The Circulatory System: Its 4 Main Parts and How They Work
The cells of an organism need food, oxygen, and some other substances to carry on life processes. The total of the chemical changes that go on within the cells of an organism is known as metabolism. In performing the life processes, the cells produce waste materials. These materials are known as metabolic wastes or waste products of the cell. The transport of needed materials to the cells and waste materials from the cells is the function of the circulatory system.
In humans, the circulatory system consists of the following parts.
- The vascular system: a system of tube, or vessels, through which blood or lymph flows
- The pumping organ, or heart, which pumps blood through the blood vessels
1. Vascular System
The system of tubes, or vascular system, through which blood flows consists of three types of blood vessels. Those that bring blood from the heart (arteries), those very fine tubes into which an artery branches (capillaries), and those that bring blood back to the heart (veins). The relationship between those three types of blood vessels is illustrated in the figure above. The diagram shows how blood travels in the body of a vertebrate - it leaves the heart by way of an artery, enters an organ through a network of capillaries, and returns to the heart by way of a vein.
Substances dissolved in the blood simply diffuse from thin-walled capillaries to the nearby cells. Similarly, substances such as waste materials from the cells diffuse through the capillary walls and into the bloodstream. This type of circulatory system is described as a closed transport system.
The power that propels the blood through the blood vessels comes from the heart. Man's heart is about the size of the fist. It is located at the center of the chest cavity, with the lower tip slightly pointed toward the left. It is protected by a tough sac of connective tissue, the pericardium. It is also protected from external injury by the rib cage. Below is the flow of blood in the heart.
The atria receive blood from various parts of the body. Hence, they are referred to as the receiving chambers of the heart. The ventricles pump blood to the different parts of the body. They are referred to as the pumping chambers of the heart. The chambers are labeled right atrium (RA), left atrium (LA), right ventricle (RV), and left ventricle(LV). A thick wall, or septum, separates the left and right chambers of the heart. The right atrium leads to the right ventricle, the right ventricle leads to an artery. The left atrium leads to the left ventricle, the left ventricle leads to an artery.
Blood flows in this direction and not backward because of the presence of the flaps of muscles (valves), which allow blood to flow in one direction only.
2a. Pulmonary and Systemic Circulation of Blood
- Blood from all over the body enters the heart by way of blood vessels that open into the right atrium.
- When the wall of the right atrium contracts, blood goes to the right ventricle.
- When the wall of the right ventricle contracts, blood rushes to the lungs.
- Blood from the lungs returns to the heart by entering the left atrium, When the wall of the left atrium contracts, blood goes to the left ventricle.
- When the wall of the left ventricle contracts, blood rushes to all parts of the body.
- The right ventricle pumps blood to the lungs, passing through the pulmonary arteries.
- As the blood reaches the capillaries of the lungs, oxygen diffuses into the blood, while excess carbon dioxide leaves the bloodstream.
- The oxygenated blood returns to the heart by way of pulmonary veins. The flow of blood from the heart (RV) to the capillaries of the lungs, and back to the heart (LA) is known as pulmonary circulation.
- The largest chamber of the heart, the left ventricle, pumps blood to all parts of the body.
- The blood leaves the left ventricle by way of the largest blood vessels in the body, aorta. As the blood reaches the capillaries of the different organs of the body, oxygen, food, and other substances diffuse out of the blood and into the tissues.
- At the same time, waste materials from the cells diffuse into the bloodstream.
- The blood returns to the heart by way of veins.
- The flow of blood from the heart (LV) to the capillaries of the body organs, and back to the heart (RA) is known as systemic circulation.
Heartbeat refers to the rhythmic contraction of heart muscles. The average rate of heartbeat is about 70 times per minute. It is slightly faster in children. Heartbeat is raised very much by exercise. A heartbeat consists of the following sequence of events.
- The right atrium contracts followed closely by the left atrium. Blood passes on to the ventricles. This is followed by relaxation of the atria, allowing blood to enter the heart, and closing the valves between each atrium and its ventricle.
- Next, both right and left ventricles contract. Blood passes on to the arteries. This is followed by relaxation of the ventricles.
- A short pause, or period of inactivity, follows. And then, the cycle is repeated.
