Unit Four: The Circulation

Chapter 17: Local and Humoral Control of Tissue Blood Flow

Guyton and Hall, Textbook of Medical Physiology, 12 edition

Local Control in Response to Tissue Needs

1. Delivery of oxygen to the tissues

2. Delivery of other nutrients, such as glucose, amino acids, and fatty acids

3. Removal of carbon dioxide from the tissues

4. Removal of hydrogen ions from the tissues

5. Maintenance of proper concentrations of other ions in the tissues

6. Transport of various hormones and other substances to the different tissues

Variations in Blood Flow in Different Tissues and OrgansTissue or

Organ% of Cardiac

Outputml/min Ml/min per

100 grams tissue wt.

Brain 14 700 50

Heart 4 200 7

Bronchi 2 100 25

Kidneys 22 1100 360

Liver 27 1350 95

Muscle 15 750 4

Bone 5 250 3

Skin 6 300 3

Thyroid gland 1 50 160

Adrenal glands

0.5 25 300

Other tissues 3.5 175 1.3

Total 100.0 5000Table 17.1 Blood flow to different organs and tissues under basal conditions

Mechanism of Blood Flow Control

• Acute Control- achieved by rapid changes in local vasodilation or vasoconstriction of the arterioles,metarterioles, precapillary sphincters; occurs inseconds to minutes

• Long Term Control- slow, controlled changes thatoccur over a period of days, weeks, or even months; due to an increase or decrease in the physical sizes or number of blood vessels supplying the tissues

Blood Flow Control (cont.)

• Acute Control

a. Effect of tissue metabolism on local blood flow

Fig. 17.1 Effect of increasing rate of metabolism on tissue blood flow

Blood Flow Control (cont.)

• Acute Control

b. Regulation when oxygen availability changes

Fig. 17.2 Effect of decreasing arterial oxygen saturation on blood flow

Blood Flow Control (cont.)

• Acute Control

c. Two theories for when either (a) or (b) occurs

1. Vasodilator theory

2. Oxygen lack theory

Blood Flow Control (cont.)

• Vasodilator Theory

a. The greater the rate of metabolism or the less oxygenavailable, the greater the rate of formation of vasodilators

b. Examples: adenosine, carbon dioxide, adenosine phosphate, histamine, potassium ions, hydrogen ions

Blood Flow Control (cont.)

• Oxygen Lack Theory

a. In the absence of oxygen or other nutrients, the bloodvessels simply relax and therefore naturally dilate

Fig. 17.3 Diagram of a tissue unit area for explanation of acute local control of blood flow.

Blood Flow Control (cont.)

• Special Examples of Acute “Metabolic” Control

a. Reactive hyperemia: increase flow after a temporaryblock

b. Active hyperemia: increase flow due to activity

Blood Flow Control (cont.)

• Autoregulation

a. Metabolic theory

b. Myogenic theory

Fig. 17.4 Effect of different levels of arterial pressure on blood flow through a muscle.

Blood Flow Control (cont.)

• Special Mechanisms

a. Kidneys: tubuloglomerular feedback

b. Brain: concentrations of carbon dioxide and hydrogen ions

c. Skin: closely linked to the regulation of body temperature

Blood Flow Control (cont.)

• Endothelial-Derived Relaxing or Constricting Factors

a. Nitric oxide-vasodilator from healthy endothelial cells

b. Endothelin-vasoconstrictor from damaged endothelial cells

Blood Flow Control (cont.)

• Long Term Regulation

a. Changes in tissue vascularity (i.e. angiogenesis)

b. Role of oxygen

c. Role of vascular endothelial growth factor

• Vascularity is Determined by Maximum Blood Flow, Notby Average Need

• Development of Collateral Circulation

Humoral Control of the Circulation

• Vasoconstrictor Agents

a. Norepinephrine and epinephrine

b. Angiotensin II

c. Vasopressin

Humoral Control of the Circulation

• Vasodilator Agents

a. Bradykinin

b. Histamine

Humoral Control of the Circulation

• Vascular Control by Ions and Other Chemical Factors

a. Vasoconstriction: increase in Ca ion concentration, carbon dioxide concentration increase in the brain, slightdecrease in H ions

b. Vasodilation: increases in K ion, Mg ion concentrations,anions (acetate and citrate), H ions on arterioles