Regulation of Blood Flow and Regional Circulation

Comprehensive vocabulary flashcards covering intrinsic and extrinsic mechanisms of blood flow regulation, regional circulations (coronary, cerebral, cutaneous, pulmonary), reflexes (Cushing, Bainbridge), and cardiovascular hemodynamics during exercise.

Intrinsic Regulation (Autoregulation)

The local control of blood flow occurring entirely within an organ itself by altering arteriolar resistance through vascular smooth muscle, independent of nerves or circulating humoral factors.

Extrinsic Regulation

The control of blood flow and arteriolar diameter mediated by neural factors (such as sympathetic stimulation) and humoral agents originating from outside the organ.

Metabolic Theory of Autoregulation

The hypothesis stating that accumulation of local tissue metabolites (such as adenosine, lactic acid, $H^+$ ions, and hypoxia) during increased metabolic activity acts to relax arteriolar smooth muscle, inducing vasodilation and elevating blood flow.

Myogenic Theory of Autoregulation

An intrinsic mechanism wherein an increase in perfusion pressure stretches arteriolar vascular smooth muscle, inducing reflex contraction and decreasing luminal radius to elevate precapillary resistance and maintain constant blood flow.

Autoregulatory Range

The arterial pressure window (typically between $60$ and $160\,\text{mmHg}$ mean arterial pressure) across which blood flow to autoregulating organs remains relatively constant.

Extrinsic Vasoconstrictors

External stimuli and substances that induce arteriolar contraction, specifically sympathetic stimulation, norepinephrine acting on $\alpha$-adrenergic receptors, angiotensin II, and endothelin.

Extrinsic Vasodilators

External factors that induce arteriolar dilation, including withdrawal of sympathetic activity, epinephrine acting on $\beta$-adrenergic receptors, and atrial natriuretic peptide (ANP).

Biphasic Vascular Action of Epinephrine

The dual response where physiological concentrations of circulating epinephrine produce vasodilation via $\beta_2$-adrenergic receptors, whereas high circulating levels stimulate $\alpha_1$-adrenergic receptors to produce vasoconstriction.

Endothelial Nitric Oxide (NO) Pathway

A local vasodilator mechanism in which shear stress or acetylcholine stimulates endothelial nitric oxide synthase ($\text{eNOS}$) to convert L-arginine into NO, activating soluble guanylyl cyclase ($\text{GC}$) to increase $\text{cGMP}$ and cause vascular smooth muscle relaxation.

Resting Coronary Blood Flow

The baseline coronary perfusion rate, measuring approximately $250\,\text{ml/min}$ ($70\,\text{ml/100\,g/min}$), which accounts for approximately $5\%$ of resting cardiac output and can increase $3\text{--}4\text{ fold}$ during exercise.

Phasic Left Coronary Blood Flow

The cyclic variation in left coronary arterial perfusion characterized by severe reduction during systole due to strong intramyocardial mechanical compression, with the vast majority of perfusion occurring during ventricular diastole.

Phasic Right Coronary Blood Flow

Coronary perfusion of the right ventricle where modest intramyocardial mechanical compression permits significant blood flow during ventricular systole, though maximum flow still occurs during diastole.

Myocardial Oxygen Extraction

The near-maximal extraction of oxygen from blood by cardiac myocytes even at rest, resulting in extremely low coronary venous $PO_2$ and necessitating increased coronary blood flow to meet any increment in myocardial oxygen consumption.

Adenosine

The primary metabolic mediator responsible for coronary vasodilation in response to increased myocardial work or hypoxia.

Myocardial Ischemia

A clinical state that develops when coronary blood supply fails to meet myocardial metabolic oxygen demand, typically caused by structural narrowing of the coronary vessels.

Circle of Willis

The anastomotic arterial ring at the base of the brain formed by the internal carotid and vertebral-basilar systems, providing collateral circulatory channels for cerebral perfusion.

