Vasoconstrictor Agents

Norepinephrine and Epinephrine Norepinephrine is an especially powerful

vasoconstrictor hormone; epinephrine is less so When the sympathetic nervous system is

stimulated, the sympathetic nerve endings in the individual tissues release norepinephrine, which excites the heart and contracts the veins and arterioles

In addition, the sympathetic nerves to the adrenal medullae cause these glands to secrete both norepinephrine and epinephrine into the blood

Angiotensin II

Angiotensin II is powerful vasoconstrictor substance

The effect of angiotensin II is to constrict powerfully the small arterioles and decreases the blood flow

Angiotensin II increases the total peripheral resistance, thereby increasing the arterial pressure

Vasopressin

Vasopressin, also called antidiuretic hormone, one of the body's most potent vascular constrictor substances

The concentration of circulating blood vasopressin after severe hemorrhage can rise high enough to increase the arterial pressure

In many instances this canbring the arterial pressure almost back up to normal

Other chemical influences on blood flow

Histamine

Most of the histamine is derived from mast cells in the damaged tissues

Histamine has a powerful vasodilator

Has the ability to increase greatly capillary porosity, allowing leakage of both fluid and plasma protein into the tissues

Bradykinin

Bradykinin causes both powerful arteriolar dilation and increased capillary permeability

Bradykinin play special roles in regulating blood flow and capillary leakage of fluids in inflamed tissues

Vascular Control by Ions and Other Chemical Factors

1. An increase in calcium ion concentration causes vasoconstriction

2. An increase in potassium ion concentration causes vasodilation

3. An increase in magnesium ionconcentration causes powerful vasodilation

4. An increase in hydrogen ion concentration causes dilation of the arterioles

5. An increase in carbon dioxide concentrationcauses moderate vasodilation in most tissues, but marked vasodilation in the brain

Vascular Control by Ions and Other Chemical Factors

Pharmacologic Stimuli that Cause Contraction or Relaxation of Vascular Smooth Muscle

Vasoconstrictors Norepinephrine and

epinephrine Endothelin Angiotensin II Vasopressin Calcium

Vasodilator agents Histamine Adenosine Nitric Oxide (NO) Carbon Dioxide Potassium Magnesium Hydrogen Ion Prostaglandins Bradykinin

Distribution of Blood in the Body Organs

Flow rate in muscle

Skeletal muscle constitute 40 – 45% body weight

5 ml/min/100 g to 80-100 ml/min/100 g

Intermittent as a result of contraction of muscle

Exercise opens capillaries

Control of blood flow through the skeletal muscle

Local regulation- decrease oxygen in muscle greatly enhance flow

During muscle activity, muscle uses oxygen thus decreasing oxygen in the tissue fluid

Vasodilator – adenosine, potassium ions, carbon dioxide

Nervous control of muscle blood flow

Skeletal muscles are provided with sympathetic vasoconstrictor nerves

Sympathetic vasoconstrictor secrete norepinephrine in their nerve ending (important during shock and stress)

Epinephrine – secreted by adrenal medullae gives vasodilator effect during exercise

Total Body Circulatory Readjustments During Exercise

Three major effects occur during exercise

(1) mass discharge of the sympathetic nervous system throughout the body

(2) increase in arterial pressure

(3) increase in cardiac output

Effects of Mass Sympathetic Discharge

Three major circulatory effects result

1. The heart is stimulated to greatly increased heart rate and increased pumping strengthas a result of the sympathetic drive to the heart plus release of the heart from normal parasympathetic inhibition

Effects of Mass Sympathetic Discharge

2. Most of the arterioles of the peripheral circulation are strongly contracted, except for the arterioles in the active muscles, which are strongly vasodilated by the local vasodilator

Two of the peripheral circulatory systems, the coronary and cerebral systems, are spared this vasoconstrictor effect

3. The muscle walls of the veins are contracted powerfully, which greatly increases the mean systemic filling pressure

This is one of the most important factors in promoting increase in venous return of blood to the heart and, therefore, in increasing the cardiac output

Effects of Mass Sympathetic Discharge

Effect of Arterial Pressure Rise

Increases force to drive blood (by 30%)

Dilates vessels, decreasing resistance (can double flow rate)

Local vs Whole Body Exercise

Local (e.g. lifting weight): Mainly vasoconstriction – high increase in BP (up to 170 mm Hg).

Whole body (e.g. running): vasodilation in a large mass of muscles leads to more slight increase in BP (maybe 20-40 mm Hg).

Effects of Exercise on Muscle Circulation

Increased heart rate & pumping strength

Aterioles constricted in most of periphery (but not in coronary and cerebral systems).

Active muscle arterioles dilated

Vein muscle walls constricted (increased filling pressure, hence, increased venous return).

12-15

Normal Coronary Blood Flow

The resting coronary blood flow in the human being averages about 225 ml/min, which is about 4 to 5 per cent of the total cardiac output

Coronary circulation is the functional blood supply to the heart muscle itself

Collateral routes ensure blood delivery to heart even if major vessels are occluded

Coronary Circulation

Left coronary arterysupplies the anterior and left lateral of the left ventricle

Right coronary artery supplies most of the right ventricle and posterior part of the left ventricle in 80 to 90% of the people

Coronary Circulation: Venous Supply Most of the coronary venous

blood from the left ventricular muscle return to the right atrium by coronary sinus

From the right ventricular muscle return through small anterior cardiac veins

Very small amount of coronary venous blood flows back into the heart through thebesian veins

Phasic changes in coronary blood flow during systole and diastole

Right coronary blood flow

Left coronary blood flow