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 Delivery of oxygen to the tissues Delivery of other nutrients, such as glucose, amino acids, and fatty acids Removal of carbon dioxide from the tissues Removal of hydrogen ions from the tissues Maintenance of proper concentrations of other ions in the tissues Transport of various hormones and other substances to the different tissues

Variations in Blood Flow in Different Tissues and Organs Tissue or Organ % of Cardiac Output ml/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 Bone 5 250 3 Skin 6 300 Thyroid gland 1 160 Adrenal glands 0.5 Other tissues 3.5 175 1.3 Total 100.0 5000 Table 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 in seconds to minutes Long Term Control- slow, controlled changes that occur 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 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 Two theories for when either (a) or (b) occurs Vasodilator theory Oxygen lack theory

Blood Flow Control (cont.) Vasodilator Theory The greater the rate of metabolism or the less oxygen available, the greater the rate of formation of vasodilators Examples: adenosine, carbon dioxide, adenosine phosphate, histamine, potassium ions, hydrogen ions

Blood Flow Control (cont.) Oxygen Lack Theory In the absence of oxygen or other nutrients, the blood vessels 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 Reactive hyperemia: increase flow after a temporary block b. Active hyperemia: increase flow due to activity

Blood Flow Control (cont.) Autoregulation Metabolic theory Myogenic theory Fig. 17.4 Effect of different levels of arterial pressure on blood flow through a muscle.

Blood Flow Control (cont.) Special Mechanisms Kidneys: tubuloglomerular feedback Brain: concentrations of carbon dioxide and hydrogen ions Skin: closely linked to the regulation of body temperature

Blood Flow Control (cont.) Endothelial-Derived Relaxing or Constricting Factors Nitric oxide-vasodilator from healthy endothelial cells Endothelin-vasoconstrictor from damaged endothelial cells

Blood Flow Control (cont.) Long Term Regulation Changes in tissue vascularity (i.e. angiogenesis) Role of oxygen Role of vascular endothelial growth factor Vascularity is Determined by Maximum Blood Flow, Not by Average Need Development of Collateral Circulation

Humoral Control of the Circulation Vasoconstrictor Agents Norepinephrine and epinephrine Angiotensin II Vasopressin

Humoral Control of the Circulation Vasodilator Agents Bradykinin Histamine

Humoral Control of the Circulation Vascular Control by Ions and Other Chemical Factors Vasoconstriction: increase in Ca ion concentration, carbon dioxide concentration increase in the brain, slight decrease in H ions Vasodilation: increases in K ion, Mg ion concentrations, anions (acetate and citrate), H ions on arterioles