Microcirculation and Local Blood Flow Regulation

Comprehensive vocabulary flashcards covering microcirculation structure, substance exchange dynamics, local blood flow regulation mechanisms, endothelial factors, and humoral controls based on the lecture transcript.

Microcirculation

The network of the smallest blood vessels within organ tissues, responsible for the exchange of substances between the blood and the tissues.

Terminal arterioles

The smallest arteries within the microcirculation that lead to the metarterioles.

Metarterioles

Microvessels that connect terminal arterioles to capillaries.

Capillaries

Microvessels that serve as the primary sites for the exchange of oxygen, nutrients, and waste products between blood and tissues.

Venules

Small veins that collect blood from capillaries.

Pericytes

Contractile cells surrounding many microvessels that can influence vessel diameter and contribute to the blood-brain barrier.

Macrocirculation

The larger blood vessels involved in the circulation of blood to and from organs.

Capillary bed

A network of capillaries within a tissue.

Vascular shunts

Direct arteriovenous (AV) connections that allow blood to bypass capillaries and skip the exchange of gases, nutrients, or wastes.

Precapillary sphincters

Rings of smooth muscle at the origin of true capillaries from metarterioles that control blood flow into the capillaries.

Vasomotion

The regular contraction and relaxation of the smooth muscle in metarterioles and precapillary sphincters, causing intermittent blood flow in capillaries.

Fenestrated capillaries

Capillaries with endothelial cells containing gaps (fenestrations) that significantly increase water conductivity, found in structures like the glomerulus and intestinal mucosa.

Diffusion (in microcirculation)

The primary mechanism for net movement of gases, nutrients, and wastes down concentration gradients, involving huge bidirectional volumes of water ($80,000\,\text{liters/day}$) with normally zero net water movement.

Ultrafiltration (Filtration)

The net movement of fluid driven by an imbalance of hydrostatic and oncotic pressures (Starling forces), producing a whole-body net outward flux of about $2\,\text{liters/day}$ returned as lymph.

Fick's Law of Diffusion

A physical law stating that the rate of diffusion (flux) across a membrane is proportional to the permeability constant ($k$), surface area ($A$), and concentration difference ($C_2 - C_1$), and inversely proportional to thickness ($t$).

Acute control of local blood flow

Rapid changes occurring within seconds to minutes in local vasodilation or vasoconstriction of arterioles, metarterioles, and precapillary sphincters.

Long-term control of local blood flow

Slow adjustments occurring over days to months that regulate blood flow through changes in the physical size and number of blood vessels supplying a tissue.

Vasodilator Theory

A theory proposing that increased metabolism or decreased oxygen/nutrient availability causes tissue cells to form vasodilator substances (such as adenosine, $CO_2$, $K^+$) that diffuse to local microvessels causing dilation.

Adenosine

A key local vasodilator substance released from active or oxygen-deprived cells, such as heart muscle.

Oxygen Lack Theory

A theory proposing that a lack of adequate oxygen leads directly to smooth muscle relaxation and vessel dilation because oxygen and nutrients are required for vascular smooth muscle contraction.

Reactive hyperemia

A significant increase in blood flow ($4$ to $7$ times normal) that occurs after a temporary blockage of blood supply is released to repay the accumulated oxygen deficit.

Active hyperemia

An increase in local blood flow that occurs when a tissue becomes highly active (such as exercising muscle) to meet elevated metabolic demands.

Autoregulation

The intrinsic ability of most tissues to maintain a relatively constant blood flow despite changes in arterial pressure.

Metabolic Theory of Autoregulation

The concept that high arterial pressure washes out local vasodilators causing vasoconstriction, whereas low pressure leads to vasodilator accumulation causing vasodilation.

Myogenic Theory of Autoregulation

The concept that sudden stretch of small blood vessels due to increased pressure causes vascular smooth muscle to contract, whereas reduced stretch causes relaxation.

Tubuloglomerular feedback

A specialized renal autoregulatory mechanism where the macula densa senses fluid composition in the distal tubule and signals the afferent arteriole to regulate blood flow and glomerular filtration rate.

Nitric Oxide (NO)

A potent vasodilator synthesized from L-arginine and oxygen by eNOS in endothelial cells, acting locally via cGMP to relax vascular smooth muscle.

Endothelin

A potent vasoconstrictor peptide released from damaged endothelial cells that helps prevent excessive bleeding.

Angiogenesis

The formation of new blood vessels in response to prolonged increases in tissue metabolism and nutrient demand.

Vascular Endothelial Growth Factor (VEGF)

A key angiogenic factor that stimulates new blood vessel growth by promoting endothelial cell proliferation and tube formation.

Collateral circulation

The enlargement and growth of pre-existing small vascular loops to reroute blood flow around a blocked artery or vein.

Humoral control

The regulation of circulation by hormones and locally produced chemical factors secreted into body fluids.

Angiotensin II

A powerful vasoconstrictor hormone that increases total peripheral resistance and arterial pressure.

Vasopressin (Antidiuretic Hormone - ADH)

A very potent vasoconstrictor that raises arterial pressure during severe hemorrhage and promotes water reabsorption in the kidneys.

Bradykinin

A powerful vasodilator agent that increases capillary permeability and helps regulate blood flow in inflamed tissues, skin, and glands.

Histamine

A vasodilator substance released during inflammation and allergic reactions that increases capillary permeability, contributing to edema.