Cardiovascular System

Blood Pressure

Anisha ValliLeave a comment
  • Blood pressure is expressed as the difference, or change in, pressures between two points along a vessel.
  • As this statement implies, blood pressure is not constant throughout the cardiovascular system.

Pressure profile graph

Graph Set-Up:

Y-axis =

  • Pressure (mm Hg); values 0-120

X-axis =

  • Left atrium and left ventricle, which are the chambers of the heart that pump oxygenated blood
  • Aorta, which is the largest artery in the body that receives blood directly from the left ventricle
  • Large arteries, small arteries, and arterioles
  • Capillaries, site of gas exchange
  • Veins
  • Right atrium and ventricle, which receive deoxygenated blood from the body and send it to the lungs via the pulmonary arteries

Pressure Curve:

Plots two key principles of blood pressure:

  • BP changes as blood moves through the body
  • BP is pulsatile because of the rhythmic contractions of the heart during the cardiac cycle

Key points of the curve

  • In the left atrium, blood pressure is relatively low, at about 5 mm Hg
  • It is much higher in the left ventricle and aorta, where it oscillates between 120 and 80 mm Hg
  • Arterial pressure rises slightly as it passes through the large arteries, then begins to fall
  • Pressure drops significantly as blood moves through the arterioles because they are the site of highest resistance to blood flow
  • Within the capillary networks, blood pressure falls to about 4 mmHg by the time it reaches the venous system
  • Blood pressure remains low in the right atrium at about 4-6 mm Hg, and rises slightly in the right ventricle
  • Within the pulmonary arteries, blood pressure is around 2-4 mm Hg
    — Recall that blood traveling through these vessels travels only a short distance, to the lungs; the low blood pressure is sufficient for lung tissue perfusion, but not so high as to cause damage

Stressed volume:

  • Blood within the arterial system is the “stressed” volume; it is under high pressure.

Unstressed volume:

  • Blood within the venous system is the “unstressed” volume, as it is under significantly less pressure.
    — Recall that the majority of the blood volume is within the venous, unstressed portion of the circulatory system.

Pulsatility:

  • Create a new graph to show the arterial pressure changes of a single cardiac cycle:

Y-axis =

  • Pressure (mm Hg); values 80 and 120.

X-axis =

  • Time

This curve shows:

  • Highest arterial pressure, 120 mmHg, is reached during systole, when the left ventricle contracts and blood is ejected to the aorta
  • Lowest pressure, 80 mmHg, is reached during diastole, the period of ventricular relaxation
  • Dicrotic notch, (aka, incisura) reflects a temporary drop in pressure after systolic contraction; it is caused by the backflow of blood after the aortic valve closes.

Clinical info:

  • Blood pressure is usually reported as systolic pressure over diastolic pressure; 120/80 mm Hg is considered to be a healthy blood pressure.

Pulse Pressure

  • Pulse pressure = systolic pressure – diastolic pressure
  • Stroke volume is the volume of blood ejected from the left ventricle per beat
  • Arterial compliance reflects the ability of the vessel wall to contract or expand to accommodate changes blood flow.

Mean arterial pressure (MAP)

  • Mean arterial pressure (MAP) is equal to the diastolic pressure plus 1/3rd of the pulse pressure;
  • Is NOT simply the average of the systolic and diastolic pressures; the equation accounts for the fact that the period of diastole is longer than that of systole.
  • Mean arterial pressure is determined by cardiac output and peripheral arterial resistance (aka, systemic vascular resistance).

Arterial pressure gradients

Driving pressure gradient

  • Refers to the change in pressure between two points along the longitudinal axis of a vessel; P 1 – P 2
  • Driving pressure can refer to changes in the overall systemic cardiovascular system or simply within a single organ; for example, knowing the driving pressure at the kidneys is important for understanding renal function and pathology.

Hydrostatic pressure gradient

  • Refers to the change in pressure between two points along the axis of a non-horizontal; for example, the femoral arteries, which deliver blood vertically from the heart to the lower extremities; h 1 – h

Transmural pressure gradient

  • Refers to change in pressure across the vessel wall; r 1 – r 2
  • Transmural pressure is important because it influences vessel diameter, and, therefore, resistance to blood flow.

Leave a comment