Cardiovascular Physiology: Veins Study Notes

Cardiovascular Physiology Study Notes

• Cardiology focuses on the heart and blood vessels
• Cardiovascular physiology describes how the cardiovascular system functions
• The cardiovascular system consists of the heart, blood vessels, and blood. Its function is to distribute oxygen and nutrients to body tissues and remove wastes from those tissues.

Veins

• Veins have a large diameter, thin smooth muscle layer, and are compliant.
• Their thin walls allow them to accommodate large blood volumes without significant pressure change
• Veins lack elastic recoil/elasticity
• Veins act as blood reservoirs, important during blood loss
• Venoconstriction, when stimulated sympathetically, decreases vein diameter and increases venous return to the heart.
• Venous return (VR) is the volume of blood brought back to the right atrium per unit time.

Veins as Blood Reservoir

• Over 60% of circulating blood resides in veins.
• Veins are compliant, acting as a reservoir for blood, enabling the body to maintain normal circulatory function during substantial blood loss.

Venous Capacity

• Venous capacity depends on vein wall distensibility and external pressure.
• Increased venous capacity means more blood remains in veins, decreasing effective circulating blood volume.
• Decreased venous capacity means more blood returns to the heart, increasing the effective circulating blood volume.

Venous Valves

• Venous valves ensure unidirectional blood flow towards the heart.
• Movement of the legs/muscle contractions propels blood upward, known as the venous pump.
• Valves prevent backflow of blood

Varicose Veins

• Stretching veins increases their cross-sectional area but not the valve size.
• Valves fail to close completely, increasing venous pressure in the legs.
• The increased pressure causes edema, pain, skin ulceration, and possible thrombosis.
• Continuous leg elevation and compression stockings help minimize complications.

Factors Enhancing Venous Return

• Cardiac contraction drives venous return.
• Sympathetically induced venoconstriction decreases vein capacitance, increasing pressure, and accelerating blood return to the heart.
• Skeletal muscle pump is another factor that aids in venous return
• Venous valves assist in unidirectional flow of blood.
• The respiratory pump generates a pressure gradient by varying intra-thoracic pressure.
• Cardiac suction happens when ventricles contract, drawing blood through the veins.

Skeletal Muscle Pump

• Skeletal muscle contractions and relaxation squeeze veins, helping propel blood toward the heart.
• Valves prevent backward flow, making this mechanism efficient for venous return.

Respiratory Pump

• Breathing movements alter pressure within the chest cavity causing blood flow.
• During inhalation, reduced pressure in the chest cavity favors venous blood flow into the heart.

Cardiac Suction Effect

• Atrial contraction lowers pressure when ventricles push blood into circulation.
• A pressure gradient from veins to atria promotes blood flow.
• The heart, by reducing intrathoracic pressure, helps facilitate venous return.

Right Atrial Pressure

• Right atrial pressure is also called central venous pressure.
• It's the pressure from blood returning to heart from systemic circulation.
• Balance between heart's pumping ability and veins flowing into the heart controls this pressure.

Venous Return

• Factors that increase venous return:
• Increased blood volume
• Increased large vessel tone
• Dilation of arterioles
• Factors that decrease venous return:
• Decreased blood volume
• Constriction of veins
• Venous return is a balance between these forces.

Vascular Function Curve

• The curve in the graph illustrates the inverse relationship between venous return and right atrial pressure.
• Lower right atrial pressure results in higher venous return.
• As pressure increases, venous return decreases.
• This curve intersects the X axis where venous return is zero.

Mean Systemic Filling Pressure

• The mean systemic filling pressure (MSFP) is the pressure when there's no blood flow in circulation
• Blood volume, and the distribution of blood between the unstressed volume, and the stressed volume influence MSFP.
• An increase or decrease in blood volume will cause a shift in the curve

Combining Cardiac and Vascular Function Curves

• Combining these curves helps predict cardiac output changes when altering cardiovascular parameters.
• They interact at the point where cardiac output and venous return are equal.
• A new steady state ensues

Inotropic Effect

• An increase in contractility increases stroke volume, increasing cardiac output.
• Positive inotropic agents cause upward shift of the cardiac output curve but not the vascular curve.
• Cardiac output and right atrial pressure increases
• This will occur at the new point of intersection

Blood Volume Effect

• Increases in blood volume cause an increase in MSFP and shift the venous return curve to the right.
• Consequently, a higher cardiac output and right atrial pressure are measured in the new equilibrium state.

Total Peripheral Resistance (TPR) Effect

• Increases in TPR increases arterial pressure, resulting in reduced cardiac output.
• The curves intersect at a point of lower cardiac output and right atrial pressure in a new steady state.
• Decreases in TPR will shift the curves to balance the changes.

Explore the intricate details of cardiovascular physiology, focusing on the structure and functionality of veins. Understand how veins function as blood reservoirs and their role in venous return to the heart. This quiz will enhance your knowledge of an essential aspect of cardiovascular health.