Muscle and Respiratory Pumps Help Maintain Central Blood Volume

Although standing would appear to be a perfect situation for increased venoconstriction (which could return some of the blood from the legs to the central blood volume), reflex venoconstriction is a relatively minor part of the response to standing. A more powerful activation of the barorecep-tor reflex, as occurs during severe hemorrhage is required to cause significant venoconstriction. However, two other mechanisms return blood from the legs to the central blood volume. The more important mechanism is the muscle pump (Fig. 18.8). If the leg muscles periodically contract while an individual is standing, venous return is increased. Muscles swell as they shorten, and this compresses adjacent veins. Because of the venous valves in the limbs, the blood in the compressed veins can flow only toward the heart. The combination of contracting muscle and venous valves provides an effective pump that transiently increases venous return relative to cardiac output. This mechanism shifts blood volume from the legs to the central blood volume, and end-diastolic volume is increased. Even mild exercise, such as walking, returns the central blood volume and stroke volume to recumbent values (Fig. 18.9).

The respiratory pump is the other mechanism that acts to enhance venous return and restore central blood volume (Fig. 18.10). Quiet standing for 5 to 10 minutes invariably leads to sighing. This exaggerated respiratory movement lowers intrathoracic pressure more than usually occurs with inspiration. The fall in intrathoracic pressure raises the transmural pressure of the intrathoracic vessels, causing these vessels to expand. Contraction of the diaphragm simultaneously raises intraabdominal pressure, which compresses the abdominal veins. Because the venous valves prevent the backflow of blood into the legs, the raised intraabdominal pressure forces blood toward the intratho-racic vessels (which are expanding because of the lowered intrathoracic pressure). The seesaw action of the respiratory pump tends to displace extrathoracic blood volume toward the chest and raise right atrial pressure and stroke volume. Figure 18.11 provides an overview of the main cardiovascular events associated with a short period of standing.

Just before contraction

90 mm Hg added hydrostatic pressure

Arterial pressure y 90 + 93 mm Hg

Just before contraction

Arterial pressure y 90 + 93 mm Hg

Venous Return Mechanisms

During contraction

Just after contraction

Vein

Venous pressure 90 + 10 mm Hg

^Muscle pump. This mechanism increases venous return and decreases venous volume. The valves (which are closed after contraction) break up the hydro

Vein

Venous pressure 90 + 10 mm Hg

Skeletal Muscular And Respiratory

Just after contraction

Skeletal Muscular And Respiratory

'^¿f + 10 mm Hg static column of blood, lowering venous (and capillary) hydrostatic pressure.

Arterial blood pressure (mm Hg)

130 110 90f 70

130 110 90f 70

Right atrial mean pressure (mm Hg)

Cardiac output (L/min)

Stroke volume (mL)

Central blood volume (L)

Heart rate (beats/min)

Forearm blood flow (mL-100 mL^-min-1)

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Responses

  • mungo
    What is the purpose of skeletal muscle and the respiratory pumps?
    8 years ago
  • jean
    What are some situation that could increase blood volume?
    6 years ago

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