Active control of local blood flow

2021-05-27 01:56 PM

The mechanism by which changes in tissue metabolism or blood oxygen levels alter blood flow are not fully understood, but two main hypotheses have been proposed so far: the vasoconstriction hypothesis and the oxygen demand hypothesis.

Increased tissue metabolism - increased blood flow to tissues

Transient effect on blood flow with increased local metabolic rate in tissues, such as skeletal muscle. Note that an increase in metabolism 8 times higher than normal results in 4 times increase in blood flow.

Decreasing tissue oxygen will increase blood flow to the tissue

One of the most essential nutrients is oxygen. Whenever oxygen to the tissue is reduced such as (1) at the top of a mountain (2) Pneumonia (3) CO poisoning or (4) cyanide poisoning, blood through the tissue increases markedly.

Figure. Effect of arterial oxygen saturation on blood flow to one leg

The figure shows that arterial oxygen saturation is reduced by 25% of normal, blood flow through one leg of the dog is increased 3 times. The blood increases almost enough but not enough, in response to a decrease in blood oxygenation, so that most of the time it remains at a relatively constant level of tissue oxygenation.

Total cyanide poisoning using oxygen by the local tissue can increase blood flow sevenfold, thus demonstrating the impact of hypoxia on blood flow.

The mechanism by which changes in tissue metabolism or blood oxygen levels alter blood flow are not fully understood, but two main hypotheses have been proposed so far: the vasoconstriction hypothesis and the oxygen demand hypothesis.

The hypothesis of vasoconstriction with rapid regulation of local blood flow - The special role of adenosine

According to the vasoconstrictor hypothesis, increasing the rate of metabolism or reducing oxygen or other nutrients to the tissues will accelerate the formation of vasoconstrictor substances in the cells. Vasoconstrictors are thought to diffuse through the tissue to the precapillary sphincter and capillaries to induce vasoconstriction. Some substances that cause vasoconstriction such as adenosine, CO2, adenosine phosphate, histamine, potassium ions and H+ ions.

Vasoconstrictors may be released from cells in response to oxygen deprivation. For example, experiments have shown that tissue hypoxia causes the release of adenosine and lactic acid (both H+ ions) from the intercellular space. These substances can cause very rapid vasoconstriction, which completely or partially regulates blood flow to tissues. Vasoconstrictors such as CO2, lactic acid, and potassium ions tend to increase in cells when blood flow to tissues is reduced and cellular metabolism will continue at the same rate or when cellular metabolism is suddenly increased. Increased concentration of metabolites causes arteriolar constriction, thereby increasing tissue blood flow and concentration of metabolites returning to normal.

Many physiologists believe that Adenosine is an important local vasoconstrictor that helps control local blood flow. For example, small amounts of adenosine are released from cardiomyocytes when the coronary arteries are constricted, and adenosine release causes sufficient vasoconstriction of the coronary arteries to return to normal. Similarly, whenever the heart is working harder than usual and the heart's metabolism increases by an amount, there is an increase in oxygen utilization, in ways that (1) decrease oxygen concentration in the myocardium ( 2) decreased adenosine triphosphate degradation (3) increased adenosine release. More adenosine leaks out of cardiomyocytes to cause arterial constriction, reducing the arterial blood supply to meet the nutritional needs of the working heart.

Although the research evidence is not clear, many physiologists also suggest that adenosine-like mechanism is the most important regulation of blood flow to striated muscle and other tissues. however, it is difficult to demonstrate that sufficient quantities of individual vasoconstrictors including adenosine are actually formed in the tissue to cause a steady increase in blood flow. It is likely that a combination of several vasoconstrictors released from the tissue contributes to the regulation of blood flow.

The hypothesis of oxygen demand with tissue blood control

Although the vasoconstriction hypothesis is widely accepted, there is another hypothesis that is accepted by physiologists, the oxygen demand hypothesis, more precisely the nutritional requirement of tissues (because nutrients are other than oxygen is also involved). Oxygen is a metabolic nutrient that is required for vasoconstriction, muscle contraction (as well as other nutrients). Due to the absence of an appropriate amount of oxygen in the tissue, blood vessels can dilate. Likewise, tissue oxygen utilization because of increased metabolism would (theoretically) deplete smooth muscle oxygen, causing vasoconstriction.

