Mechanisms of Venous Return

The flow of blood back to the heart, called venous return, is achieved by five mechanisms:

1. The pressure gradient. Pressure generated by the heart is the most important force in venous flow, even though it is substantially weaker in the veins than in the arteries. Pressure in the venules ranges from 12 to 18 mmHg, and pressure at the point where the venae cavae enter the heart, called central venous pressure, averages 4.6 mmHg. Thus, there is a venous pressure gradient (AP) of about 7 to 13 mmHg favoring the flow of blood toward the heart. The pressure gradient and venous return increase when blood volume increases. Venous return decreases when the veins constrict (venoconstriction) and oppose flow, and it increases when they dilate and offer less resistance. However, it increases if all the body's blood vessels constrict, because this reduces the "storage capacity" of the circulatory system and raises blood pressure and flow.

2. Gravity. When you are sitting or standing, blood from your head and neck returns to the heart simply by "flowing downhill" by way of the large veins above the heart. Thus the large veins of the neck are normally collapsed or nearly so, and their venous pressure is close to zero. The dural sinuses, however, have more rigid walls and cannot collapse. Their pressure is as low as —10 mmHg, creating a risk of air embolism if they are punctured (see insight 20.3).

3. The skeletal muscle pump. In the limbs, the veins are surrounded and massaged by the muscles. They squeeze the blood out of the compressed part of a vein, and the valves ensure that this blood can go in only one direction—toward the heart (fig. 20.18).

4. The thoracic (respiratory) pump. This mechanism aids the flow of venous blood from the abdominal to the thoracic cavity. When you inhale, your thoracic cavity expands and its internal pressure drops, while downward movement of the diaphragm raises the pressure in your abdominal cavity. The inferior vena cava (IVC), your largest vein, is a flexible tube passing through both of these cavities. If abdominal pressure on the IVC rises while thoracic pressure on it drops, then blood is squeezed upward toward the heart. It is not forced back into the lower limbs because the venous valves there prevent this. Because of the thoracic pump, central venous pressure fluctuates from 2 mmHg when you inhale to 6 mmHg when you exhale, and blood flows faster when you inhale.

Skeletal Muscle Pump Lymph

Figure 20.18 The Skeletal Muscle Pump. (a) When the muscles contract and compress a vein, blood is squeezed out of it and flows upward toward the heart; valves below the point of compression prevent backflow of the blood. (b) When the muscles relax, blood flows back downward under the pull of gravity but can only flow as far as the nearest valve.

Figure 20.18 The Skeletal Muscle Pump. (a) When the muscles contract and compress a vein, blood is squeezed out of it and flows upward toward the heart; valves below the point of compression prevent backflow of the blood. (b) When the muscles relax, blood flows back downward under the pull of gravity but can only flow as far as the nearest valve.

5. Cardiac suction. During ventricular systole, the chordae tendineae pull the AV valve cusps downward, slightly expanding the atrial space. This creates a slight suction that draws blood into the atria from the venae cavae and pulmonary veins.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • Fre-Swera Robel
    What is the most important force in venous flow?
    8 years ago
  • Sandy
    What mechanisms aids in returning venous?
    7 years ago
  • claudia
    What are the mechanisms of venous return of blood flow?
    4 years ago
  • Viola Bianchi
    How to achieve venous return?
    2 years ago
  • jere hakola
    What aids venous return?
    1 year ago
  • willie
    What mechanisms allow venous return to take place?
    11 months ago
  • Jonah
    When they say resistance decreases venous return are they refering to veins?
    7 months ago
  • Wiseman
    What are the 5 mechanisms involved in venous return?
    6 months ago
  • Rosamunda
    What physiologic mechanism support venous return to the heart?
    5 months ago
  • ANJA
    What mechanisms assist venous return?
    4 months ago
  • matteo longo
    What are three mechanisms responsible for venous blood return to the heart?
    2 months ago
  • greg
    What mechanisms aid in the Phoenix return of blood to the heart?
    29 days ago
  • pervinca
    What is the correct mechanism for blood to return to the heart?
    26 days ago
  • Laurie
    What are the various mechanisms that contribute to venous return of blood?
    10 days ago

Post a comment