Local Controls

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The term local controls denotes mechanisms independent of nerves or hormones by which organs and tissues alter their own arteriolar resistances, thereby self-regulating their blood flows. It does include changes caused by autocrine/paracrine agents. This self-regulation includes the phenomena of active hy-peremia, flow autoregulation, reactive hyperemia, and local response to injury.

Active Hyperemia Most organs and tissues manifest an increased blood flow (hyperemia) when their metabolic activity is increased (Figure 14-39a); this is termed active hyperemia. For example, the blood flow to exercising skeletal muscle increases in direct proportion to the increased activity of the muscle. Active hyperemia is the direct result of arteriolar dilation in the more active organ or tissue.

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

Active Hyperemia

f Metabolic

K

| O2 , f metabolites

K

Arteriolar

activity

>

in organ

>

dilation

of organ

1/

interstitial fluid

1/

in organ

t Blood flow to organ

Flow autoregulation

Flow autoregulation

FIGURE 14-39

Local control of organ blood flow in response to (a) increases in metabolic activity, and (b) decreases in blood pressure. Decreases in metabolic activity or increases in blood pressure would produce changes opposite those shown here.

FIGURE 14-39

Local control of organ blood flow in response to (a) increases in metabolic activity, and (b) decreases in blood pressure. Decreases in metabolic activity or increases in blood pressure would produce changes opposite those shown here.

The factors acting upon arteriolar smooth muscle in active hyperemia to cause it to relax are local chemical changes in the extracellular fluid surrounding the arterioles. These result from the increased metabolic activity in the cells near the arterioles. The relative contributions of the various factors implicated vary, depending upon the organs involved and on the duration of the increased activity. Therefore, we shall name but not quantify some of these local chemical changes that occur in the extracellular fluid: decreased oxygen concentration; increased concentrations of carbon dioxide, hydrogen ion, the metabolite adenosine, potassium (as a result of enhanced potassium movement out of muscle cells during the more frequent action potentials) and eicosanoids (Chapter 7); increased osmolarity (resulting from the increased breakdown of high-molecular-weight substances); and, in some glands, increased concentration of a peptide known as bradykinin. The last substance is generated locally from a circulating protein called kininogen by the action of an enzyme, kallikrein, secreted by the active gland cells.

Local changes in all these chemical factors have been shown to cause arteriolar dilation under controlled experimental conditions, and they all probably contribute to the active-hyperemia response in one or more organs. It is likely, moreover, that additional important local factors remain to be discovered. It must be emphasized that all these chemical changes in the extracellular fluid act locally upon the arteriolar smooth muscle, causing it to relax. No nerves or hormones are involved.

It should not be too surprising that active hyper-emia is most highly developed in skeletal muscle, cardiac muscle, and glands, tissues that show the widest range of normal metabolic activities in the body. It is highly efficient, therefore, that their supply of blood be primarily determined locally.

Flow Autoregulation During active hyperemia, increased metabolic activity of the tissue or organ is the initial event leading to local vasodilation. However, locally mediated changes in arteriolar resistance can occur when a tissue or organ suffers a change in its blood supply resulting from a change in blood pressure (Figure 14-39b). The change in resistance is in the direction of maintaining blood flow nearly constant in the face of the pressure change and is therefore termed flow autoregulation. For example, when arterial pressure in an organ is reduced, say, because of a partial occlusion in the artery supplying the organ, local controls cause arteriolar vasodilation, which tends to maintain flow relatively constant.

What is the mechanism of flow autoregulation? One mechanism is the same metabolic factors described for active hyperemia. When an arterial pressure reduction lowers blood flow to an organ, the supply of oxygen to the organ is diminished, and the local extracellular oxygen concentration decreases. Simultaneously, the extracellular concentrations of carbon dioxide, hydrogen ion, and metabolites all increase because they are not removed by the blood as fast as they are produced. Also, eicosanoid synthesis is increased by still unclear stimuli. Thus, the local metabolic

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

changes occurring during decreased blood supply at constant metabolic activity are similar to those that occur during increased metabolic activity. This is because both situations reflect an initial imbalance between blood supply and level of cellular metabolic activity. Note then that the vasodilations of active hyperemia and of flow autoregulation in response to low arterial pressure do not differ in their major mechanisms, which involve local metabolic factors, but in the event—altered metabolism or altered blood pressure—that brings these mechanisms into play.

Flow autoregulation is not limited to circumstances in which arterial pressure decreases. The opposite events occur when, for various reasons, arterial pressure increases: The initial increase in flow due to the increase in pressure removes the local vasodilator chemical factors faster than they are produced and also increases the local concentration of oxygen. This causes the arterioles to constrict, thereby maintaining local flow relatively constant in the face of the increased pressure.

Although our description has emphasized the role of local chemical factors in flow autoregulation, it should be noted that another mechanism also participates in this phenomenon in certain tissues and organs. Some arteriolar smooth muscle responds directly to increased stretch, caused by increased arterial pressure, by contracting to a greater extent. Conversely, decreased stretch, due to decreased arterial pressure, causes this vascular smooth muscle to decrease its tone. These direct responses of arteriolar smooth muscle to stretch are termed myogenic responses. They are due to changes in calcium movement into the smooth-muscle cells through stretch-sensitive calcium channels in the plasma membrane.

Reactive Hyperemia When an organ or tissue has had its blood supply completely occluded, a profound transient increase in its blood flow occurs as soon as the occlusion is released. This phenomenon, known as reactive hyperemia, is essentially an extreme form of flow autoregulation. During the period of no blood flow, the arterioles in the affected organ or tissue dilate, owing to the local factors described above. Blood flow, therefore, is very great through these wide-open arterioles as soon as the occlusion to arterial inflow is removed.

Response to Injury Tissue injury causes a variety of substances to be released locally from cells or generated from plasma precursors. These substances make arteriolar smooth muscle relax and cause vasodilation in an injured area. This phenomenon, a part of the general process known as inflammation, will be described in detail in Chapter 20.

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Responses

  • flavus gamgee
    Does active hyperemia decrease arterial pressure?
    8 years ago
  • amina afwerki
    What is local control in the body?
    8 years ago
  • Alberic Proudfoot
    What chemical factors that increase in the blood during increase metabolic activity?
    8 years ago
  • Doroteo
    Why does eicosanoids increase in blood during metabolic activity?
    8 years ago
  • SHUKORNIA
    Is active hyperaemia autoregulation?
    8 years ago
  • meghan
    What are local controls in the body?
    8 years ago
  • ADELARD
    What are the chemical factors that increase in the blood during increased metabolic activity?
    8 years ago

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