Notes on Blood Pressure

How is blood flow measured in a given period? Cardiac Output: the amount of blood that the heart pumps into the aorta each minute Measured by multiplying heart rate by stroke volume (volume of blood pumped with each heartbeat) Heart rate (beats/minute) x 70 ml / beat HR Healthy adult at rest ~ 75 beats/minute & stroke volume averages ~ 70 ml per beat. Resting CO then is ~ 5.25 liters/minute Normal blood volume is 5 liters so entire blood supply passes through the heart every minute!

What is blood pressure? How is it expressed? The force that blood exerts on the wall of a blood vessel due to pumping of heart Expressed in millimeters of mercury (mm Hg) and measured in reference to systemic arterial BP in large arteries near the heart Systolic / diastolic

11) Where is systemic blood pressure highest? Where is it lowest? Systolic = highest pressure of cycle when ventricles contract to eject blood from heart Highest in arteries Diastolic = lowest pressure when ventricles relax Lowest in capillaries

What does arterial blood flow reflect? Arterial BP reflects two factors of the arteries close to the heart: 1. Their elasticity (distensibility) 2. The amount of blood forced into them at any given time

What is the difference between systolic pressure and diastolic pressure? Arteries store the energy generated by the heart during systole (ventricles contract) During diastole arteries use that stored energy to supply blood to the tissues (ventricles relax)

Why is low capillary pressure desirable? Capillary BP ranges from 20 to 40 mm Hg Low capillary pressure is desirable because high BP would rupture fragile, thin-walled capillaries Low BP is sufficient to force filtrate out into interstitial space and distribute nutrients, gases, and hormones between blood and tissues

Venous Blood Pressure Venous BP is steady and changes little during the cardiac cycle The pressure gradient in the venous system is only about 20 mm Hg A cut vein has even blood flow; a lacerated artery flows in spurts

What factors aid venous return? Venous BP alone is too low to promote adequate blood return and is aided by the: Respiratory “pump” – pressure changes created during breathing suck blood toward the heart by squeezing local veins Muscular “pump” – contraction of skeletal muscles “milk” blood toward the heart Valves prevent backflow during venous return

How does blood pressure vary in a 24-hour period? BP peaks in the morning due to waxing and waning levels of retinoic acid (Vitamin A) [Most heart attacks occur in the early morning] Other factors such as age, sex, weight, race, mood, posture, socioeconomic status, smoking and physical activity may also cause BP to vary