The mechanism by which tissue oxygen can act, a unit of tissue, including capillaries along with neighbouring capillaries and tissue around it. At the base of the capillary is the precapillary sphincter and surrounding the precapillary are several smooth muscle fibres. On microscopic examination of the tissue, the normal precapillary sphincters can be either completely dilated or completely contracted. the number of dilated precapillary sphincters at any given time is proportional to the nutrient requirements of that tissue. The precapillary sphincters and capillaries open and close periodically several times per minute, with the duration of the open phase commensurate with the metabolic demands of tissue oxygen. The cycle of opening and closing is called vasomotion.

Figure. Schematic of a single tissue unit that accounts for instantaneous blood flow feedback, showing capillaries running through the tissue and capillaries with precapillary sphincters with capillary blood flow control.

Since smooth muscle requires oxygen to maintain muscle contraction, it is assumed that the strength of sphincter contraction increases with increased oxygen concentration. Therefore, when the oxygen concentration in the tissue increases above some level, the precapillary sphincter will probably close until, however, when oxygen limitation has occurred, and the tissue cell oxygen concentration is exhausted. absorb excess oxygen. However, when the excess oxygen is depleted, and the oxygen concentration falls below the required level, the sphincter opens more than once to restart the cycle.

Thus, based on baseline data, either the adenosine hypothesis or the oxygen demand would explain the immediate regulation of tissue blood flow in response to the metabolic demands of the tissue. In fact, it certainly lies in a combination of these two mechanisms.

The role of nutrients other than oxygen in controlling blood flow to tissues

Under special conditions, a lack of glucose in the blood can cause local vasoconstriction. It may have the same effect that occurs when other nutrients such as amino acids or fatty acids are deficient, although this issue has not been fully studied. In addition, vasoconstriction occurs with vitamin deficiency in beriberi, patients with vitamin B deficiency (thiamine, niacin riboflavin). In this disease, the amount of blood to the peripheral vessels everywhere in the body is usually increased 2 to 3 times. Because all the primary vitamins are required for the phosphorylation to generate ATP for the cell, it is possible to know how much vitamin deficiency will lead to a decrease in smooth muscle contraction, vasoconstriction also occurs.

A special example of instantaneous metabolic control of blood flow

The mechanism we describe for local blood flow control is called a metabolic mechanism because the functions of all of them are responsible for the metabolic demands of the tissue. Two particularly added examples of metabolic control of blood flow are reactive and active congestion.

Reactive congestion occurs after tissue blood supply is briefly blocked

When the blood supply to the tissue is blocked for a few seconds to hours or more, so that blood flow is allowed to flow, the blood flow through the tissue usually increases immediately 4 to 7 times normal. This increased blood flow will continue for a few seconds if the blockage lasts only a few seconds but sometimes continues for hours if blood flow is blocked for an hour or more, a phenomenon is known as a pulse. reactive blood.

Reactive congestion is another manifestation of local metabolic blood flow regulation. After a brief period of vasoconstriction, blood flow is enhanced during reactive congestion that lasts long enough to respond almost exactly to the tissue hypoxia accumulated during the embolism. This mechanism emphasizes the close connection between oxygen and other nutrients to the tissue.

Figure. Tissue reactive congestion after transient arterial occlusion and active congestion when tissue is metabolically active

Active congestion occurs when the rate of tissue metabolism increases

When any tissue increases activity such as muscle, gastrointestinal gland during periods of increased secretion or even when the brain increases mental activity, the flow rate through the tissue increases. The increase in local metabolism leads to the rapid absorption of nutrients and the release of large quantities of vasoconstrictors. The result is dilation of local capillaries and increased blood flow to the tissues. In this way, active tissues receive additional nutrients. As noted above, active congestion in skeletal muscle can increase blood flow to the tissue 20-fold during high-intensity exercise.