as reservoir for blood. The arteries regulate the blood pressure. The capillaries allow exchange of gases to take place.
Heat dissipation through the flow of blood is also a function of the circulatory system. The various components of blood also flow to spots of injury and infection, acting on the sites to produce inflammation, which is a protective response.
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