Regulatory Mechanisms

REGULATORY MECHANISMS OF CARDIOVASCULAR SYSTEM AIM The aim of regulatory mechanism of cardiovascular system is maintain blood flow to active tissues. To maintain blood flow to vital organs during emergencies such as bleeding. Local auto regulatory mechanisms Systemic regulatory mechanisms 1. Chemical 2. Neural --AUTONOMIC MEDULLARY • Chemoreceptors • Baroreceptors

LOCAL REGULATORY MECHANISMS Tissues of the vital organs of the body have capacity to regulate their own blood flow by Basal Myogenic Tone( BMT) Factors affecting BMT Vasodilator metabolites. Following factors cause vasodilatation 1.Acidosis – decrease pH 1. Hypoxia- decrease pO2 2. Hypercapnia- increase pCO2 3. Increased temperature 4. K+ - hyperkalemia 5. Adenosine 6. Lactic acidosis

ENDOTHELIAL CELLS Endothelial cells secretes following substances in blood z Prostaglandins z Thromboxanes z Nitric oxide NO z Endothelins z Growth factors Prostaglandins—inhibits platelet aggregation ----vasodilation Thromboxanes --- promotes platelet aggregation—vasoconstrictor

Nitric oxide NO Released by the endothelium of blood vessels- --- vasodilatation. Binds to guanylyl cyclase that converts GTP to cGMP ----- relaxation of smooth muscles. Actions 1. Relaxation of vascular smooth muscle. 3. Smooth muscle relaxation in GIT. 2. Important for cytotoxic activity of macrophages.

ENDOTHELINS Three forms Endothelins I, II,III. Endothelins I is most potent vasoconstrictor. Factors affecting Endothelins secretion Stimulating factors 1. Catecholamine 2. Hypoxia 3. Angiotensin II

4.

Stress

Inhibitory factors Nitric oxide ANP Prostaglandins E ACTIONS z z z z z z

Increase myocardial contractibility and heart rate. Contraction of vascular smooth muscle. Bronchoconstriction. Regulates transport of substances across blood brain barrier. Decrease GFR and RPF. Regulates GIT blood flow.

SYSTEMIC REGULATORY MECHANSISMS

CHEMICAL Vasodilator z Kinins z VIP z ANP ACTIONS z z z z z

Contraction of smooth muscles. Vasodilation of vascular smooth muscles. Bronchoconstriction. Increase vascular permeability. Promotes migration of leucocytes from blood to tissues.

Vasoconstrictors z Catecholamines

NEURAL REGULARTORY MECHANISMS Autonomic through Sympathetic and Parasympathetic system Cardiac innervation Sympathetic supply Causes z Chromotropic action -- increase Heart Rate z Ionotropic action ---increase Force of myocardial contraction z Dromotropic action--- increase Conduction z Bathmotropic action -- increase Excitability

IF CR BE DC

Parasympathetic system Via vagus nerves to SA node , AV node and atria. NOTE – Ventricles are not innervated by vagus nerve ACTIONS Negative Ionotropic action --

Decrease force of Myocardial contraction Negative Chromotropic action --- Decrease Heart Rate Negative Bathmotropic action -- Decrease Excitability Negative Dromotropic action --- Decrease Conduction Peripheral vascular innervation 1.Vasoconstriction 2. Vasodilatation

MEDULLARY REGULATION VASOMOTOR CENTRE VMC CARDIAC VAGAL CENTRE VASOMOTOR CENTRE -1. Venterolateral portion of medulla (also called Pressor area) exerts excitatory effects on sympathetic pre ganglionic neurons in the intermediolateral grey column of spinal cord through glutamate. ACTIONS 1. 2. 3. 4.

Increase in Heart Rate Increase in stroke volume Increase in Blood pressure Venoconstriction

2. Medial and caudal portion of fourth ventricle in medulla (also called depressor area ). Stimulation of neurons in this area decrease sympathetic activity. ACTIONS z Decrease in heart rate. z Decrease in stroke volume. z Decrease in blood pressure. z Venodilation. CARDIAC VAGAL CENTRE Inhibitory pathways descend from medulla from three different Nuclei 1. Nucleus of tractus solitarius 2. Nucleus of ambiguous. 3. Dorsal motor nuclei of the vagus Baroreceptors -carotid sinus ----------------Nucleus of tractus nd solitarius---2 order neurons----nucleus ambiguous (cardiac vagal centre)-------vagus to heart .

