Velocity of Capillary Blood Flow

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Figure 14-44 illustrates a simple mechanical model of a series of 1-cm-diameter balls being pushed down a single tube that branches into narrower tubes. Although each tributary tube has a smaller cross section than the wide tube, the sum of the tributary cross sections is three times greater than that of the wide tube. Let us assume that in the wide tube each ball moves 3 cm/min. If the balls are 1 cm in diameter and they move two abreast, six balls leave the wide tube per minute and enter the narrow tubes, and six balls leave the narrow tubes per minute. At what speed does each ball move in the small tubes? The answer is 1 cm/min.

This example illustrates the following important principle: When a continuous stream moves through consecutive sets of tubes, the velocity of flow decreases as the sum of the cross-sectional areas of the tubes increases. This is precisely the case in the cardiovascular system (Figure 14-45). The blood velocity is very great in the aorta, slows progressively in the arteries and ar-terioles, and then slows markedly as the blood passes through the huge cross-sectional area of the capillaries. The velocity of flow then progressively increases in the venules and veins because the cross-sectional area decreases. To reemphasize, flow velocity is not dependent on proximity to the heart but rather on total cross-sectional area of the vessel type.

The huge cross-sectional area of the capillaries accounts for another important feature of capillaries:

PART THREE Coordinated Body Functions

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

III. Coordinated Body Functions

PART THREE Coordinated Body Functions

14. Circulation

© The McGraw-Hill Companies, 2001

Intercellular clefts

Endothelial cell

Intercellular clefts

Endothelial cell

Intercellular Cleft Image

Enlargement of capillary

To veins

Metarteriole

FIGURE 14-43

Diagram of microcirculation. Note the absence of smooth muscle in the capillaries. %

Adapted from Chaffee and Lytle.

Enlargement of capillary

To veins

Precapillary sphincters

Metarteriole

FIGURE 14-43

Diagram of microcirculation. Note the absence of smooth muscle in the capillaries. %

Adapted from Chaffee and Lytle.

-Smooth muscles

- Arteriole

Precapillary sphincters

-Capillaries

-Venule

Blood Flow Velocity Capillaries

FIGURE 14-44

Relationship between total cross-sectional area and flow velocity. The total cross-sectional area of the small tubes is three times greater than that of the large tube. Accordingly, velocity of flow is one-third as great in the small tubes.

FIGURE 14-44

Relationship between total cross-sectional area and flow velocity. The total cross-sectional area of the small tubes is three times greater than that of the large tube. Accordingly, velocity of flow is one-third as great in the small tubes.

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

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

3000

Is 8

2000

iu 1000

0 30

3000

2000

iu 1000

0 30

Aorta

Arteries and arterioles

Capillaries

Venules and veins

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Responses

  • KERSTIN
    Why is total cross sectional area of vessel greatest at capillaries?
    8 years ago
  • trahand hornblower
    Why is slower blood velocity important in the capillaries?
    8 years ago
  • Kerttu
    Why does velocity decrease with total cross sectional area?
    8 years ago
  • konsta
    What accounts for blood flow in venules and veins?
    8 years ago
  • Indro
    Why is it important that capillaries have a large cross sectional area?
    8 years ago
  • Alanna
    When blood flows from venules to veins, the crosssectional area?
    8 years ago
  • Melissa
    Is the blood velocity slow in veins?
    8 years ago
  • ANNA
    Why does the velocity of blood flow decreases upon entering venules from the capillaries?
    8 years ago
  • lia
    Is velocity of blood flow in veins greater than velocity of blood flow in capillaries?
    8 years ago
  • ana
    What is the velocity of capillary blood flow?
    8 years ago
  • Asfaha
    Why velocity decrease from aorta to cappilary?
    7 years ago
  • henrik paarma
    Why do capillaries have a larger area than the aorta?
    7 years ago
  • jo
    Why the velocity of blood flow decrease from aorta to the capillaries?
    7 years ago
  • aurelio longo
    Why the velocity of blood flow decreases from the aorta to the capillaries?
    7 years ago
  • Mustafa
    Why does velocity increase in the capillaries?
    7 years ago
  • Cora
    Why capillaries having small velocity and large crosssectional area?
    7 years ago
  • VERONICA
    How does velocity crosssectional area affect capillary exchange?
    7 years ago
  • lucie
    Does blood velocity increase from capillaries to veins?
    7 years ago
  • peony
    Does large cross sectional area in capillaries decrease blood velocity?
    7 years ago
  • Athos Fiorentino
    Why capillaries have a greater velocity than arteries?
    7 years ago
  • aamu
    What is the function of low velocity in capillaries?
    5 years ago
  • victor
    Why is does blood velocity increase slightly as it moves from the capillaries to the veins?
    5 years ago
  • assunta davide
    Why does blood slow down in capillaries?
    4 years ago
  • Adelbert
    Why is slow blood velocity important in the capillaries?
    4 years ago
  • SARAH
    Why is it that the blood loses velocity upon entering a capillary?
    3 years ago
  • nuguse
    Is the velocity in the capillary greater than the aorta?
    3 years ago
  • laurie
    How to explain speed of blood flow in capillary?
    2 years ago
  • elisa
    Why is the velocity of blood flow slower in capillaries than in arteries?
    1 year ago
  • Rosetta Greece
    What happens to bllod velocity doen to aorta?
    1 year ago
  • sabina
    Does capillary action increase velocity?
    1 year ago
  • anna-liisa niinist
    Why the velocity of aorta is bigger than capillaries?
    1 year ago
  • Meneaduc
    Why is the velocity of blood flow significantly slower in capillaries?
    10 months ago
  • craig
    Why is low velocity through the capillaries a good thing?
    10 months ago
  • evie
    Why is the velocity of blood flow the lowest in capillaries?
    9 months ago
  • holman
    Why is my blood velocity speed elevated?
    7 months ago
  • archibald
    Why is it important that the velocity and pressure of blood are lowest in capillary beds?
    5 months ago
  • asmait
    Why is blood velocity lower in capilaries?
    4 months ago
  • daryl shaw
    How does velocity affect how blood flows?
    3 months ago
  • ronja
    What are the blood flow velocity for aorta and capillaries and the difference in velocity?
    3 months ago
  • lelia
    Why does blood flow decreae in the capillaries and then increase again in the vein?
    25 days ago

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