Page 122 - Alaska A & P Primer

20.3 Capillary Exchange
20.4 Homeostatic Regulation of the Vascular System
20.3 OBJECTIVES
1. Explain the fate of fluid that is not reabsorbed from the tissues into the vascular capillaries
20.4 OBJECTIVES
1. Discuss how hypertension, hemorrhage, and circulatory shock affect vascular health
The primary purpose of the cardiovascular system is to circulate gases, nutrients, wastes, and other substances to and from the cells of the body. Small molecules, such as gases, lip- ids, and lipid-soluble molecules, can diffuse directly through the membranes of the endo- thelial cells of the capillary wall. Glucose, amino acids, and ions—including sodium, potas- sium, calcium, and chloride—use transporters to move through specific channels in the membrane by facilitated diffusion. Glucose, ions, and larger molecules may also leave the blood through intercellular clefts. Larger molecules can pass through the pores of fenes- trated capillaries, and even large plasma proteins can pass through the great gaps in the sinusoids. Some large proteins in blood plasma can move into and out of the endothelial cells packaged within vesicles by endocytosis and exocytosis. Water moves by osmosis.
Net filtration occurs near the arterial end of the capillary since capillary hydrostatic pres- sure (CHP) is greater than blood colloidal osmotic pressure (BCOP). There is no net move- ment of fluid near the midpoint since CHP = BCOP. Net reabsorption occurs near the ve- nous end since BCOP is greater than CHP. Since overall CHP is higher than BCOP, it is inevitable that more net fluid will exit the capillary through filtration at the arterial end than enters through reabsorption at the venous end. Considering all capillaries over the course of a day, this can be quite a substantial amount of fluid: Approximately 24 liters per day are filtered, whereas 20.4 liters are reabsorbed. This excess fluid is picked up by capil- laries of the lymphatic system. These extremely thin-walled vessels have copious numbers of valves that ensure unidirectional flow through ever-larger lymphatic vessels that eventu- ally drain into the subclavian veins in the neck. An important function of the lymphatic system is to return the fluid (lymph) to the blood. Lymph may bethought of as recycled blood plasma. (Seek additional content for more detail on the lymphatic system.)
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Any disorder that affects blood volume, vascular tone, or any other aspect of vascular func- tioning is likely to affect vascular homeostasis as well. That includes hypertension, hemor- rhage, and shock.
Chronically elevated blood pressure is known clinically as hypertension. It is defined as chronic and persistent blood pressure measurements of 140/90 mm Hg or above. Pres- sures between 120/80 and 140/90 mm Hg are defined as prehypertension. About 68 mil- lion Americans currently suffer from hypertension. Unfortunately, hypertension is typi- cally a silent disorder; therefore, hypertensive patients may fail to recognize the serious- ness of their condition and fail to follow their treatment plan. The result is often a heart attack or stroke. Hypertension may also lead to an aneurism (ballooning of a blood vessel caused by a weakening of the wall), peripheral arterial disease (obstruction of vessels in peripheral regions of the body), chronic kidney disease, or heart failure.
In response to blood loss, stimuli from the baroreceptors trigger the cardiovascular centers to stimulate sympathetic responses to increase cardiac output and vasoconstriction. This typically prompts the heart rate to increase to about 180–200 contractions per minute, restoring cardiac output to normal levels. Vasoconstriction of the arterioles increases vas- cular resistance, whereas constriction of the veins increases venous return to the heart. Both of these steps will help increase blood pressure.
MOVIE 1.32 Three Types of Capillaries 9:16 minutes Khan Acad- emy Medicine
The loss of too much blood may lead to cir- culatory shock, a life-threatening condition in which the circulatory system is unable to maintain blood flow to adequately supply sufficient oxygen and other nutrients to the tissues to maintain cellular metabolism.
Typically, the patient in circulatory shock will demonstrate an increased heart rate but decreased blood pressure, but there are cases in which blood pressure will remain normal.
Unfortunately, shock is an example of a positive-feedback loop that, if uncorrected, may lead to the death of the patient.
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