Blood Vessels Ch. 19 Module 2: Section 19.03-19.05 Dynamic Study Module Flashcards

Thoroughfare channel

Terminal arteriole

continuous capillaries

Ex.

The vessels that exhibit the lowest level of permeability are the continuous capillaries .

Continuous capillaries are the least permeable and most common.

Continuous capillaries are:

• Abundant in skin, muscles, lungs, and CNS.
• Often have associated pericytes.
• Pinocytotic vesicles ferry fluid across the endothelial cell.
• Brain capillary endothelial cells lack intercellular clefts and have tight junctions around their entire perimeter.

Smallest vessels leading away from capillaries

Ex.

Venules are the smallest vessels leading away from capillaries .

Capillaries unite to form venules, which range from 8 to 100 μm in diameter. The smallest venules, the postcapillary venules , consist entirely of endothelium around which pericytes congregate. Postcapillary venules are extremely porous (more like capillaries than veins in this way), and fluid and white blood cells move easily from the bloodstream through their walls. Indeed, a well recognized sign of inflammation is adhesion of white blood cells to the postcapillary venule endothelium, followed by their migration through the wall into the inflamed tissue. Larger venules have one or two layers of smooth muscle cells (a scanty tunica media) and a thin tunica externa as well.

Anterior communicating artery

Contain valves to assist blood flow back toward heart

Ex.

Veins contain valves to assist blood flow back toward heart.

Venous valves prevent blood from flowing backward in veins just as valves do in the heart, and represent another adaptation to compensate for low venous pressure. They are formed from folds of the tunica intima and resemble the semilunar valves of the heart (see Figure 19.2). Venous valves are most abundant in the veins of the limbs, where gravity opposes the upward flow of blood. They are usually absent in veins of the thoracic and abdominal body cavities.

The effectiveness of venous valves is demonstrated by this simple experiment: Hang one hand by your side until the blood vessels on its dorsal aspect distend with blood. Next place two fingertips against one of the distended veins, and pressing firmly, move the superior finger proximally along the vein and then release that finger. The vein will remain collapsed (flat) despite the pull of gravity. Finally, remove your distal fingertip and watch the vein refill with blood.

Running in place

Ex.

Running in place would not increase the chances of developing varicose veins.

Varicose veins are veins that are tortuous and dilated because of incompetent (leaky) valves. More than 15% of adults suffer from varicose veins, usually in the lower limbs.

Several factors contribute, including heredity and conditions that hinder venous return, such as prolonged standing in one position, obesity, or pregnancy. Both the “potbelly” of an overweight person and the enlarged uterus of a pregnant woman exert downward pressure on vessels of the groin, restricting return of blood to the heart. Consequently, blood pools in the lower limbs, and with time, the valves weaken and the venous walls stretch. Superficial veins, which receive little support from surrounding tissues, are especially susceptible.

Elevated venous pressure can also cause varicose veins. For example, straining to deliver a baby or have a bowel movement raises intra-abdominal pressure, preventing blood from draining from anal veins. The resulting varicosities in the anal veins are called hemorrhoids (hem’ŏ-roidz).

veins

Ex.

Up to 65% of the body's blood supply is found in veins.

With their large lumens and thin walls, veins can accommodate a fairly large blood volume. Veins are called capacitance vessels and blood reservoirs because they can hold up to 65% of the body’s blood supply at any time. Even so, these distensible vessels are usually not filled to capacity.

Arterioles

Ex.

Arterioles do not contain intercellular clefts.

Structurally, there are three types of capillaries—continuous, fenestrated, and sinusoid. As you study their properties in the figure below, notice that all three types have tight junctions that join their endothelial cells together. However, these junctions are usually incomplete and leave gaps of unjoined membrane called intercellular clefts, which allow limited passage of fluids and small solutes. Leakier capillaries have specialized passageways that increase fluid movement.

Venule

diameter of arterioles

Ex.

Minute-to-minute blood flow into the capillary beds is determined by arteriolar diameter , which varies in response to changing neural, hormonal, and local chemical influences.

Changing diameter changes resistance to blood flow, and so arterioles are called resistance vessels . When arterioles constrict, the tissues served are largely bypassed. When arterioles dilate, blood flow into the local capillaries increases dramatically.

Smallest blood vessels with thin walls that allow exchange between blood and tissue cells

Ex.

Capillaries are the smallest blood vessels with thin walls that allow exchange between blood and tissue cells.

If we compare arteries and arterioles to expressways and roads, capillaries are the back alleys and driveways that provide direct access to nearly every cell in the body. Given their location and thin walls, capillaries are ideally suited for their role—exchange of materials (gases, nutrients, hormones, and so on) between the blood and the interstitial fluid.

Posterior communicating artery

Precapillary sphincters

pericytes

Ex.

The presence of pericytes stabilizes the wall of capillaries.

The microscopic capillaries are the smallest blood vessels. Their exceedingly thin walls consist of just a thin tunica intima. In some cases, one endothelial cell forms the entire circumference of the capillary wall. At strategic locations along the outer surface of some capillaries are spider-shaped pericytes, contractile stem cells that can generate new vessels or scar tissue, stabilize the capillary wall, and help control capillary permeability.

Anterior cerebral artery

capillaries and tissue cells

Ex.

Exchange of gases and nutrients occurs by diffusion between the capillaries and tissue cells.

Oxygen, carbon dioxide, most nutrients, and metabolic wastes pass between the blood and interstitial fluid by diffusion. Recall that in diffusion, net movement always occurs along a concentration gradient—each substance moving from an area of its higher concentration to an area of its lower concentration.

Hence, oxygen and nutrients pass from the blood, where their concentration is fairly high, through the interstitial fluid to the tissue cells. Carbon dioxide and metabolic wastes leave the cells, where their content is higher, and diffuse into the capillary blood.

capillaries

Ex.

The only vessels that provide direct access to nearly every cell in the body are the capillaries.

Of all the blood vessels, only the capillaries have intimate contact with tissue cells and directly serve cellular needs. Exchanges between the blood and tissue cells occur primarily through the gossamer-thin capillary walls.

Terminal arterioles

Ex.

Terminal arterioles regulate blood flow through most capillary beds.

Capillaries do not function independently. Instead they form interweaving networks called capillary beds. The flow of blood from an arteriole to a venule is called the microcirculation. In most body regions, a terminal arteriole branches into 10 to 20 capillaries (exchange vessels) that form the capillary bed. These then drain into a postcapillary venule. Blood flow through the capillary bed is controlled by the diameter of the terminal arteriole as well as by all of the arterioles upstream from it.

Posterior cerebral artery

net hydrostatic pressure (HP) is greater than net osmotic pressure (OP)

Ex.

Fluids would be likely to leave or filter out of the capillary if net hydrostatic pressure (HP) is greater than net osmotic pressure (OP).

Arteriole

Ex.

The part of a capillary bed indicated by “A” is the terminal arteriole .