Circulatory system: physiological properties

2021-05-31 04:16 PM

The function of arteries is to deliver blood under pressure to tissues. To ensure this function, arteries have thick walls, and blood moves at high speed through the lumen.

The function of the circulatory system is to supply the blood needed by tissues - transport nutrients to organ tissues, and at the same time transport waste products, transport hormones from some organs in the body to other places and maintain stability. The concentration of substances in the homeostasis in the body helps cells survive and perform their functions well.

The circulatory system, pictured, includes systemic circulation and pulmonary circulation. Because the circulatory system supplies blood to all tissues in the body, including the lungs, it is also known as great circulation or peripheral circulation.

Functions of the circulatory system

Before discussing the functional characteristics of the circulatory system, it is important to understand the role each part of the circulatory system plays.

The function of arteries is to deliver blood under pressure to tissues. To ensure this function, arteries have thick walls, and blood moves at high speed through the lumen.

The arterioles are the terminal branches of the arteries, which act as conduits to control the flow of blood through the capillary lumen. Arterioles with thick muscular layers can completely close the arterioles, but they can also dilate the whole vessels, thus, the ability to turn blood flow to tissues is great when needed.

The function of capillaries is to exchange fluids, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid. To ensure this function, capillary walls are thin and have many capillary slits that allow water and low-molecular substances to pass through.

Small veins receive blood from capillaries and then combine, emptying into large veins.

The function of the veins is the system of tubes that carry blood from the venules back to the heart, It is like an auxiliary blood reservoir. Because the blood pressure in the veins is very small, the vein wall is thin. Even so, its muscular layer is sufficient to contract or relax, thereby satisfying the regulation of blood storage function, large or small depending on the needs of the circulatory system.

Amount of blood in different parts of the circulatory system

The figure gives an overview of the circulatory system and the percentage of total blood volume in major parts of the circulatory system. For example, 84% of the body's total blood volume is in the systemic circulation, 16% in the heart and lungs, 13% in the arteries, and 7% in the arterioles and capillaries. The heart contains 7% of the blood and the pulmonary veins account for 9%.

Figure. Blood distribution (percentage of total blood volume) in different parts of the circulatory system.

What is surprising is the small amount of blood in the capillaries. However, the most important function of the circulation is the place where the metabolism between blood and tissues takes place.

Cross-sectional area and blood flow rate

If all circuits of each type are placed side by side, the total mean cross-sectional area is approximate:

Blood vessels: Cross-sectional area (cm2)

Aorta: 2.5

Small arteries: 20

Arteries: 40

Capillaries: 2500

Varicose veins: 250

Small veins: 80

Vein: 8

The cross-sectional area of ​​the vein can be seen to be much larger than that of the artery, averaging about 4 times that of the corresponding artery. This difference explains the greater blood capacity of the venous system when compared to the arterial system.

Because the same amount of blood must flow through each segment in the same minute (F), the flow rate (v) is inversely proportional to the cross-sectional area of ​​the vessel.

       v = F/A

Therefore, at rest, the average speed of blood flow is about 33cm/s in the arteries, but the velocity in the capillaries is only 1/1000 of that, about 0.3mm/s. However, because capillaries are typically about 0.3 to 1mm long, blood stays in the capillaries for only about 3 seconds. This is surprising because all diffusion of nutrients and electrolytes through the capillary wall must take place in this short time.

Blood pressure in different parts of the circulatory system

Because the heart pumps into the arteries, the blood pressure in the arteries is high. About 100 mmHg. Also, because the heart pumps blood according to the beat of the heart, arterial blood pressure fluctuates between a systolic pressure of about 120 mmHg and a diastolic pressure of about 80 mmHg.

With blood flowing in the circulatory system, it means that the blood pressure gradually drops to 0 mmHg by the time the blood reaches the end of the superior and inferior vena cava to empty into the right atrium.

Blood pressure in the capillary system is about 35 mmHg near the arterioles and about 10 mmHg near the venules, but the mean "functional" blood pressure level for most capillary beds is about 17 mmHg, which is low enough to Small particles in the plasma can pass through the clefts of the capillary walls, although nutrition can easily diffuse through these fissures far from the tissue.

Figure. Normal blood pressure in various parts of the circulatory system when a person lies in a horizontal position.

Note on the right side of the figure is the corresponding blood pressure of the different parts of the pulmonary circulation. In the pulmonary artery, blood pressure fluctuates with the beat, diastolic blood pressure is about 25 mmHg, systolic blood pressure is about 8 mmHg. Mean arterial blood pressure is only about 16 mmHg, mean pulmonary capillary blood pressure is about 7 mmHg. The total volume of blood passing through the pulmonary circulation per minute is equal to the total volume of blood passing through the systemic circulation per minute. The low blood pressure in the pulmonary circulation is consistent with the needs of the lungs because its function is to push blood through the capillaries to exchange oxygen and other gases in the alveoli.