If

The Big Heart Disease Lie

Alternative Ways to Treat Cardiovascular Disease

Get Instant Access

Time

FIGURE 14-61

Five simultaneous graphs showing the time course of cardiovascular effects of hemorrhage. Note that the entire decrease in arterial pressure immediately following hemorrhage is secondary to the decrease in stroke volume and, hence, cardiac output. This figure emphasizes the relativeness of the "increase" and "decrease" arrows of Figure 14-59. All variables shown are increased relative to the state immediately following the hemorrhage, but not necessarily to the state prior to the hemorrhage.

only by the reflex response to hemorrhage—heart rate and total peripheral resistance—are increased above their prehemorrhage values. The increased peripheral resistance results from increases in sympathetic outflow to the arterioles in many vascular beds but not those of the heart and brain. Thus, skin blood flow may decrease markedly because of arteriolar vasoconstric-tion—this is why the skin becomes cold and pale. Kidney and intestinal blood flow also decrease.

A second important type of compensatory mechanism (one not shown in Figure 14-59) involves the movement of interstitial fluid into capillaries. This occurs because both the drop in blood pressure and the increase in arteriolar constriction decrease capillary hydrostatic pressure, thereby favoring absorption of interstitial fluid (Figure 14-62). Thus, the initial event—blood loss and decreased blood volume—is in large part compensated for by the movement of interstitial fluid into the vascular system. Indeed, 12 to 24 h after a moderate hemorrhage, the blood volume may be restored virtually to normal by this mechanism (Table 14-9). At this time, the entire restoration of blood volume is due to expansion of the plasma volume.

The early compensatory mechanisms for hemorrhage—the baroreceptor reflexes and interstitial fluid absorption are highly efficient, so that losses of as much as 1.5 L of blood—approximately 30 percent of total blood volume—can be sustained with only slight reductions of mean arterial pressure or cardiac output.

We must emphasize that absorption of interstitial fluid only redistributes the extracellular fluid. Ultimate replacement of the fluid lost involves the control of fluid ingestion and kidney function. Both processes are described in Chapter 16. Replacement of the lost erythrocytes requires stimulation of erythropoiesis by eryth-ropoietin. These replacement processes require days to weeks in contrast to the rapidly occurring reflex compensations described in Figure 14-62.

Hemorrhage is a striking example of hypotension due to decrease in blood volume, but loss of large quantities of cell-free extracellular fluid rather than whole blood is also a common cause of low-blood-volume hypotension. In such cases the basic loss is of salts, particularly sodium (along with chloride or bicarbonate) and water. Such fluid loss may occur via the skin, as in severe sweating (Chapter 18) or burns. It may occur via the gastrointestinal tract, as in diarrhea or vomiting (Chapter 17), or via unusually large urinary losses (Chapter 16). Regardless of the route, the loss of fluid decreases circulating blood volume and produces symptoms and compensatory cardiovascular changes similar to those seen in hemorrhage.

Hypotension may be caused by events other than blood or fluid loss. One major cause is a depression of cardiac pumping ability (for example, during a heart attack).

Another cause is strong emotion, during which hypotension can cause fainting. Somehow, the higher brain centers involved with emotions inhibit sympathetic activity to the cardiovascular system and enhance parasympathetic activity to the heart, resulting in a markedly decreased arterial pressure and brain blood flow. This whole process is usually transient. It should be noted that the fainting that sometimes occurs in a person donating blood is usually due to hypotension brought on by emotion, not the blood loss, since losing 0.5 L of blood will not itself cause serious hypotension in normal individuals.

Massive liberation of endogenous substances that relax arteriolar smooth muscle may also cause hypotension by reducing total peripheral resistance. An important example is the hypotension that occurs during severe allergic responses (Chapter 20).

Shock

The term shock denotes any situation in which a decrease in blood flow to the organs and tissues damages them. Arterial pressure is usually, but not always, low

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

Erythropoietic Response Blood Loss

FIGURE 14-62

Mechanisms compensating for blood loss by movement of interstitial fluid into the capillaries.

FIGURE 14-62

Mechanisms compensating for blood loss by movement of interstitial fluid into the capillaries.

in shock, and the classification of shock is quite similar to what we have already seen for hypotension: (1) Hypovolemic shock is caused by a decrease in blood volume secondary to hemorrhage or loss of fluid other than blood; (2) low-resistance shock is due to a decrease in total peripheral resistance secondary to excessive release of vasodilators, as in allergy and infection; and (3) cardiogenic shock is due to a marked decrease in cardiac output from a variety of factors (for example, during a heart attack).

The cardiovascular system, especially the heart, suffers damage if shock is prolonged. As the heart deteriorates, cardiac output declines markedly and shock becomes progressively worse and ultimately irreversible even though blood transfusions and other appropriate therapy may temporarily restore blood pressure.

TABLE 14-9 Fluid Shifts after Hemorrhage

Normal

Percent of Normal Value

Immediately 18 h after after Hemorrhage Hemorrhage

Total blood

5000

80

98

volume, ml

Erythrocyte

2300

80

80

volume, ml

Plasma

2700

80

113

volume, ml

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

Was this article helpful?

0 0
Your Heart and Nutrition

Your Heart and Nutrition

Prevention is better than a cure. Learn how to cherish your heart by taking the necessary means to keep it pumping healthily and steadily through your life.

Get My Free Ebook


Post a comment