Venous Return

60-- without epinephrine tiv 4G

Skeletal muscle

Cardiac muscle-with epinephrine

(inotropic effect)

Cardiac muscle

Skeletal muscle

Cardiac muscle-with epinephrine

(inotropic effect)

60-- without epinephrine

Cardiac muscle

Human Physiology

50 60 70 80 90 100

Length of muscle (as percent of optimum at 100%)

50 60 70 80 90 100

Length of muscle (as percent of optimum at 100%)

■ Figure 14.4 The effect of muscle length and epinephrine on contraction strength. In this schematic comparison, all three curves demonstrate that each muscle contracts with its maximum force (100% relative tension) at its own optimum length (100% optimum length). As the length is decreased from optimum, each curve demonstrates a decreased contraction strength. Notice that the decline is steeper for cardiac muscle than for skeletal muscle, demonstrating the importance of the Frank-Starling relationship in heart physiology. At any length, however, epinephrine increases the strength of myocardial contraction, demonstrating a positive inotropic effect.

The end-diastolic volume—and thus the stroke volume and cardiac output—is controlled by factors that affect the venous return, which is the return of blood to the heart via veins. The rate at which the atria and ventricles are filled with venous blood depends on the total blood volume and the venous pressure (pressure in the veins). It is the venous pressure that serves as the driving force for the return of blood to the heart.

Veins have thinner, less muscular walls than do arteries; thus, they have a higher compliance. This means that a given amount of pressure will cause more distension (expansion) in veins than in arteries, so that the veins can hold more blood. Approximately two-thirds of the total blood volume is located in the veins (fig. 14.6). Veins are therefore called capacitance vessels, after electronic devices called capacitors that store electrical charges. Muscular arteries and arterioles expand less under pressure (are less compliant), and thus are called resistance vessels.

Although veins contain almost 70% of the total blood volume, the mean venous pressure is only 2 mmHg, compared to a mean arterial pressure of 90 to 100 mmHg. The lower venous pressure is due in part to a pressure drop between arteries and capillaries and in part to the high venous compliance.

The venous pressure is highest in the venules (10 mmHg) and lowest at the junction of the venae cavae with the right atrium (0 mmHg). In addition to this pressure difference, the venous return to the heart is aided by (1) sympathetic nerve activity, which stimulates smooth muscle contraction in the venous walls and thus reduces compliance; (2) the skeletal muscle pump, which squeezes veins during muscle contraction; and (3) the pressure difference between the thoracic and

Mean arterial pressure

Cardiac output

Cardiac rate

Stroke volume e

Sympathetic nerves

Parasympathetic nerves

■ Contraction strength

End-diastolic volume Stretch (EDV)

Frank-Starling

■ Figure 14.5 The regulation of cardiac output. Factors that stimulate cardiac output are shown as solid arrows; factors that inhibit cardiac output are shown as dashed arrows.

Fox: Human Physiology, I 14. Cardiac Output, Blood I Text I © The McGraw-Hill

Eighth Edition Flow, and Blood Pressure Companies, 2003

412 Chapter Fourteen

Blood Distribution Rest

■ Figure 14.6 The distribution of blood within the circulatory system at rest. Notice that the venous system contains most of the blood; it functions as a reservoir from which more blood can be added to the circulation under appropriate conditions (such as exercise).

abdominal cavities, which promotes the flow of venous blood back to the heart.

Contraction of the skeletal muscles functions as a "pump" by virtue of its squeezing action on veins (described in chapter 13; see fig. 13.28). Contraction of the diaphragm during inhalation also improves venous return. The diaphragm lowers as it contracts, thus increasing the thoracic volume and decreasing the abdominal volume. This creates a partial vacuum in the thoracic cavity and a higher pressure in the abdominal cavity. The pressure difference thus produced favors blood flow from abdominal to thoracic veins (fig. 14.7).

Test Yourself Before You Continue

Describe how the stroke volume is intrinsically regulated by the end-diastolic volume. Why is this regulation significant? Describe the effects of autonomic nerve stimulation on the cardiac rate and stroke volume. 3. Define the terms preload and afterload and explain how these factors affect the cardiac output.

List the factors that affect venous return. Using a flowchart, show how an increased venous return can result in an increased cardiac output.

Intrathoracic Pressure And Breathing

Negative intrathoracic pressure

Tissue-fluid volume

End-diastolic volume

Venous return

Negative intrathoracic pressure

Breathing

Breathing

Venoconstriction

Skeletal muscle pump

Tissue-fluid volume

Venoconstriction

Sympathetic nerve stimulation

Skeletal muscle pump

■ Figure 14.7 Variables that affect venous return and thus end-diastolic volume. Direct relationships are indicated by solid arrows; inverse relationships are shown with dashed arrows.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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Responses

  • tammie
    Why does veins have a higher compliant than arteries?
    8 years ago
  • Monika
    How does epinephrine affect venous blood return to the heart?
    8 years ago
  • arsi
    When venous return highest?
    8 years ago
  • Guerrino
    Which of the three factors affecting stroke volume is determined by the amount of venous return?
    8 years ago
  • jasmin kovalainen
    Does increased venous return negatively affect blood pressure?
    7 years ago
  • Maxima Noakes
    When venous pressure is to what stimulates smooth muscle?
    6 years ago
  • ailidh
    Do veins or arteries depend on the skeletal muscle pump and changes during breathing?
    6 years ago
  • Pearl
    Which depends on the skeletal muscle pump and changes during breathing Explain this dependence?
    5 years ago
  • alina
    Which depends on the skeletal muscle pump and changes during breathing veins explain the dependence?
    5 years ago
  • MULU
    Do veins depend on skeletal muscle contraction?
    5 years ago
  • marcho diggle
    Which depends on the skeletal muscle pump and changes during breathing quizlet?
    5 years ago
  • walter
    Which vesssel depends on the skeletal muscle pump?
    5 years ago
  • Tauno
    Which depends on the skeletal muscle group and changes during breathing Explain this dependence.?
    5 years ago
  • reginald shibata
    Which depends on the skelatal muscle pump and changes during breathing?
    4 years ago
  • jorma
    Which depends on the skeletal muscle pump and changes during breathining?
    4 years ago
  • Lotho Goodbody
    Which one the veins or the arteries depends on the skeletal muscle pump and changes during breathing?
    4 years ago
  • veli-matti
    Which dpends on the skeletal muscle pump and changes during breathing?
    4 years ago
  • HELI
    Which depends on skeletal muscle pump and changes during breahting?
    4 years ago
  • Aziz
    What is the effect of cardiac vacuum on venous return to the heaet?
    3 years ago
  • raymond
    How does a sketetal muscle pump increase mean arterial pressure?
    3 years ago

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