Normal distribution of blood flow to the tissues during the cardiac cycle

LOCAL CONTROL OF BLOOD FLOW

1) BY THE TISSUES:

Anatomy

• Metarterioles & precapillary sphincters are poorly innervated; contraction is controlled by local metabolites

a) SHORT TERM [minutes]

• Here the flow is controlled according to the tissue’s need for flow

• Include needs for nutrition: oxygen, fatty acids, glucose, amino acids etc

• Also local flow include special needs: heat loss in skin, removal of wastes by kidney,

• Local blood flow can be increased to increased CO₂, increased H⁺, decreased O₂, plus other metabolites —> this is graphically demonstrated during exercise

• During rest: note the high blood flows via the liver (abdomen flow), brain & kidneys despite their small % contribution to total body mass. Yet even during rest, the metabolic needs of these organs are great to maintain their functions.

Role of Oxygen [& other nutrients] in local blood flow control

• reduced availability of O₂ to tissues —> increased blood flow to tissues

• Due to 1) Production of a vasodilator substance [?adenosine, H⁺, CO₂, histamine]—> acts on arterioles, metarterioles, precapillary sphincters

&/or 2) Increased O2 demands in tissue (& other nutrient demands) —> reduced local concentrations of these substances —> dilatation of blood vessels [as blood vessel muscle tone is also dependent on these substances]

• High BP —> excessive local blood flow —> flush out vasodilator substances &/or provide too many local nutrients —> constriction of blood vessels

• Low BP —> low local blood flow —> ischaemia —> increased vasodilator substances &/or provide too few local nutrients —> dilatation of blood vessels

reactive hyperemia

• Cease blood flow to tissue —> resume blood flow —> local hyperemia

active hyperemia

• Increase activity in tissue (eg muscle during exercise, brain during increased mental activity) —> increased rate of local blood flow

autoregulation in kidney

• blood flow remains relatively constant despite changes in BP due to action of juxtaglomerular apparatus operating via Na⁺

autoregulation in brain

• local regulation of blood flow is based on [CO₂] & [H⁺]

• increased [CO₂] & [H⁺] —> increased local blood flow

• neuronal activity is very sensitive to changes in [CO₂] & [H⁺]

b) LONG TERM [hours, days, weeks]

• decreased blood flow —> decreased tissue O2 —>

i) dilatation of blood vessels —> increased tension in vascular wall —> increased stretch in vessel wall —> permanent enlargement of vessel & outgrowth of new vessels

ii) increased blood flow via collateral blood vessels

—> The vascularity of most tissues is directly proportional to the local metabolism. If there are changes to this proportion, long term regulation automatically readjust the degree of vascularity over weeks to months.

2) BY THE AUTONOMIC NERVOUS SYSTEM

• The nervous system can function extremely rapidly

• It provides a means for controlling large parts of the circulation simultaneously often overriding local blood flow mechanisms

Anatomy

• Arteries, veins, arterioles & (to a lesser extent) the venules are supplied by extensive innervation from the sympathetic nervous system —> contraction influenced by CNS_{_}

[the parasympathetic nervous system only influences the blood flow via the heart]

• Innervation of small arteries & arterioles —> increase resistance

• Innervation of large arteries & veins —> changes in distribution of blood volume

vasomotor tone

• vasomotor centre (in medulla) innervates blood vessels via sympathetic system. Maintains a constant output —> maintain blood vessels in a partial state of contraction: Vasomotor tone

3) BY HUMORAL REGULATION

[regulation by hormones & ions in body fluids}

vasoconstrictors

1)Ad & NAd Sympathetic nerve endings release NAd, adrenals release NAd & Ad. Note that Ad causes mild vasodilation in skeletal & cardiac muscle

2)Angiotensin Reduced BP/Na+ —> release of renin from kidney —> Angiotensin I —> Angiotensin II. A potent vasoconstrictor

3)Vasopressin (ADH) formed in hypothalamus & secreted by posterior pituitary. Increased release during circulatory shock

4) Increased Ca²⁺ —> direct effect to stimulate smooth muscle contraction

5) Decreased osmolality

6) Increased CO₂ —> Vasomotor centre: sympathetic vasoconstriction

vasodilators

1) Bradykinin Small polypeptide derived from alpha globulins by action of enzyme kallikrein found in blood & tissue fluids. See increased kallikrien 2° blood trauma, dilution, contact with foreign substances. Both bradykinin & Kallikrein have very short half lifes. Bradykinins also increase capillary permeability.

2) Serotonin Found in high concentrations in platelets & certain tissues of gut. Can also have a vasoconstrictor effect depending on the area of the circulation

3) Histamine Released by almost all damaged tissues especially mast cells & eosinophiles. Also increase capillary permeability

4) Prostaglandins released by all tissues. Some types cause vasoconstriction.

5) Increased K⁺ —> inhibit smooth muscle

6) Increased Mg²⁺ —> inhibit smooth muscle

7) Increased Na⁺ /glucose —> increased osmolality —> arteriolar dilatation

8) Increased H⁺ —> arteriolar dilatation

9) Increased CO₂ —> Local: arteriolar dilatation; 2) vasomotor centre: sympathetic vasoconstriction

K. C. Potger
Copyright © 2001