Blood flow through the vessels is determined by two factors: the difference in blood pressure in the arteries and veins, which is supported by contractions of the heart, and the resistance of the walls of the vascular bed to blood flow. Blood flow rate is inversely proportional to the cross sectional area of blood vessels. Thus, the cross-sectional area of the aorta is about 1000 times smaller than the area of the total cross-section of the capillaries. Therefore, the rate of blood flow in the aorta of approximately 0.5 m / sec and capillaries – 0.5 mm / sec. With such a small speed in the capillaries of the blood time to meet its metabolic function, although the average red blood cell passes the capillary for 0.1-0.2 seconds. Despite the fact that the blood ejected from the heart into the bloodstream pushes it moves through the vessels continuous flow that is caused by the elasticity of the artery walls. The arteries during systole of the heart to fill with blood, their walls are stretched, and then during diastole artery pumps blood into smaller vessels.
Low pressure in the veins, and promotion of blood through them contribute to the above-described valves; contraction of skeletal muscles surrounding the veins; suction effect of the chest during inspiration, and others.
Blood pressure is most conveniently measured in the brachial artery. In this section the limbs of a healthy person, it is the time of systole 120 mm Hg. Art., and at the time of diastole – 80 mm Hg. Art. The aortic pressure is significantly higher, and in vessels brushes – are significantly lower. In the large veins of the chest cavity is almost equal to the atmospheric pressure.
At the time of the next portion of systole blood flows into the aorta push, then the tremors spread through the vessels, and they are called arterial pulse. According to the frequency and strength of the pulse can judge the state of the heart and blood vessels. The most convenient way to record heart rate in those places where arterial blood vessels closest to the surface of the body: neck, forearm, and so the temple. D.
The lumen of blood vessels, and along with it, and blood pressure regulated by neural and humoral routes. The main nerve center called vasomotor regulation of blood flow and is located in the medulla oblongata. From his excitement spreads sympathetic and parasympathetic nerves. Sympathetic influence of peripheral blood vessels constrict, increasing the pressure in them. The exception here make vessels of the brain, heart and lungs. The parasympathetic influence vasodilator effect.
Information about what the pressure in the vessels and what the composition of the blood therein is supplied to the brain by numerous receptors located in the walls of blood vessels. The most important group of receptors located in the walls of the aortic arch in a branching point of the common carotid artery into internal and external, in the walls of the pulmonary artery. Chemical vascular receptors respond to changes in the concentration of O2, CO2, H +, et al. Blood. Blood pressure is also regulated by numerous chemical factors, blood-borne, ie humoral. Adrenaline has a strong effect, narrowing the arterial vessels of the lungs, kidneys, digestive organs and the skin dilate the arteries and skeletal muscles and smooth muscles of the bronchi. When emotional and physical stress adrenaline enhances the blood flow through the brain, heart and skeletal muscles.
Has pronounced vascular effect of vasopressin (antidiuretic hormone) – peptide secreted by cells of the hypothalamus, stored in the posterior lobe of the pituitary and thence into the blood. Vasopressin improves blood flow in the brain and heart, the blood vessels extending these bodies, but also narrows the arteries and arterioles of the abdominal cavity and lung.
Kidney cells produce renin – an enzyme that catalyzes the formation of peptide angiotensin II, has a very strong vasoconstrictor effect on the arteries and raises blood pressure. If the damage of the skin and mucous membranes of basophils release histamine, causing local expansion of arterioles and venules.
The lymphatic system, as mentioned above, is a system of additional outflow of interstitial fluid from the bodies. Lymph performs several important functions. Almost all of the lymph vessels, the largest of which are provided with valves, flow into the thoracic duct. And only the lymphatic vessels of the right half of the head, right side of his chest and right hand going into the right lymphatic duct. The lymph ducts empty into the vena cava. Reasons motion lymph: rhythmic contractions of the walls of large lymphatic vessels, the presence of the valves, contraction of skeletal muscle surrounding lymph vessels, respiratory movements of the thorax.