Vagal fibers exert inhibitory action on heart. REGULATION OF MEDULLARY CENTRES BARORECEPTORS Arterial Baroreceptors- carotid sinus and arch of aorta Carotid Baroreceptors Afferents from Baroreceptors end in NTS and the terminate on pressor area of VMC Applied –increased Baroreceptors discharge has inhibitory effect on pressor area of VMC and stimulatory effect on vagal centre

ARTERIAL BARORECEPTORS They respond to rise of pressure stretching the arterial wall. The afferents from these receptors terminate on vasomotor centre and cardiac vagal centre causing reflex inhibition of VMC and stimulation of CVC. APPLIED BP decrease ↓ Baroreceptor discharge decrease ↓ Decrease inhibition of VMC and decrease stimulation of CVC ↓ Reflex increase in sympathetic activity ↓ Leading to increase BP. Conversely true for increase BP leading to fall in sympathetic activity and decrease in BP INCREASE IN BP

causes INCREASED BARORECEPTORS DISCHARGE

Inhibits VMC

Excites CIS

Decrease BP Atrial stretch receptors Type A : Discharge primarily in atrial systole Type B : Discharge primarily in atrial diastole Bezold-Jarisch Reflex Receptors located in left ventricle when stimulated can cause apnea, hypotension and bradycardia

CHEMORECEPTORS Carotid bodies and aortic bodies

CHEMORECEPTORS Sensitive to changes in blood chemistry Aim is to maintain normal level of arterial pO2, pCO2, pH Carotid bodies located near common carotid artery bifurcation and innervated by carotid sinus nerve. Aortic bodies located around aortic arch and innervated by aortic nerve branch of vagus nerve.

Chemoreceptors get stimulated by • Hypoxia • Hypercapnia • Acidosis Afferents from Chemoreceptors terminate on NTS.

Stimulation of these chemoreceptor, stimulate VMC and respiratory neurons in medulla causing increase in rate and depth of respiration.

Multiple Choice Questions: 1. Amount of blood pumped by each ventricle per minute into the circulation is called a) Cardiac output b) Blood pressure c) Stroke volume d) Heart rate 2. According to Frank Starling Law a) Preload is directly proportional to End Diastolic Volume b) Preload is inversely proportional to End Diastolic Volume c) Preload is directly proportional to End Systolic Volume d) Afterload is directly proportional to End Diastolic Volume 3. Myocardial contractibility is increased by a) Increase in parasympathetic stimulation b) Hypoxia c) Increase in heart rate d) Acidosis 4. One of the following increase End Diastolic Volume a) Standing b) Increase in intrathoracic pressure c) Increase venous return d) Decrease venous return 5. One of the following increase cardiac output except a) Anxiety b) Tachycardia c) Walking d) Sleep

6. Chromotropic action of heart is a) Increased force of myocardial contraction b) Increased conduction c) Increased heart rate d) Increased excitability 7. Match the following a) Chromotropic action b) Ionotropic action c) Dromotropic action d) Bathmotropic action

I. Increased excitability II. Increased conduction III. Increased heart rate IV. Increased force of myocardial contraction

8. Role of Vasomotor centre is a) Increase heart rate b) Decrease heart rate c) Increase in parasympathetic stimulation d) Decrease in blood pressure 9. Vasomotor centres is located in a) Venterolateral portion of medulla b) Medial portion of medulla c) Caudal portion of medulla d) Pons 10. Sympathetic stimulation of heart has one of the following effect a) Decrease excitability of SA node b) Increase atrial contractions c) Increase AV conduction time d) Decrease heart rate 11. Baroreceptors respond to a) Hypoxia

b) Hypercapnia c) High blood pressure d) Acidosis