Our hearts pump between 4 and 6 litres of blood per minute through our circulatory systems in a resting state. An extensive network of blood vessels ensures that the body is supplied with blood, supplying all organs and tissues of the body down the very last cell with oxygen and vital nutrients, at the same time carrying away products of metabolism and degradation.
Blood vessels become more branched and smaller in size the farther away they are from the heart. Blood vessels with large diameters serve mainly to transport the blood and ensure constant blood flow, whereas the small vessels – also referred to as microvessels – supply needed substances to the cells of the body and carry away the products of metabolism and degradation.
Microvessels include arterioles, capillaries and venules (see Figure). The capillaries are the vessels of our body in which gases and other substances are exchanged. They form a netlike structure, the so-called capillary region. When the blood, arriving from the larger arterioles, is distributed in the capillary region among the many capillary vessels, blood pressure drops and the blood flow rate is considerably slowed.
Gases and other substances can then be exchanged through thin, semi-permeable vascular walls of the capillaries between the blood and the surrounding tissues. The blood then flows on into the postcapillary venules into which the capillary region transitions. Blood pressure and flow rate then rise again as the blood moves into the ever larger branches of the venous system and is finally returned to the heart.
Movement of the blood in the vessels is known as blood flow. The blood flow in the blood vessels with diameters of less than 0.1 millimetre is known in the medical field as microcirculation.
The flow characteristics of the blood follow the laws of physics, with influences coming from a variety of factors. Crucial roles are played by blood pressure, vascular diameter, vascular resistance and blood thickness (viscosity).
Our body has a variety of control mechanisms for the individual factors so as to keep the various organs and body segments supplied with blood according to their fluctuating performance requirements. For example, when we are engaged in sporting activities, our musculature temporarily demands a higher supply volume, i.e. more blood must flow into the microcirculation there.
The arterioles contribute decisively to blood flow regulation. When the vascular musculature contracts and relaxes this changes the vascular cross-section (lumen), thus determining the blood pressure and flow rate in the capillary region. If more blood supply is needed, e.g. due to an increased performance load, the blood pressure and flow rate in the capillary region should be as low as possible.
Blood flow regulation may vary from one vascular segment to another. There are numerous control mechanism, both centralized and local.
The signal to change the vascular lumen width may originate from vascular nerves, hormones or due to local mechanical effects of muscle contraction (autoregulation).
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