Chapter 20: The Circulatory System - Blood Vessels and Circulation

Comprehensive vocabulary flashcards covering the structural anatomy of blood vessels, hemodynamics, capillary exchange, regulatory reflexes, venous return mechanisms, circulatory shock, and systemic vascular circuits.

Tunica Interna (Tunica Intima)

The innermost layer of a blood vessel wall that lines the lumen, composed of endothelium over a basement membrane and internal elastic membrane, acting as a selectively permeable barrier to materials entering or leaving the bloodstream.

Tunica Media

The middle and usually thickest layer of a blood vessel wall, composed of smooth muscle, collagen, and elastic fibers, responsible for vasomotion (vasoconstriction and vasodilation).

Tunica Externa (Tunica Adventitia)

The outermost layer of a blood vessel wall consisting of loose connective tissue that anchors the vessel to adjacent tissues and provides passage for small nerves, lymphatic vessels, and smaller blood vessels.

Conducting (Elastic) Arteries

The largest arteries (such as the aorta, common carotid, subclavian, pulmonary trunk, and common iliac arteries) that expand during ventricular systole to absorb blood pressure and recoil during diastole to maintain downstream blood flow.

Distributing (Muscular) Arteries

Medium-sized arteries, such as the brachial, femoral, renal, and splenic arteries, that deliver blood to specific organs and have thick layers of smooth muscle constituting most of the wall.

Resistance (Small) Arteries

Small arterial vessels, including arterioles, that are too variable in number and location to have individual names; they control the relative amount of blood directed to various organs.

Arterioles

The smallest resistance arteries that deliver blood to capillary beds; they are the primary point of control over peripheral resistance and blood flow, producing half of total peripheral resistance.

Metarterioles

Short transitional vessels that link arterioles directly to capillaries or provide a thoroughfare channel to a venule, featuring individual muscle cells spaced apart forming precapillary sphincters.

Precapillary Sphincters

Smooth muscle cuffs encircling the entrance to a capillary; constriction reduces or shuts off flow through the capillary bed, leaving approximately three-fourths of the body's capillaries shut down at any given time.

Aneurysm

A weak point in the wall of an artery or the heart that forms a thin-walled, bulging sac which pulsates with each heartbeat and may rupture, most commonly occurring in the abdominal aorta, renal arteries, and arterial circle at the base of the brain.

Dissecting Aneurysm

A specific type of aneurysm in which blood pools between the tunics of an arterial wall and separates them, typically resulting from degeneration of the tunica media.

Carotid Sinuses

Baroreceptors located in the walls of the internal carotid artery that monitor blood pressure and signal the brainstem via the glossopharyngeal nerve to decrease heart rate and promote vasodilation in response to elevated pressure.

Carotid Bodies

Oval chemoreceptor structures located near the bifurcation of the common carotid arteries that monitor blood chemistry ($\text{pH}$, $\text{CO}_2$, and $\text{O}_2$) and transmit signals to respiratory centers in the brainstem to adjust ventilation.

Aortic Bodies

One to three chemoreceptors located in the wall of the aortic arch that monitor blood chemistry and transmit regulatory signals to brainstem respiratory centers.

Continuous Capillaries

Capillaries found in most tissues whose endothelial cells are joined by tight junctions to create a continuous tube, separated by narrow intercellular clefts that allow the passage of small solutes such as glucose.

Pericytes

Elongated contractile cells that wrap around continuous capillaries, containing the same contractile proteins as muscle to help regulate blood flow and vessel permeability.

Fenestrated Capillaries

Capillaries found in organs engaged in rapid absorption or filtration (such as the kidneys and small intestine) featuring endothelial cells riddled with filtration pores (fenestrations).

Sinusoids (Discontinuous Capillaries)

Irregular, blood-filled spaces in the liver, bone marrow, and spleen with wide gaps between endothelial cells that allow large proteins, clotting factors, and whole blood cells to enter or exit circulation.

Thoroughfare Channel

A central vessel continuing from a metarteriole that runs straight through a capillary bed to empty directly into a venule.

Postcapillary Venules

The smallest veins, measuring up to $1\,\text{mm}$ in diameter, that are even more porous than capillaries and serve as the primary site where leukocytes emigrate from the bloodstream into surrounding tissues.

Venous Valves

Infoldings of the tunica interna found in medium-sized veins that ensure unidirectional blood flow toward the heart and prevent retrograde pooling under the force of gravity.

Venous Sinuses

Veins characterized by especially thin walls, large lumens, and a complete absence of smooth muscle, rendering them incapable of vasomotion (such as coronary and dural venous sinuses).

Varicose Veins

Distended, tortuous superficial veins that develop when pooled blood stretches vessel walls and pulls the cusps of venous valves apart, allowing progressive backflow and stretching.

Portal System

A specialized circulatory route in which blood passes through two consecutive capillary networks before returning to the heart, occurring between the hypothalamus and anterior pituitary, in the kidneys, and between the intestines and liver.

Arteriovenous Anastomosis (Shunt)

A vascular connection through which arterial blood flows directly into a vein without passing through a capillary bed.

Arterial Anastomosis

The convergence of two arteries that provides collateral (alternative) routes of blood supply to a tissue, common in coronary circulation and around movable joints.

Blood Flow

The volume of blood flowing through a vessel, tissue, or organ in a given period of time, typically quantified in units of $\text{mL/min}$.

