Blood Vessel Compliance and Function Quiz

Vascular Distensibility and Functions

• Blood vessels are distensible, meaning they can stretch.
• Arteries accommodate the heart's pulsatile output, smoothing blood flow.
• Veins are highly distensible, acting as reservoirs for extra blood. A rise in venous pressure causes a significant volume increase (0.5-1.0 liters).
• Vascular distensibility is the fractional increase in volume per mm Hg rise in pressure.

Vascular Distensibility Formula/Calculation

• Vascular distensibility = (Increase in volume) / (Increase in pressure × Original volume)
• Example: If a 10 ml vessel increases by 1 ml with a 1 mm Hg increase, distensibility is 0.1 per mm Hg or 10% per mm Hg.

Arteries and Veins: Distensibility Comparison

• Arteries are stronger than veins.
• Veins are roughly eight times more distensible than arteries.
• A given pressure increase causes more blood volume increase in veins than in arteries of similar size.

Compliance or Capacitance

• Compliance (or capacitance) is more important than distensibility in hemodynamic studies.
• It measures the total blood volume a part of the circulation can hold for a given pressure increase.
• Vascular compliance = (Increase in volume) / (Increase in pressure)
• Compliance and distensibility are not the same; compliance is equal to distensibility times volume.
  • A highly distensible vessel with a small volume has potentially lower compliance than a less distensible vessel with a larger volume.
• Compliance for Systemic Veins
  • Systemic veins are 24 times more compliant than corresponding arteries. This high compliance is due to both an eight-fold increase in distensibility and a three-fold increase in volume.

Volume-Pressure Curves

• Reflect the relationship between volume and pressure in blood vessels.
• The arterial system in an average adult, holding 700ml of blood at a mean arterial pressure of 100mm Hg, falls to zero when the volume decreases to 400ml.
• The venous system has a volume range of 2000-3500ml and a pressure change from 3-5mm Hg.
• Sympathetic stimulation leads to increased vascular smooth muscle tone and pressure at each volume. Inhibiting the same causes a lowering of pressure.
• Changes in vascular tone shifts blood volume to different segments of the circulation.
• Blood shift to the heart is one of the primary methods to increase cardiac output
• Vascular tone changes, particularly in veins, maintain circulation even with large blood loss

Delayed Compliance

• Delayed Compliance: A vessel initially displays a large pressure increase with rising volume. But the pressure eventually returns to a normal level over several minutes or hours due to gradual smooth muscle stretching.
• Increased blood volume results in a large initial pressure increase.
• The pressure subsequently decreases, returning to normal levels over time due to delayed smooth muscle adaptation.
• Immediate stretching of the vessel in response to volume expansion is followed by a delayed effect of increasing tensile strength in the vessel.

Arterial Pressure Pulsations

• Each heartbeat injects blood. Inertia initially restricts movement, causing proximal aorta stretching.

• Distention spreads progressively, transmitting the pressure pulse throughout the artery.

• Compliance minimizes pressure fluctuations in the smaller vessels, leading to continuous tissue blood supply.

• Normal systolic pressure in young adults is around 120 mm Hg. Diastolic pressure is roughly 80 mm Hg. Pulse pressure is 40mm Hg (120-80)

• Age and arterial hardening (arteriosclerosis) impacts pulse pressure through lowered compliance

Transmission of Pressure Pulses

• The velocity of pressure wave movement depends upon the compliance of the vessels.
• Arterial compliance is a significant factor in transmitting pressure waves with the speed influenced by the resistance of the vessel and vessels’ compliance.
• Damping of pressure waves is due to resistance of the vessels and compliance. The more compliant a vessel, the more blood required for a pressure increase.

Veins and Their Functions

• Veins can constrict or enlarge to hold more/less blood, making blood available when necessary.
• Peripheral veins regulate cardiac output by acting as a venous pump.
• The pressure in the right atrium (central venous pressure) relies on heart pumping ability and the flow of blood from peripheral veins.
• The right atrial pressure decreases with stronger heart pumping, conversely weak heart pumping increases right atrial pressure. Blood entering the right atrium from peripheral veins also increases pressure.
• Several factors influence venous return (increased blood volume, vascular tone, and arteriolar dilation)

Specific Blood Reservoirs

• Selected vascular regions (e.g., spleen, liver, abdominal veins, subcutaneous veins) function as reservoirs to regulate blood volume by storing or releasing different blood volumes.
• The heart and lungs, although not directly parts of the systemic venous system, can also act as blood reservoirs, expanding or shrinking to accommodate blood demands.

Venous Valves and the "Venous Pump"

• Valves in veins prevent backflow.
• Leg muscle contractions are crucial to venous return.
• The muscle pump or venous pump efficiently returns blood against gravity in standing positions or exertion.
• The gravitational effects on venous pressure within the lower body will cause an increase to 90 mm Hg if a person is standing completely still for 30 seconds.