Functions and Pressures of Veins

Functions of Veins

• Veins act as blood reservoirs, capable of holding significant volumes of blood with minimal changes in pressure.
• Veins are transport vessels offering minimal resistance to blood flow.

Venous Pressures

• Central Venous Pressure (CVP): Is equivalent to right atrial pressure, typically ranging from 0-2 mm Hg.

• CVP is crucial for ventricular filling.

• It serves as an indicator of blood volume, declining in cases of hemorrhage.

• Elevated CVP indicates right-sided heart failure.

• Mean Systemic Filling Pressure (MSFP): The pressure driving venous return to the heart.

Vascular Compliance

• Venous compliance is significantly higher than arterial compliance, allowing veins to accommodate more blood volume per pressure change.
• Venous compliance is primarily regulated by venous tone. Sympathetic stimulation increases venous tone, reducing compliance.

Regulation of Venous Return

• Venous return is the volume of blood flowing back to the heart per minute.
• Under steady conditions, venous return equals cardiac output (approximately 5 liters/minute).
• Venous return depends on the difference between MSFP and right atrial pressure (RAP). This difference is considered the pressure gradient for venous return.

Factors Affecting Venous Return

• Blood Volume: Decreased blood volume reduces venous return. Conversely, increased blood volume increases venous return.

• Vein Capacity: Decreased capacity (e.g., from sympathetic stimulation) increases venous return. Conversely, increased capacity decreases venous return.

• Right Atrial Pressure (RAP): High RAP impedes venous return, as it makes it harder for MSFP to return blood to the right atrium.

Resistance to Venous Return (RVR)

• Represents vascular resistance to blood flow back to the heart.

Venous Return Curve

• Increasing RAP decreases venous return. Venous return reaches zero when RAP equals MSFP.
• Conversely, decreasing RAP increases venous return.

Effect of Changes in MSFP on Venous Return Curve

• Increasing MSFP (e.g., increased blood volume or decreased circulation capacity) increases venous return.
• Decreasing MSFP decreases venous return.

Effect of Changes in RVR on Venous Return Curve

• Decreasing RVR increases venous return.
• Increasing RVR decreases venous return
• Venous return is zero when RAP equals MSFP, regardless of RVR.

Interaction Between Cardiac Factors and Venous Factors

• Venous return and cardiac output must be equal.
• The right atrial pressure (RAP) needs to be the same for cardiac output and venous return to occur.
• At point A on the graph, venous return equals cardiac output (both are 5 liters/minute).
• RAP is maintained at 0 mmHg at this point.

Effects of Exercise on Cardiac Output and Venous Return

• Exercise increases heart rate and contractility.
• Exercise decreases afterload by decreasing total peripheral resistance. This causes the cardiac function curve to shift up and to the left.

Exercise and Venous Return

• Increased venous return during exercise is driven by factors like increased MSFP, muscle pumping, thoracic and cardiac suction, and decreased resistance (RVR).

Muscle Blood Flow During Exercise

• Muscle blood flow can increase dramatically (up to 25 times) during strenuous exercise, largely due to intramuscular vasodilation in response to increased muscle metabolism.
• Some additional increase is caused by a moderate increase in arterial blood pressure during exercise.