Perfusion

The flow of blood per given volume or mass of tissue over a specific time, expressed in $\text{mL/min/g}$.

Systolic Pressure

The peak arterial blood pressure measured during ventricular contraction (ventricular systole).

Diastolic Pressure

The minimum arterial blood pressure recorded during ventricular relaxation (ventricular diastole) between heartbeats.

Peripheral Resistance

The opposition to blood flow encountered in vessels away from the heart, governed primarily by three variables: blood viscosity, vessel length, and vessel radius.

Laminar Flow

The normal concentric-layer movement of blood through a smooth vessel, where fluid moves at its fastest velocity in the center and slowest near the vessel wall due to friction.

Autoregulation

The intrinsic ability of tissues to adjust their own local blood flow according to metabolic needs via the accumulation of metabolic wastes that induce vasodilation.

Reactive Hyperemia

A pronounced, transient increase in blood flow above normal resting levels that occurs immediately after the blood supply to a tissue has been temporarily occluded and then restored.

Angiogenesis

The growth of new blood vessels from pre-existing vasculature, stimulated by growth factors in exercised muscle, malignant tumors, regrowth of the uterine lining, and around coronary arterial obstructions.

Baroreflex

An autonomic, negative feedback reflex initiated by stretch-sensitive baroreceptors in the carotid sinuses that lowers blood pressure by inhibiting sympathetic tone and stimulating vagal cardioinhibitory neurons.

Medullary Ischemic Reflex

An emergency autonomic response to inadequate brainstem perfusion in which the medulla oblongata triggers widespread sympathetic vasoconstriction and accelerates heart rate to restore cerebral blood flow.

Venoconstriction

The sympathetic nerve-induced contraction of smooth muscle fibers in medium-sized veins, reducing venous volume and shifting blood into the arterial system to sustain arterial pressure during volume loss.

Angiotensin II

A potent circulating vasoconstrictor peptide that elevates blood pressure and stimulates renal sodium and water retention to increase total blood volume.

Atrial Natriuretic Peptide (ANP)

A hormone released by cardiac muscle cells in response to stretch that promotes urinary sodium excretion, induces generalized vasodilation, and lowers blood volume and pressure.

Venous Return

The flow of blood returning to the heart from the venous system, driven by a pressure gradient, gravity, the skeletal muscle pump, the thoracic (respiratory) pump, and cardiac suction.

Skeletal Muscle Pump

The mechanism by which contracting skeletal muscles compress embedded veins in the limbs, forcing blood through one-way venous valves toward the heart.

Thoracic (Respiratory) Pump

A venous return mechanism in which inhalation lowers intrathoracic pressure to $2\,\text{mm}\,\text{Hg}$ while raising abdominal pressure, squeezing the inferior vena cava and drawing venous blood toward the thoracic cavity.

Edema

The abnormal accumulation of excess interstitial fluid in a tissue, occurring when capillary filtration rate exceeds the rate of lymphatic drainage and capillary reabsorption.

Circulatory Shock

Any physiological state in which cardiac output is inadequate to satisfy the metabolic and oxygenation demands of the body's tissues.

Cardiogenic Shock

A category of circulatory shock resulting directly from the failure of the heart to pump blood effectively, most commonly caused by myocardial infarction.

Hypovolemic Shock

The most common form of low venous return shock, produced by a critical reduction in blood volume due to hemorrhage, extensive burns, severe dehydration, or trauma.

Neurogenic Shock

A type of venous pooling shock characterized by a sudden, systemic loss of sympathetic vasomotor tone, leading to widespread vasodilation from severe emotional trauma or brainstem damage.

Septic Shock

A distributive shock state triggered by bacterial toxins circulating in the bloodstream that cause widespread vasodilation and pathologically increased capillary permeability.

Anaphylactic Shock

A severe, systemic allergic reaction triggered by antigen exposure that induces massive basophil and mast cell histamine release, generalized vasodilation, and widespread vascular permeability.

Compensated Shock

An early stage of circulatory shock in which homeostatic negative feedback mechanisms (such as baroreflex-mediated tachycardia and vasoconstriction) successfully restore adequate blood pressure.

Decompensated Shock

A late, life-threatening stage of circulatory shock that develops when compensation mechanisms fail, activating destructive positive feedback loops that worsen cardiac and cerebral ischemia.

Transient Ischemic Attacks (TIAs)

Brief, temporary episodes of cerebral ischemia caused by arterial spasms that result in dizziness, weakness, loss of vision, or aphasia, commonly serving as warning signs of impending stroke.

Stroke (Cerebrovascular Accident, CVA)

The sudden infarction and death of brain tissue caused by prolonged ischemia, typically arising from atherosclerosis, thrombosis, or a ruptured aneurysm.

Aortic Arch

The curved segment of the aorta that gives rise to three major arterial branches supplying the head, neck, and upper limbs: the brachiocephalic trunk, left common carotid artery, and left subclavian artery.

Circle of Willis (Cerebral Arterial Circle)

An arterial anastomosis situated on the base of the brain encircling the pituitary gland, formed by branches of the basilar and internal carotid arteries to supply blood to the brain and orbital structures.

Great Saphenous Vein

A prominent superficial vein ascending the medial aspect of the leg and thigh that is frequently harvested for use in coronary artery bypass graft surgeries.