Blood Pressure Hypertension Notes

Semester 2

Case 6: Too Much Pressure

• What is the normal anatomy of the blood vessels?

[1] Most vessels of the circulatory system have 3 coats, or tunics:

• Tunica intima: Inner lining consisting of a single layer of extremely flattened epithelial cells, the endothelium, supported by delicate connective tissue.

• Tunica media: Middle layer consisting primarily of smooth muscle. The most variable coat. Arteries, veins, & lymphatic ducts are distinguished by the thickness of this layer relative to the size of the lumen, its organisation, & in the case of arteries, the presence of variable amounts of elastic fibres.

• Tunica adventitia: Outer connective tissue layer or sheath.

Arteries

Blood vessels that carry blood under relatively high pressure from the heart & distribute it to the body. Different types of arteries are distinguished from each other on the basis of overall size, relative amounts of elastic tissue or muscle in the tunica media, thickness of the wall relative to the lumen, & function. There are 3 types of arteries:

• Large elastic arteries – AKA: Conducting arteries. Have many sheets of elastic fibres in their walls, which enable them to expand when they receive the cardiac output from the ventricles, minimising the pressure change. They return to normal size between ventricular contractions, as they continue to push the blood into the medium arteries downstream. This maintains arterial blood pressure between ventricular contractions.

• Medium muscular arteries – AKA: Distributing arteries. Have walls that consist mainly of circularly-disposed smooth muscle fibres. Their ability to constrict regulates the flow of blood to different parts of the body as required. Pulsatile contractions of their muscular walls temporarily & rhythmically constrict their lumina in progressive sequence, propelling & distributing blood to various parts of the body.

• Small arteries & arterioles: Have relatively narrow lumina & thick muscular walls. Degree of filling of capillary beds & level of arterial pressure within the vascular system are mainly regulated by degree of tonus (firmness) in the smooth muscle of arteriolar walls. (Tonus above normal hypertension).

Anastomoses between multiple branches of an artery provide numerous potential detours for blood flow in case the usual pathway is obstructed by compression due to the position of a joint, pathology, or surgical ligation. If a main channel is occluded, smaller alternate channels can usually increase in size over a period of time, providing a collateral circulation that ensures blood supply to the structures distal to the blockage. These are usually insufficient to compensate for sudden occlusion or ligation. However, true terminal arteries do not anastomose with any adjacent arteries (eg. Those in the retina). Functional terminal arteries have ineffectual anastomoses & supply segments of the brain, liver, kidneys, spleen, & intestines.

Veins

Venous system has lower blood pressure so the walls (specifically the tunica media) are thinner than those of their companion arteries. Normally, veins do not pulsate & do not squirt or spurt blood when severed. There are 3 sizes of veins:

• Venules: Drain capillary beds & join similar vessels to form small veins. Magnification is required to observe venules. Small veins are the tributaries of larger veins that unite to form venous plexuses.

• Medium veins: Drain venous plexuses & accompany medium arteries. In locations where the flow of blood is opposed by the pull of gravity (eg. In the limbs), medium veins have venous valves, passive flap valves that permit blood to flow toward the heart but not in the reverse direction.

• Large veins: Characterised by wide bundles of longitudinal smooth muscle & a well-developed tunica adventitia.

Veins are more abundant than arteries. Their walls are thinner, giving them a large capacity for expansion. However, their diameters are usually larger than those of the corresponding artery. Veins tend to be double or multiple. Those that accompany deep arteries surround them in an irregular branching network. This arrangement serves as a countercurrent heat exchanger, the warm arterial blood warming the cooler venous blood. The accompanying veins occupy a relatively unyielding fascial vascular sheath with the artery they accompany. As a result, they are stretched & flattened as the artery expands during contraction of the heart, which aids in driving venous blood toward the heart – an arteriovenous pump. Anastomoses occur more commonly between veins than arteries.

Blood Capillaries

Simple endothelial tubes connecting the arterial & venous sides of the circulation that allow the exchange of materials with the interstitial or extracellular fluid. Generally arranged in capillary beds, networks that connect the arterioles & venules.

As the hydrostatic pressure in the arterioles forces blood into & through the capillary bed, it also forces fluid containing oxygen, nutrients, & other cellular materials out of the blood at the arterial end of the capillary bed into the extracellular spaces, allowing exchange with cells of the surrounding tissue. However, walls are relatively impermeable to plasma proteins.

At the venous end of the capillary bed, most of this extracellular fluid – now containing waste products & carbon dioxide – is reabsorbed into the blood as a result of the osmotic pressure from the higher concentrations of proteins within the capillary. (Starling hypothesis)

Arteriolovenular anastomoses (AV shunts) exist in some regions (eg. Fingers) between the small arterioles & venules proximal to the capillary beds they supply & drain. These permit blood to pass directly from the arterial to the venous side of the circulation.

A portal venous system is where blood passes through 2 capillary beds before returning to the heart (Eg. Hepatic portal system).

• How is cardiac output...

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