Integration Of Cardiovascular Function Regulation Of Systemic Arterial Pressure

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In Chapter 7 we described the fundamental ingredients of all reflex control systems: (1) an internal environmental variable being maintained relatively constant, (2) receptors sensitive to changes in this variable, (3) afferent pathways from the receptors, (4) an integrating center that receives and integrates the afferent inputs, (5) efferent pathways from the integrated center, and (6) effectors "directed" by the efferent pathways to alter their activities. The control and integration of cardiovascular function will be described in these terms.

The major cardiovascular variable being regulated is the mean arterial pressure in the systemic circulation. This should not be surprising since this pressure is the driving force for blood flow through all the organs except the lungs. Maintaining it is therefore a prerequisite for ensuring adequate blood flow to these organs.

The mean systemic arterial pressure is the arithmetic product of two factors: (1) the cardiac output and (2) the total peripheral resistance (TPR), which is the sum of the resistances to flow offered by all the systemic blood vessels.

Mean systemic Cardiac Total peripheral arterial pressure = output X resistance

These two factors, cardiac output and total peripheral resistance, set the mean systemic arterial pressure because they determine the average volume of blood in the systemic arteries over time, and it is this blood volume that causes the pressure. This relationship cannot be emphasized too strongly: All changes in mean arterial pressure must be the result of changes in cardiac output and/or total peripheral resistance. Keep in mind that mean arterial pressure will change only if the arithmetic product of cardiac output and total peripheral resistance changes. For example, if cardiac output doubles and total peripheral resistance goes down 50 percent, mean arterial pressure will not change because the product of cardiac output and total peripheral resistance has not changed.

That the volume of blood pumped into the arteries per unit time—the cardiac output—is one of the two direct determinants of mean arterial blood volume and hence mean arterial pressure should come as no surprise. That the total resistance of the blood vessels to flow—the TPR—is the other determinant may not be so intuitively obvious but can be illustrated by using the model introduced previously in Figure 14-38.

As shown in Figure 14-52, a pump pushes fluid into a container at the rate of 1 L/min. At steady state, fluid also leaves the container via outflow tubes at a total rate of 1 L/min. Therefore, the height of the fluid column (AP), which is the driving pressure for outflow, remains stable. We then disturb the steady state by loosening the cuff on outflow tube 1, thereby increasing its radius, reducing its resistance, and increasing its flow. The total outflow for the system immediately becomes greater than 1 L/min, and more fluid leaves the reservoir than enters from the pump. Therefore the volume and hence the height of the fluid column begin to decrease until a new steady state between inflow and outflow is reached. In other words, at any given pump input, a change in total outflow resistance must produce changes in the volume and hence the height (pressure) in the reservoir.

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

III. Coordinated Body Functions

14. Circulation

© The McGraw-Hill Companies, 2001

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

Heart

Arteries

1 L/min

1 L/min

Organ blccd flows

1 L/min Steady state

1275 ml/min Outflow > Inflow

-133 ml

1 L/min New steady state

FIGURE 14-52

Dependence of arterial blood pressure upon total arteriolar resistance. Dilating one arteriolar bed affects arterial pressure and organ blood flow if no compensatory adjustments occur. The middle panel indicates a transient state before the new steady state occurs.

This analysis can be applied to the cardiovascular system by again equating the pump with the heart, the reservoir with the arteries, and the outflow tubes with various arteriolar beds. As described earlier, small arteries and capillaries offer some resistance to flow, but the major site of resistance in the systemic blood vessels is the arterioles; moreover changes in total resistance are normally due to changes in the resistance of arterioles. Therefore, in our discussions we equate total peripheral resistance with total arteriolar resistance.

A physiological analogy to opening outflow tube 1 is exercise: During exercise, the skeletal-muscle arte-rioles dilate, thereby decreasing resistance. If the cardiac output and the arteriolar diameters of all other vascular beds were to remain unchanged, the increased runoff through the skeletal-muscle arterioles would cause a decrease in systemic arterial pressure.

It must be reemphasized that it is the total arteriolar resistance that influences systemic arterial blood pressure. The distribution of resistances among organs is irrelevant in this regard. Figure 14-53 illustrates this

FIGURE 14-53

Compensation for dilation in one bed by constriction in others. When outflow tube 1 is opened, outflow tubes 2 to 4 are simultaneously tightened so that total outflow resistance, total runoff rate, and reservoir pressure all remain constant.

FIGURE 14-53

Compensation for dilation in one bed by constriction in others. When outflow tube 1 is opened, outflow tubes 2 to 4 are simultaneously tightened so that total outflow resistance, total runoff rate, and reservoir pressure all remain constant.

Bodily Functions

PART THREE Coordinated Body Functions

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

PART THREE Coordinated Body Functions

Activity of sympathetic nerves to veins

Peripheral veins

Venous pressure

Venous return

Atrial pressure

End-diastolic ventricular volume

Cardiac muscle

Stroke volume

Blood

volume

Activity of sympathetic nerves to heart

Skeletal muscle pump

Inspiration

movements

Plasma epinephrine

Activity of parasympathetic nerves to heart

SA node

Heart rate

Cardiac output

Mean

Mean arterial pressure

FIGURE 14-54

Summary of factors that determine systemic arterial pressure, an amalgamation of Figures 14-32, 14-41, and 14-50, with the addition of the effect of hematocrit on resistance.

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

III. Coordinated Body Functions

14. Circulation

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

Local controls

Vasoconstrictors Internal blood pressure (myogenic response) Endothelin-1

Vasodilators i Oxygen K+, CO2, H+ Osmolarity Adenosine Eicosanoids Bradykinin

Substances released during injury Nitric oxide

Peripheral Resistance Equat

Total peripheral resistance

Hematocrit

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Responses

  • Semhar
    How is total peripheral resistance related to systemic arterial blood pressure?
    8 years ago
  • hugo vanhala
    When arterial outflow becomes greater than inflow?
    8 years ago
  • HADDAS
    Why must systemic arterial blood pressure be maintained?
    8 years ago
  • lia
    WHAT HAPPPENS WHEN ARTERIAL OUTFLOW BECOMES GREATER THAN INFLOW?
    7 years ago
  • elizabeth
    When arterial outflow becomes greater than inflow arterial pressure?
    7 years ago
  • simon
    When arterial outflow becomes greater than arterial in flow the pressure will?
    7 years ago

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