Peripheral Circulation and the Microcirculation

Comprehensive vocabulary flashcards covering the structural components, physics, local autoregulation, neural and endocrine control, and microcirculatory exchange mechanisms of the peripheral vasculature.

Resistance Vessels

Small arteries and arterioles that provide the greatest resistance to blood flow, maintain an underlying basal vascular tone, and are critical for the maintenance of arterial blood pressure.

Autoregulation

The intrinsic ability of a vascular bed to maintain a relatively constant blood flow despite fluctuations in perfusion pressure (arterial blood pressure) by adjusting vascular resistance ($Q = \frac{\Delta P}{R}$).

Myogenic Mechanism

An intrinsic vascular response where vascular smooth muscle contracts when experiencing an increase in transmural pressure and relaxes when transmural pressure decreases, acting as a protective mechanism against excessive wall stress.

Law of Laplace (Thin-Walled Vessels)

The relationship governing vessel wall tension defined as $T = P \times r$, where $T$ is wall tension, $P$ is intramural or transmural pressure, and $r$ is vessel radius.

Law of Laplace (Thick-Walled Vessels)

The modified relationship for thicker vascular structures (such as the aorta and cardiac ventricles) expressed as $\sigma = \frac{P \times r}{w}$, where $\sigma$ is wall stress, $P$ is pressure, $r$ is radius, and $w$ is wall thickness.

Reactive Hyperemia

An above-normal increase in blood flow to a vascular bed following a brief period of arterial occlusion, followed by a gradual return to baseline flow.

Capacitance Vessels

Veins that store large volumes of blood and constrict under sympathetic stimulation (e.g., during exercise) to redistribute blood back toward the heart.

Baroreceptors

Stretch receptors responsible for short-term control of blood pressure that respond to elevated pressure by inducing reflexive vasodilation and lowering heart rate, thereby decreasing vascular tone and cardiac output ($Q$).

Chemoreceptors (Vascular Regulation)

Sensory receptors that trigger systemic vasoconstriction in response to hypoxia (decreased $O_2$), hypercapnia (increased $CO_2$), and elevated $H^+$ concentration when arterial blood pressure drops significantly.

Renin-Angiotensin-Aldosterone System (RAAS)

An extrinsic endocrine pathway activated by a drop in renal blood flow or blood pressure that orchestrates fluid balance and long-term arterial blood pressure control.

Angiotensin II

A potent vasoconstrictor produced when angiotensin-converting enzyme (ACE) cleaves Angiotensin I; it acts on Angiotensin II type 1 receptors to elevate blood pressure, decrease glomerular filtration rate, stimulate thirst, and trigger aldosterone and ADH release.

Aldosterone

A steroid hormone synthesized by the adrenal glands atop the kidneys that promotes sodium ($Na^+$) and water reabsorption in distal renal tubules to expand blood volume and increase blood pressure.

Antidiuretic Hormone (ADH)

A peptide hormone synthesized in the hypothalamus and secreted by the posterior pituitary that promotes water reabsorption by inserting aquaporins into nephron collecting duct cells and increases sodium reabsorption in the renal medulla.

Atrial Natriuretic Peptide (ANP)

A cardiac hormone produced by atrial myocytes that acts antagonistically to the angiotensin pathway, reducing blood volume and blood pressure by increasing glomerular filtration rate, reducing nephron $Na^+$ reabsorption, and inhibiting renin, aldosterone, and ADH release.

Alpha Adrenergic Receptors

Vascular smooth muscle receptors primarily activated by sympathetic stimulation via norepinephrine and epinephrine to cause vasoconstriction.

Beta-2 Adrenergic Receptors

Vascular receptors that mediate smooth muscle relaxation and vasodilation in response to adrenergic stimulation.

Dual Control of Peripheral Circulation

The complementary regulation of peripheral blood flow by extrinsic neural/hormonal mechanisms and local tissue-derived metabolic/intrinsic mechanisms, where intrinsic control dominates in the brain and heart, and extrinsic control dominates in the skin.

Microcirculation

The terminal vascular network consisting of arterioles, metarterioles, capillaries, arteriovenous (AV) shunts, and venules organized to facilitate tissue fluid and nutrient exchange.

Nutritional Flow

The delivery of oxygen ($O_2$) and essential nutrients to parenchymal tissue cells via capillaries and the concomitant removal of metabolic wastes for excretion by the kidneys and lungs.

Arterioles ("Stopcocks" of Circulation)

Small precapillary resistance vessels that dynamically constrict or dilate to regulate downstream perfusion, rendering capillary blood flow non-uniform.

Arteriovenous (AV) Shunt

A direct microvascular anastomosis connecting an arteriole directly to a venule, allowing blood to bypass intermediate capillary beds.

Prostacyclin ($\text{PGI}_2$)

An endothelial metabolite derived from arachidonic acid via cyclooxygenase and $\text{PGI}_2$ synthase that elevates smooth muscle cAMP to induce vasodilation and inhibit platelet aggregation.

Nitric Oxide (NO)

An endothelium-derived relaxing factor (EDRF) synthesized from L-arginine that diffuses to adjacent vascular smooth muscle, activating guanylyl cyclase to form cGMP and induce vasodilation.

Endothelin

An endothelium-derived peptide that functions as a potent local vasoconstrictor.

Capillary Hydrostatic Pressure

The physical blood pressure within the capillary lumen that serves as the primary force promoting filtration of water and small solutes outward into the interstitial space.

Colloid Oncotic Pressure

The osmotic force generated by impermeable plasma proteins (principally albumin) within capillaries that restricts fluid extravasation and promotes absorption into the vascular compartment.