Blood Vessels and Blood Pressure: Physiology and Regulation

Blood Flow

Blood is transported to all parts of the body through a system of vessels, bringing fresh supplies to the vicinity of all cells while also removing their wastes.

Homeostasis

To maintain homeostasis, reconditioning organs receive blood flow in excess of their needs, like the digestive tract, kidneys, and skin.

Flow Rate

Volume of blood passing through per unit of time, directly proportional to the pressure gradient and inversely proportional to vascular resistance.

Flow Rate Equation

F = Delta P / R, where F is the flow rate of blood through a vessel, Delta P is the change in pressure gradient, and R is the resistance of blood vessels.

Pressure Gradient

The difference in pressure between the beginning and end in a vessel.

Resistance

Hindrance to blood flow through a vessel.

Viscosity

Friction developed between molecules of a fluid as they slide over each other during fluid flow.

Poiseuille's Law

Integrates factors affecting flow rate through a vessel.

Vascular Tree

The body's hierarchy consisting of arteries, arterioles, capillaries, venules, and veins.

Arteries

Arteries serve as rapid-transit passageways to the organs and as a pressure reservoir.

Systolic Pressure

Averages 120 mm Hg.

Diastolic Pressure

Averages 80 mm Hg.

Blood Pressure Measurement

Blood pressure can be measured indirectly by using a sphygmomanometer, which is an externally applied inflatable cuff attached to a pressure gauge.

Mean Arterial Pressure (MAP)

The main driving force for blood flow, representing the average pressure driving blood into the tissues throughout the cardiac cycle.

Arterioles

Major resistance vessels with a small radius that offers considerable resistance to flow.

Vasoconstriction

Narrowing of a vessel.

Vasodilation

Enlargement of a vessel.

Vascular Tone

State of partial constriction of arteriolar smooth muscle, establishing a baseline of arteriolar resistance.

Active Hyperemia

Increased blood flow in response to enhanced tissue activity.

Local Metabolic Influences

Changes such as decreased O2, adenosine release, increases in CO2, acid, K+, and osmolarity that influence arteriolar radius.

Vasoactive Paracrines

Endothelial-derived vasoactive substances that include endothelin and nitric oxide (NO), which promote vasodilation.

Angiogenesis

The process stimulated by vascular endothelial growth factor (VEGF) which promotes new vessel growth.

Extrinsic Control of Arteriolar Radius

Neural and hormonal influences on blood pressure, including sympathetic nervous system activation.

Norepinephrine and Epinephrine

Hormones that cause vasoconstriction.

ADH

Hormone that causes vasoconstriction and regulates water balance.

Angiotensin II

Hormone that causes vasoconstriction and regulates salt balance.

Total Peripheral Resistance (TPR)

The primary determinant of TPR is the adjustable arteriolar radius.

Local (intrinsic) control

Primarily important in matching blood flow through a tissue with the tissue's metabolic needs and is mediated by local factors acting on the arteriolar smooth muscle.

Extrinsic control

Important in regulating blood pressure and is mediated primarily by sympathetic influence on arteriolar smooth muscle.

Capillaries

Sites of exchange that are ideally suited for diffusion due to their narrowness, allowing red blood cells to squeeze through.

Diffusion across capillaries

Enhanced by the short distance diffusing molecules have to travel between blood and surrounding cells.

Total cross-sectional area

The area that influences the velocity of blood flow, which is inversely proportional to the total cross-sectional area of all the vessels at a given level.

Blood flow rate

Identical through all levels of the circulatory system and is equal to the cardiac output (5 L/min at rest).

Velocity of flow

Varies throughout the vascular tree and is slowest in the capillaries, which have the largest total cross-sectional area.

Capillary walls

Composed of a single layer of endothelium with water-filled pores that permit passage of small, water-soluble substances.

Precapillary sphincters

Wisps of spiraling smooth muscle cells that act as stopcocks to control blood flow through the particular capillary that each one guards.

Continuous capillary wall

Characterized by slit-like gaps between adjacent endothelial cells that form pores for the exchange of small water-soluble substances.

Interstitial fluid

Makes up 80% of extracellular fluid and acts as a passive intermediary between blood and cells.

Diffusion across capillary walls

Important in solute exchange, with the extent of exchanges for each solute determined by the magnitude of its concentration gradient.

Veins

Serve as a blood reservoir and passageways back to the heart, capable of storing extra blood due to their passive distensibility.

Mean arterial pressure

Determined by cardiac output (CO) and total peripheral resistance (TPR).

Short-term control of blood pressure

Involves adjustments in cardiac output and total peripheral resistance.

Long-term control of blood pressure

Involves adjusting total blood volume by restoring normal salt and water balance.

Baroreceptor Reflex

A short-term mechanism for regulating blood pressure that influences the heart and blood vessels to adjust cardiac output and TPR.

Hypertension

A national public-health problem with largely unknown causes, categorized into primary and secondary types.

Baroreceptor adaptation during hypertension

Adapt to operate at a higher level.

Complications of hypertension

Include left ventricular hypertrophy, stroke, heart attack, kidney failure, and progressive vision loss.

Baroreceptors

Special receptors in the walls of the carotid arteries for monitoring blood pressure.