Normal Variations of the Cardiovascular System Notes

NOTES NOTES NORMAL VARIATIONS ▪ Physiological adaptations within cardiovascular system in response to changes such as hemorrhage, exercise, postural changes CARDIOVASCULAR CHANGES DURING EXERCISE osms.it/cardiovascular-changes-exercise ▪ Involves central nervous system (CNS), local mechanisms ▫ CNS responses: changes in autonomic nervous system (ANS) due to inputs from cerebral motor cortex ▫ Local responses: exercise causes ↑ blood flow, O2 delivery to skeletal muscles ▪ Exercise results in ↑ sympathetic (ß1 receptors), ↓ parasympathetic activity to heart → ↑ cardiac output due to ↑ heart rate + ↑ stroke volume ▪ Muscle changes also occur ▫ ↑ metabolites (lactate, potassium, adenosine) are produced → metabolites stimulate local vasodilation → ↑ blood flow → ↓ overall total peripheral resistance (TPR) OVERALL RESPONSE TO EXERCISE ▪ Central command: ↑ cardiac output (CO), vasoconstriction in some vascular beds (excludes exercising skeletal muscle, cerebral, coronary circulations) ▫ ↑ CO → ↑ heart rate, contractility ▫ ↑ contractility → ↑ stroke volume → ↑ pulse pressure ▫ ↑ CO due to ↑ venous return (sympathetic vein constriction, squeezing action of skeletal muscle on veins) 146 OSMOSIS.ORG

Chapter 20 Cardiovascular Physiology: Normal Variations of the Cardiovascular System Figure 20.1 Flowchart showing cardiovascular response to exercise. CARDIOVASCULAR CHANGES DURING HEMORRHAGE osms.it/cardiovascular-changes-hemorrhage ▪ Blood loss → ↓ arterial pressure → compensatory responses to restore arterial pressure ▫ Response mediated by baroreceptor reflex, renin-angiotensin-aldosterone system (RAAS), vascular actions Decrease in arterial pressure ▪ Hemorrhage → ↓ total blood volume → ↓ venous return to heart, ↓ right atrial pressure → ↓ cardiac output → ↓ Pa as a product of cardiac output, TPR Return of arterial pressure ▪ Baroreceptors in carotid sinus detect ↓ Pa → relay information to medulla via carotid sinus nerve → ↑ sympathetic outflow to heart, blood vessels; ↓ parasympathetic outflow to heart → ↑ heart rate, ↑ contractility, ↑ TPR, constriction of veins ▪ ↓ mean arterial pressure → ↓ perfusion to kidney → response via RAAS ▫ Kidney secretes renin from renal juxtaglomerular cells → ↑ angiotensin I production → converted to angiotensin II (causes arteriolar vasoconstriction, stimulates aldosterone secretion) ▪ Capillary changes favor fluid reabsorption ▫ ↑ sympathetic outflow to blood vessels, angiotensin II → arteriolar vasoconstriction → ↓ capillary hydrostatic pressure (Pc)→ restricts filtration out of capillaries, favors absorption OTHER RESPONSES IN HEMORRHAGE ▪ Hypoxemia (↓ arterial PO2): carotid, aortic bodies chemoreceptors sense ↓ PO2 → ↑ sympathetic outflow to blood vessels → ↑ vasoconstriction, TPR, Pa ▪ Cerebral ischemia: local ↑ PCO2 OSMOSIS.ORG 147

▪ ↓ blood volume → ↓ return of blood to heart → detection by atria volume receptors → ADH secretion to maintain adequate blood pressure → water reabsorption by renal collecting ducts → arteriolar vasoconstriction 148 OSMOSIS.ORG

Chapter 20 Cardiovascular Physiology: Normal Variations of the Cardiovascular System Figure 20.2 Flowchart showing cardiovascular responses to hemorrhage. CARDIOVASCULAR CHANGES DURING POSTURAL CHANGE osms.it/cardiovascular-changes-postural ▪ Standing up quickly → lightheadedness, sometimes fainting (due to delayed constriction of lower extremity blood vessels → orthostatic hypotension) ▫ ↓ in systolic blood pressure > 20mmHg/ diastolic blood pressure > 10mmHg within three minutes of standing ▪ Initiating event: pooling of blood in extremities ▫ Moving from supine to standing position: blood pools in veins of lower extremities → ↓ venous return to heart, ↓ cardiac output → ↓ mean arterial pressure ▫ Venous pooling → ↑ hydrostatic pressure in leg veins → ↑ fluid filtration into interstitial fluid, ↓ intravascular volume ▫ Severe ↓ blood pressure → syncope Response of baroreceptor reflex ▪ Responsible for homeostatic blood pressure maintenance ▪ Carotid sinus baroreceptors detect ↓ Pa → sends information to medullary vasomotor center → inactivates medulla vagal neurons, activates sympathetic neurons → ↑ sympathetic outflow to heart, blood vessels, ↓ parasympathetic outflow to heart to normalize Pa ▪ ↑ systemic vascular resistance, cardiac output act in negative feedback mechanism to maintain Pa OSMOSIS.ORG 149

150 OSMOSIS.ORG

Chapter 20 Cardiovascular Physiology: Normal Variations of the Cardiovascular System Figure 20.3 Flowchart showing cardiovascular response to postural change. OSMOSIS.ORG 151