Blood Vessels and Circulation

Blood Vessels

Delivery system of dynamic structures that begins and ends at the heart, working with the lymphatic system to circulate fluids. Works with lymphatic system to circulate fluids.

Arteries

Carry blood away from the heart to capillaries; oxygenated except for pulmonary circulation and umbilical vessels of the fetus.

Blood Vessels

Delivery system of dynamic structures that begins and ends at the heart, working with the lymphatic system to circulate fluids. Works with lymphatic system to circulate fluids.

Arteries

Carry blood away from the heart to capillaries; oxygenated except for pulmonary circulation and umbilical vessels of the fetus.

Capillaries

Microscopic porous blood vessels in direct contact with tissue cells; directly serve cellular needs by exchanging substances between blood and tissue.

Veins

Carry blood from capillaries toward the heart; deoxygenated except for pulmonary circulation and umbilical vessels of the fetus.

Lumen

The space inside of a vessel, a central blood-containing space surrounded by a wall.

Tunica intima

Innermost layer of a blood vessel, in intimate contact with blood; includes the endothelium, which is continuous with the endocardium and reduces friction.

Tunica media

Middle layer of a blood vessel; circularly arranged smooth muscle and elastin, responsible for vasoconstriction and vasodilation.

Vasoconstriction

Contraction of the tunica media, narrows the lumen.

Vasodilation

Relaxation of the tunica media, widens the lumen.

Tunica externa

Outermost layer of a blood vessel; areolar connective tissue with elastic and collagen fibers, helps anchor the vessel to other structures and protect it.

Vasa vasorum

Small arteries required to supply very large vessels; known as the 'vessels to the vessels'.

Arteries (compared to veins)

Have a thicker tunica media and narrower lumen than veins; more resilient and resistant to changes in blood pressure.

Veins (compared to arteries)

Have a thicker tunica externa and larger lumen than arteries; wall collapses if no blood in vessel.

Capillaries (structure)

Contain only tunica intima (endothelium and basement membrane); thin wall allows for rapid gas and nutrient exchange.

Elastic arteries

Thick-walled arteries with a large, low-resistance lumen; conduct blood from the heart to muscular arteries; contain elastin in all three tunics.

Muscular arteries

Deliver blood to specific body regions/organs; have the thickest tunica media with more smooth muscle; active in vasoconstriction.

Arterioles

Smallest of all arteries; control flow into capillary beds via vasodilation and vasoconstriction of smooth muscle; regulate systemic blood pressure and blood flow.

Vasomotor tone

Smallest of all arteries contain smooth muscle that is usually somewhat constricted.

Atherosclerosis

Progressive disease of elastic and muscular arteries; thickening of tunica intima, narrowing of arterial lumen.

Aneurysm

Part of an arterial wall that thins and balloons out, prone to rupture. Most common in aorta or arteries at the base of the brain.

Continuous Capillaries

Endothelial cells form a continuous lining; tight junctions connect cells but don’t form a complete seal; found in muscle, skin, lungs, central nervous system.

Fenestrated Capillaries

Endothelial cells form a continuous lining with fenestrations (pores) that allow movement of smaller plasma proteins; found in areas where much fluid transport happens.

Sinusoids

Endothelial cells form an incomplete lining with large gaps; basement membrane is incomplete or absent; allow transport of large substances. Found in bone marrow, spleen, and some endocrine glands.

Metarteriole

Vessel branch of an arteriole that feeds a capillary bed.

Thoroughfare channel

Connects the metarteriole to the postcapillary venule.

Postcapillary venule

Drains the capillary bed.

True capillaries

Vessels that branch from the metarteriole and make up the bulk of the capillary bed.

Precapillary sphincter

Controls blood flow into the true capillaries.

• Sphincter relaxes blood flow through true capillaries

• Sphincter contracts blood flow through thoroughfare channel

Perfusion

Amount of blood entering capillaries.

Postcapillary venules

Capillaries unite to form these smallest venuoles, which are very porous and allow fluids and WBCs into tissues.

Capacitance vessels

Blood reservoirs because they contain up to 65% of blood supply.

Venous valves

Prevent blood from pooling in the limbs; ensure flow toward the heart; made of tunica intima and elastic and collagen fibers.

Venous sinuses

Flattened veins with extremely thin walls; composed only of endothelium.

Simple pathway (blood vessels)

One major artery delivers blood to an organ or region, which is then drained by one major vein.

Vascular anastomoses

Interconnections of blood vessels; provide alternative pathways (collateral channels) to ensure continuous blood flow.

Arterial anastomosis (arterial joining)

Two or more arteries converge to supply the same region.

Venous anastomosis

Two or more veins drain the same body region (more common)

Arteriovenous anastomosis (shunt)

Transports blood from an artery directly to a vein, allowing areas to be bypassed.

