A common misconception is that the blood vessels only act as a delivery system of blood through the body.
Ischemia deprives the cells of oxygen, causing gangrene. Clinically, it is important to assess all tissues and organs for signs of ischemia. In particular, ischemia to the lower extremities will produce the “five cool Ps,” all of which are caused by lack of blood and oxygen.
Typically, the body maintains blood pressure within a normal range. Recently, the normal range for blood pressure has been revised as a systolic pressure of less than 120 and a diastolic pressure of less than 80 mm Hg. This revision is based on data from the “Seventh Report of the Joint National Committee on Detection, Evaluation and Treatment of High Blood Pressure.” Why is hypertension called “the silent killer?” In its early stages, hypertension does not produce symptoms but damages organs such as the heart and kidney.
Blood pressure is frequently monitored in clinical situations. In panel A, blood is flowing through the brachial artery. In panel B, pressure from the cuff stops the blood flow through the artery and there are no sounds. Panel C shows the recoding of the systolic pressure; the first of the Korotkoff sounds is heard. Panel D shows the recording of the diastolic pressure when the cuff is loosened and the brachial artery is opened. The Korotkoff sounds are absent or muffled. Refer students to the instructions for taking a blood pressure in Figure 19-1.
Because pressure is higher in the aorta than in the venae cavae, the pressure differential drives blood from the arteries to the veins. The farther blood gets from the heart, the lower its pressure against the vessel wall. The greatest pressure drop occurs across the arterioles. Ask students to note the difference in pressure between the aorta and capillaries. This lower pressure is enough to move fluid and dissolved solute, but does not burst capillary walls. Ask students to note the difference in pressure between the aorta and large veins. Because of this, even lower pressure veins need help in moving blood.
Pressure within the large veins is insufficient to return blood t the heart, so help is needed. The skeletal muscle pump massages the vessels and helps return the blood to the heart. In addition, the respiratory pump, not shown, facilitates the return of blood to the heart; this is accomplished by changes in thoracic volume during breathing. Venoconstriction makes the diameter of the veins smaller, shifting blood from the venous system through the heart and to the arterial system.
Both the heart and the blood vessels have roles in determining blood pressure, the heart through cardiac output and the vessels through controlling resistance. Antihypertensive medications are often aimed at both the heart and the blood vessels. For example, an alpha1-adrenergic blocker dilates the blood vessel and lowers the blood pressure, whereas a beta1-adrenergic blocker decreases cardiac output and lowers blood pressure.
Both the heart and the plunger (shown as an analogy for the heart) create the pressure gradient that moves blood throughout the circulation. This also explains the cardiac contribution to blood pressure.
The nozzle is an analogy for vasodilation and vasoconstriction in the arterioles. The arterioles are the resistance vessels. The smooth muscles of the arteriole change the diameter of the vessel as a nozzle changes the diameter of the hose. These changes affect pressure. Because of the nozzle, pressure in hose B is greater than in hose A. Because of vasoconstriction, the pressure in arteriole D is higher than in arteriole C. Systemic vascular resistance (SVR) describes the contractile state of the arterioles as a group.
Both nervous reflex and chemical reflex regulates blood pressure; this discussion focuses on the nervous or baroreceptor reflex.
The baroreceptor reflex is one example of the nervous reflex arc. Ask students to use the components of the arc listed on this slide to trace the baroreceptor reflex arc in Figure 19-6 in the text.
This slide illustrates the response of the baroreceptor reflex to a sudden decline in blood pressure. Ask students to trace the pathway in the slide. What are the effector responses and their outcome? The effector responses are vasoconstriction (increase in SVR), increased heart rate, increased stroke volume, and increased cardiac output. The outcome is an increase in blood pressure.
Develop the analogy between the waiter and the capillaries by emphasizing exchange. Capillaries both deliver and take away. If the cell is deprived of blood, it is deprived of both nutrients and oxygen.
Exchange takes place at the capillary level for three reasons shown on the slide. Why wouldn’t the aorta act as an efficient exchange vessel? The cell wall of the aorta is too thick. The diffusion distances are too great, because there is only one aorta but millions of cells all over the body. Finally, the blood would be moving too rapidly for exchange to occur.
The graph shows the speed of blood flow through each type of vessel; note that it is slowest in the capillaries. The slow rate of flow through the capillaries gives oxygen, nutrients, and waste time to exchange. The lengthy time needed for exchange is the crucial point of the graph.
The plunger is an analogy for the heart, which provides the driving force for all circulation. The holes in the side of the barrel are like the pores in capillary walls. Pressure on the plunger (or the pumping of the heart) forces water and dissolved solute through the pores into the interstitium, where it bathes the cells. Plasma protein (albumin) is not filtered because it is too big to fit through the capillary pores. Because proteins trapped in the capillaries exert oncotic pressure, fluid is pulled back from the interstitium into the venous end of the capillary. Reabsorption is aided by lymphatic vessels. Oncotic pressure is the same as osmotic pressure, but the former term in used when the cause is plasma protein (albumin).
The previous slide shows how the outward and inward movement of fluid is balanced. Edema and dehydration result from imbalances of capillary exchange. The model of transcapillary exchange provides the mechanisms for edema formation. Severe edema is characterized by pitting of the skin. The following slides will provide clinical examples of edema and dehydration.
The loss of albumin is one cause of edema. If normal amounts of albumin are not present in the blood, plasma oncotic pressure decreases. This lessens the force pulling fluid out of the interstitium and into the capillary; therefore, fluid remains in the interstitium—this is called edema. Nephrotic syndrome causes loss of albumin in the urine. Severe burns cause the size of the capillary pores to increase. As a result, albumin enters the interstitium and so cannot pull fluid back into the capillaries.
A second cause of edema is poor lymphatic drainage. The illustration shows the axillary lymph nodes which may be removed during mastectomy, lessening the reabsorption of fluid and causing edema. Oncotic pressure within the blood is responsible for 75% of the reabsorption of interstitial fluid. The lymphatic vessels are responsible for the other 25%. It might be helpful to ask students to look ahead to Figure 20-1 in the text for an illustration of poor lymphatic drainage. Why is this condition called elephantiasis? The excess fluid trapped in the limb make it swell so much that it looks like the leg of an elephant.
A third cause of edema is hypervolemia or excess volume in the blood. Excess volume increases the capillary filtration pressure, forcing excess fluid into the interstitial space. This can be caused by heart failure or overhydration from IV therapy. Why might a pregnant woman who spends a lot of time on her feet develop ankle edema? The baby’s weight may impede venous drainage from the lower extremities, elevating capillary pressure and forcing excess fluid into the interstitium.
The basic cause of dehydration is hypovolemia. Any clinical condition that that leads to excess loss of fluid can cause dehydration; common examples are shown on the slide. Older people are frequently dehydrated because they have a diminished sense of thirst and do not drink enough. Dehydration is characterized by low blood pressure, dry mouth, poor skin turgor, and tending.
Vessels can constrict and dilate to distribute blood to various organ systems as is needed at any time. The most dramatic example is the increased flow to skeletal muscles during exercise. In hypovolemic shock, why is blood flow to the kidneys dramatically diminished? The body is trying to conserve water to prevent further loss of blood volume and maintain blood flow to the most vital organs, such as the heart and brain. As a result, urinary output diminishes.
Why is it unwise to pile blankets on a feverish child with a flushed face? The flushing is an attempt to lose heat, and the blankets would interfere with that process. The combination of alcohol, which dilates the blood vessels, and prolonged exposure to extreme cold can be lethal.
The Human Body in Health and
Illness, 4th edition
Functions of the Blood Vessels