Question

1. Explain the effects of pressure,resistance, and venous return on cardiac output?

2. Describe how cardiovasculr Regulatory Mechanisms respond to changes in blood pressure and blood chemistry? (This answer must include how the ANS, endocrine system, baroreceptors, chemoreceptors, all work wirh the cardiovascular system)

Answer 1

1. Factors affecting cardiac output:

a) Pressure: Pressure is caused due to myocardial contractility which depends on following:

• i) Increased ventricular muscle mass in those doing regular physical exercises increases cardiac output.
• ii) Sympathetic stimulation increases ventricular contractility and therefore cardiac output.
• iii) Hormones like catecholamines, acetylcholine, glucagon, insulin, thyroxine and chemicals like xanthines and drugs like digitalis, quinidine etc.

b) Resistance: Resistance is caused by the peripheral resistance induced by blood vessel and it is the after load. This resistance decreases the cardiac output. It mainly depends on two factors: vessel diameter and viscosity of blood.

c) Venous return: It is the amount of blood that returns to right atrium from systemic venous circulation. It depends on:

• i) Skeletal muscle pump in lower limbs which help to propel blood towards heart.
• ii) Thoracic pump, where venous return increases during inspiration.
• iii) Abdominal pump increases venous return by rising intra-abdominal pressure.
• iv) ECF volume especially plasma volume increased causes increase in cardiac output.
• v) Sympathetic stimulation causes venoconstriction which results in increased venous return.

2.Regulatory mechanisms of blood pressure:

Three mechanisms: Neural mechanism, Vascular mechanism and Hormonal mechanism.

Autonomic regulation: Mainly occurs by sympathetic control.

Sympathetic vasoconstrictor system:

Sympathetic stimulation increases blood pressure by vasoconstriction, venoconstriction, cardiac stimulation and increased secretion of catecholamines.

Sympathetic vasodilator system:

By sympathetic cholinergic fibers and by decrease in rate of discharge produces vasodilation.

Hormonal regulation:

• Stimulation of sympathetic fibers to adrenal medulla releases catecholamines.
• Renin-angiotensin aldosterone system helps in increasing blood pressure when there is a fall.
• Vasopressin increases blood pressure by increasing water reabsorption from kidney.
• Atrial natriuretic peptide (ANP) decreases blood pressure by producing diuresis and natriuresis.
• Other hormones include histamine,endothelins (endothelium derived relaxing factor) and adrenomedullin.

​​​​​​​Baroreceptor reflex:

• Increased blood pressure stretches carotid sinus which increases discharge in 9th and 10th cranial nerves and stimulates nucleus tractus solitareus which activates vagus nerve thus resulting in decrease heart rate and cardiac output.
• It also inhibits vasomotor centre and inhibits sympathetic outflow decreasing myocardial contractility and thus cardiac output, later vasodilation occurs decreasing blood pressure.
• Regulates blood pressure change within a range of 50-200mmHg.

​​​​​​​Chemoreceptor reflex:

• Occurs in conditions like hypoxia, hypercapnia, acidosis.
• The net effect is no change in heart rate or mild tachycardia and vasoconstriction that increases blood pressure.
• The range of regulation of blood pressure is 40-70 mmHg.

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