Portal system

Two capillary beds in sequence, with blood moving from an artery to a capillary bed, then to a portal vein, then to another capillary bed, and finally to a vein.

Blood Pressure

Force of blood on blood vessel walls.

Blood flow

Volume of blood flowing through a vessel, organ, or entire circulation in a given period; equivalent to the cardiac output for the entire vascular system.

Total cross-sectional area

Sum of diameters of all vessels of a certain type (artery, capillary, or vein).

Blood flow velocity

Inversely related to total cross-sectional area. Blood flow is slow in capillaries, allowing for exchange between blood and tissue fluid.

Capillary Exchange- Diffusion

Process by which molecules pass by diffusion between blood and interstitial fluid, moving down concentration gradients.

Bulk flow

Fluids flow down a pressure gradient, with large amounts of fluids and dissolved substances moving.

Hydrostatic Pressure

Force exerted by fluid pressing against a wall; capillary blood pressure that tends to force fluids through capillary walls.

Colloid osmotic pressure (COP)

The 'pull' on water due to the presence of proteins (colloid).

Opposing forces

• Hydrostatic pressures — “pushes”

• Colloid osmotic pressure — “sucks”

Filtration

Fluid moves out of blood.

Reabsorption

Fluid moves back into blood.

Net filtration pressure (NFP)

Comprises all forces acting on a capillary bed; determines net fluid flow out or in.

Local blood flow

Not all capillaries are filled simultaneously — local blood flow varies (must be high enough to maintain adequate perfusion)
Dependent upon:

• degree of tissue vascularity

• myogenic response

• local regulatory factors altering blood flow

• total blood flow

Degree of vascularization

Extent of blood vessel distribution within a tissue, which determines the ability of blood delivery.

Angiogenesis

The production of new blood vessels in tissues.

Myogenic response

Smooth muscle in blood vessel wall keeps local flow relatively constant by contracting when blood pressure rises and relaxing when it decreases.

Vasodilators

Cause smooth muscle relaxation, vasodilation, opening precapillary sphincters, and increased blood flow to capillaries.

• oxygen and nutrient levels declin

• carbon dioxide, lactic acid, H+ and K+ increase

• Medicines: nitrates, calcium channel blockers like amlodipine, ACE inhibitors like lisinopril

Vasoconstrictors

Cause smooth muscle contraction, vasoconstriction, closing precapillary sphincters, and decreased blood flow to capillaries.

Reactive hyperemia

Occurs after blood flow is restored to tissue, results in increased blood flow to tissue

Blood flow

Amount of blood transported through vasculature per unit of time.

Arterial blood pressure

Determined by two factors:

• elasticity (compliance or distensibility) of arteries close to the heart

• Volume of blood forced into them at any time

Mean arterial pressure (MAP)

The average arterial blood pressure across the entire cardiac cycle, representing the pressure that propels blood to tissues.

MAP Formula

Average arterial blood pressure is average arterial blood pressure across entire cardiac cycle: MAP = 1/3 (SBP – DBP) + DBP

Venous Return

Muscle pump: contraction of skeletal muscles “milks” blood back toward heart; valves prevent backflow, respiratory pump move blood toward heart, sympathetic venoconstriction: under sympathetic control, smooth muscles constrict, pushing blood back toward heart

Muscular pump

Contraction of skeletal muscles “milk” blood back toward the heart; valves prevent backflow

Respiratory pump

pressure changes during breathing move blood toward the heart

Sympathetic venoconstriction

under sympathetic control, smooth muscles in veins constrict, pushing blood back to the heart.

Resistance

The friction blood encounters.

Peripheral resistance

The opposition to the flow of blood in vessels. Vessel Length; Blood viscosity; Lumen size

Total Blood Flow

Volume of blood flowing through the system per unit time

Cardiovascular center

Includes cardiac center and vasomotor center; Goal: maintain MAP

Baroreceptor reflexes

Stretch receptors; Located in carotid sinuses, aortic arch, and walls of large arteries of neck and thorax

Chemoreceptor reflexes

Aortic arch and large arteries of neck detect increase in CO2, or drop in pH or O2; Signaling cardioacceleratory center to increase CO and vasoconstriction

Hypertension

Chronically elevated blood pressure; Systolic pressure > 140 mm Hg and/or diastolic > 90 mm Hg

Hypotension

Chronically low blood pressure; Symptoms of fatigue, dizziness, fainting

Orthostatic hypotension

Drop in blood pressure after sudden standing; Dizziness, light-headedness, fainting

The two main circulations

Pulmonary circulation : short loop that runs from heart to lungs and back to heart; Systemic circulation: long loop to all parts of body and back to heart