2c. Blood Flow Pressure
Put your right hand on your chest, a little to the left. The beating that you feel comes from the left ventricle. The contraction of the left ventricle imparts pressure to the blood flow. This pressure drives the blood through the blood vessels. In turn, the blood rushing out of the ventricle imparts pressure on the wall of the artery. The impact causes the wall of the artery to expand. Since the wall of the artery, is elastic, it recoils, causing a wave of expansions to pass along the length of the artery. This is the origin of the pulse which you feel from the arteries in spurts. The wave of recoil along the wall of an artery helps push blood farther on to the capillaries.
After traveling through the arteries and capillaries, the pressure of the blood flow is reduced considerably by the time the blood reaches the veins as a result of rubbing against the walls of the vessels. Since the pressure is weak, it is impossible for the blood in a large vein flows backward. The backward flow of blood is prevented by the presence of valves along the veins.
3a. Composition of Blood
The table below shows the average composition of human blood. It shows that the whole blood consists of blood cells, which is about 45%, and a liquid portion called plasma of about 55%.
The table also shows that plasma is mostly water, containing about 92%. You can see how valuable water is to the body. The plasma also contains in solution about 7% proteins, about 1% inorganic salts, and some organic substances. The organic substances dissolved in the plasma consists of digested food from the food tube, gases, waste materials from the cells, enzymes, and hormones.
I. Blood Cells
about 45% of whole blood
A. Red Blood Cells
4,500,000 to 5,000,000 per cubic milliliter of blood
B. White Blood Cells
5,000 to 10,000 per cubic milliliter of blood
C. Blood Platelets
about 250,000 per cubic milliliter of blood
II. Blood Plasma
about 55% of whole blood
about 92% of the plasma
about 7% of plasma
about 4.5% of proteins
about 2% of the proteins
about 0.5% of the proteins
C. Inorganic salts and some organic substances
about 1% of plasma
3b. Red Blood Cells
Mature red blood cells in mammals have a biconcave shape. They contain no nucleus. Because of this, red blood cells are unable to repair themselves and thus have a rather short life. They live for about 120 days. They stay in the blood for 10 days only. They are destroyed mostly in the spleen and the liver. Red blood cells contain a pigment called hemoglobin, which gives a red color to the blood. Because of this color, red blood cells are also caller erythrocytes. Erythrocytes come from the Greek word erythos, which means red, and cyte, which means cell. Hemoglobin is a complex protein that has a strong attraction for oxygen.
Because of their hemoglobin content, red blood cells are the best adapted for carrying oxygen to the body cells. Compared to the red blood cells of fish, amphibians, reptiles, and birds, those of mammals are smaller, measuring about 7 to 8 microns in diameter. Because of their small size, the red blood cells of mammals have more hemoglobin per unit volume than those of other vertebrates. Thus, they carry more oxygen in proportion to their size.
In man, one milliliter of blood contains about 5 million red blood cells. In women, it is about 4.5 million red blood cells only. Considering the functions of red blood cells, why is it advantageous for men to have a greater number of red blood cells than women? Red blood cells are made in the red marrow of flat bones, and long bones. Blood cells, including red blood cells, certain white blood cells, and blood platelets are formed from special connective tissue cells, referred to as hemocytoblasts.
4. Lymph, Lymph Vessels, and Tissue Fluid
As blood passes through the capillaries, water, and dissolved substances (oxygen, amino acids, and simple sugars) filter through the capillary walls, forming what is known as tissue fluid. Blood proteins and most blood cells remain in the blood and do not pass through the capillary walls. This tissue fluid is in direct contact with the cells.
Since the concentration of oxygen and other needed materials in the tissue fluid is greater than that inside the cells, these substances diffuse into the cells. Similarly, waste materials including carbon dioxide diffuse out of the cells into the tissue fluid and then into the blood where their concentration is least.
Two things happen to tissue fluid. Some of it enters capillaries. Some of it enters a system of vessels called lymph vessels. Inside these vessels, the fluid is known as lymph.
The very fine lymph vessels are comparable to capillaries. They lead to larger lymph vessels, in turn, lead to two large ducts: the right lymphatic duct, which received lymph from the head and right arm, and the left lymphatic duct, or thoracic duct, which receives lymph from all other parts of the body.
The two lymphatic ducts are joined to the large veins at the region of the shoulders below the neck. The ducts empty the lymph into the bloodstream in this region. Thus the lymph becomes part of the blood again. From there the blood enters the right atrium of the heart.
Located along the lymph vessels are enlargements called lymph nodes or glands. In the lymph nodes, foreign materials like bacteria are removed. White blood cells in these nodes engulf the bacteria. You can see and feel the lymph nodes near the skin when they become swollen due to infection.
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This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2020 Ray