Blood Reservoir Function of the Veins

As pointed out in Chapter 14, more than 60 per cent of all the blood in the circulatory system is usually in the veins. For this reason and also because the veins are so compliant, it is said that the venous system serves as a blood reservoir for the circulation.

When blood is lost from the body and the arterial pressure begins to fall, nervous signals are elicited from the carotid sinuses and other pressure-sensitive areas of the circulation, as discussed in Chapter 18. These in turn elicit nerve signals from the brain and spinal cord mainly through sympathetic nerves to the veins, causing them to constrict. This takes up much of the slack in the circulatory system caused by the lost blood. Indeed, even after as much as 20 per cent of the total blood volume has been lost, the circulatory system often functions almost normally because of this variable reservoir function of the veins.

Specific Blood Reservoirs. Certain portions of the circulatory system are so extensive and/or so compliant that they are called "specific blood reservoirs." These include (1) the spleen, which sometimes can decrease in size sufficiently to release as much as 100 milliliters of blood into other areas of the circulation; (2) the liver, the sinuses of which can release several hundred milliliters of blood into the remainder of the circulation; (3) the large abdominal veins, which can contribute as much as 300 milliliters; and (4) the venous plexus beneath the skin, which also can contribute several hundred milliliters. The heart and the lungs, although not parts of the systemic venous reservoir system, must also be considered blood reservoirs. The heart, for instance, shrinks during sympathetic stimulation and in this way can contribute some 50 to 100 milliliters of blood; the lungs can contribute another 100 to 200 milliliters when the pulmonary pressures decrease to low values.

The Spleen as a Reservoir for Storing Red Blood Cells.

Figure 15-13 shows that the spleen has two separate areas for storing blood: the venous sinuses and the pulp. The sinuses can swell the same as any other part of the venous system and store whole blood.



Venous sinuses

Vein Artery

Figure 15-13

Functional structures of the spleen. (Courtesy of Dr. Don W. Fawcett, Montana.)

In the splenic pulp, the capillaries are so permeable that whole blood, including the red blood cells, oozes through the capillary walls into a trabecular mesh, forming the red pulp. The red cells are trapped by the trabeculae, while the plasma flows on into the venous sinuses and then into the general circulation.As a consequence, the red pulp of the spleen is a special reservoir that contains large quantities of concentrated red blood cells. These can then be expelled into the general circulation whenever the sympathetic nervous system becomes excited and causes the spleen and its vessels to contract. As much as 50 milliliters of concentrated red blood cells can be released into the circulation, raising the hematocrit 1 to 2 per cent.

In other areas of the splenic pulp are islands of white blood cells, which collectively are called the white pulp. Here lymphoid cells are manufactured similar to those manufactured in the lymph nodes. They are part of the body's immune system, described in Chapter 34.

Blood-Cleansing Function of the Spleen—Removal of Old Cells

Blood cells passing through the splenic pulp before entering the sinuses undergo thorough squeezing. Therefore, it is to be expected that fragile red blood cells would not withstand the trauma. For this reason, many of the red blood cells destroyed in the body have their final demise in the spleen. After the cells rupture, the released hemoglobin and the cell stroma are digested by the reticuloendothelial cells of the spleen, and the products of digestion are mainly reused by the body as nutrients, often for making new blood cells.

Reticuloendothelial Cells of the Spleen

The pulp of the spleen contains many large phagocytic reticuloendothelial cells, and the venous sinuses are lined with similar cells. These cells function as part of a cleansing system for the blood, acting in concert with a similar system of reticuloendothelial cells in the venous sinuses of the liver. When the blood is invaded by infectious agents, the reticuloendothelial cells of the spleen rapidly remove debris, bacteria, parasites, and so forth. Also, in many chronic infectious processes, the spleen enlarges in the same manner that lymph nodes enlarge and then performs its cleansing function even more avidly.

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Essentials of Human Physiology

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  • Galileo
    Who do veins function as a blood reservoir?
    4 years ago
  • aija
    How veins function as blood reservoirs?
    4 years ago
  • katri
    Why is venous circulation referred to as blood reservoir?
    4 years ago
  • erik wolf
    Why is the skin listed as reservoir for blood?
    4 years ago
    Which are blood reservoirs in the circulating system?
    4 years ago
  • isembard
    Which are the blood reservoirs in circulatory system?
    4 years ago
  • Piera
    How much blood pushed out from spleen during sympathetic stimulation?
    3 years ago
  • corey
    Why vein is blood reserviours?
    3 years ago
  • Ursula
    How do veins functions as blood reservioir?
    3 years ago
  • bercilac
    How the liver function as blood reservoir?
    3 years ago
  • tommi koponen
    How many blood reservoirs are there in the body?
    3 years ago
  • Lea
    Does the lungs serve as a blood reservoir?
    3 years ago
  • Alberica Bergamaschi
    What is a blood reservoir in a human body?
    3 years ago
  • max
    Why veins acts as blood reservoirs?
    3 years ago
  • Segan Yemane
    Which of the following acts as a reservoir for blood?
    2 years ago
  • girma
    Why lung acts as blood reservoirs?
    2 years ago
    Which is the first reservior of blood?
    2 years ago
  • Conrad
    Why is the liver refer to as the reservoir of blood.?
    1 year ago
    What is the primary blood resevoir in the body?
    1 year ago
  • Lisa
    What helps the body in hemorrhagic conditions because it acts as a blood reservoir?
    1 year ago
  • Sara
    How are veins a bllood reservoir?
    1 year ago
  • Uta
    How does the venous system serve as a blood reservoir?
    1 year ago
  • hollie dickson
    What id the blood reservoirs examples?
    1 year ago
  • patrick mclean
    Why vennous store blood most of it?
    10 months ago
  • shelby
    Why is the vein a volume reservoir?
    9 months ago
  • Mandy
    Why are veins considered blood reservoirs?
    9 months ago
  • hamish
    What are the main blood reservoirs of the body?
    8 months ago
  • Calum
    How is the heart blood reservoirs in the human body?
    8 months ago
  • Crispus
    How is the large abdominal veins blood reservoir?
    8 months ago
  • pete
    Can the heart be classified as a blood reservoir?
    8 months ago
  • Carita Nyman
    Which vessels serve as the blood reservoirs of the body?
    2 months ago
  • marianna
    Which is not considered part of the "blood reservoir" system?
    1 month ago
  • brhane negassi
    What does blood reservior do in the body?
    18 days ago

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