Cerebral Blood Flow Regulation

A tightly autoregulated circulation maintaining roughly $750\,\text{mL/min}$, primarily controlled under normal physiological conditions by arterial $PCO_2$ tension, where elevated $PCO_2$ increases cerebral flow and decreased $PCO_2$ decreases flow.

Hypoxic Cerebral Vasodilation

A pronounced compensatory increase in cerebral blood flow that is triggered specifically when arterial $PO_2$ falls below $60\,\text{mmHg}$, at which point low $PO_2$ supersedes $PCO_2$ as the main determinant of flow.

Cushing Reaction

A physiological reflex to increased intracranial pressure (ICP) wherein compression of cerebral vasculature decreases cerebral perfusion, accumulating $CO_2$ and lowering pH to trigger intense sympathetic discharge that raises systemic blood pressure.

Cushing Triad

The classic clinical sign of elevated intracranial pressure consisting of systemic hypertension, reflex vagal bradycardia, and irregular or depressed respiration caused by brainstem compression.

Cardiopulmonary Baroreceptors

Low-pressure stretch receptors located in veins, cardiac atria, and pulmonary arteries (the unstressed vascular volume) that sense blood volume fullness and regulate fluid balance via renal sodium and water excretion.

Bainbridge Reflex

A compensatory reflex elicited by increased blood volume and elevated venous and atrial pressures that stimulates cardiopulmonary baroreceptors to increase ANP, reduce ADH, and trigger a reflex increase in heart rate to prevent venous congestion.

Cutaneous Microcirculation Architecture

The vascular arrangement of the skin, comprising superficial capillary loops in the dermis, deep arteriovenous anastomoses, and an extensive venous plexus in the subcutaneous layer.

Cutaneous Circulation Flow Rates

Perfusion rates through the skin varying from a resting value of $10\text{--}15\,\text{ml/100\,g/min}$, down to $1\,\text{ml/100\,g/min}$ ($\frac{1}{10}\text{th}$ of rest) during cold exposure, and up to $150\,\text{ml/100\,g/min}$ ($10\times$ rest) during heat exposure.

Glomus Body (Arteriovenous Anastomosis)

A highly coiled, richly innervated vascular shunt connecting cutaneous arterioles directly to venules that alters diameter to divert blood toward or away from superficial capillary beds for thermoregulation.

Cutaneous Response to Heat Exposure

Hypothalamic-directed response where cutaneous arterioles dilate and shunt vessels (glomus bodies) narrow, channeling large volumes of warm blood into superficial dermal capillaries to maximize radiant heat dissipation.

Cutaneous Response to Cold Exposure

Sympathetic activation that constricts cutaneous arterioles and dilates shunt vessels (glomus bodies), diverting blood away from superficial capillaries directly into deep veins to conserve core heat.

Central Command (Cardiovascular Exercise Control)

Higher brain center signaling during exercise that increases sympathetic outflow and reduces parasympathetic outflow to increase heart rate, contractility, and cardiac output, while constricting splanchnic and renal arterioles and reducing venous unstressed volume.

Active Hyperemia in Exercising Muscle

The dominant local metabolic mechanism during exercise where accumulating vasodilator byproducts (adenosine, $CO_2$, ADP, lactate, $K^+$, and $H^+$) overcome sympathetic vasoconstrictor tone at arteriolar smooth muscle to dramatically increase muscular blood flow.

Vascular Conductance in Exercise

The measure of blood flow through a vascular bed at a given pressure gradient, defined mathematically as the inverse of resistance ($\text{Conductance} \propto \frac{1}{R}$); it increases significantly in exercising muscle due to local metabolic arteriolar dilation.

Ventricular P-V Loop in Severe Exercise

The altered left ventricular pressure-volume loop showing increased end-diastolic volume (increased preload), elevated peak systolic pressure, increased stroke volume, and decreased end-systolic volume.

Pulmonary Circuit Changes in Exercise

Cardiovascular adaptations comprising a large decrease in pulmonary vascular resistance, recruitment of previously unperfused capillaries, and an increased capillary surface area that matches ventilation with enhanced cardiac output.