Cardiac Cycle and Blood Pressure

Blood Pressure During the Cardiac Cycle

• Arterial blood pressure is highest during ventricular systole, known as systolic pressure
• Each ventricular contraction causes a spike in blood pressure, stretching the walls of the arteries, and can be felt as a pulse
• The pressure surge is partly due to the narrow openings of arterioles impeding the exit of blood from the arteries
• During diastole, the elastic walls of the arteries snap back, resulting in a lower but still substantial blood pressure (diastolic pressure)

Regulation of Blood Pressure

• Homeostatic mechanisms regulate arterial blood pressure by altering the diameter of arterioles
• Vasoconstriction (narrowing of arterioles) increases blood pressure upstream in the arteries
• Vasodilation (widening of arterioles) decreases blood pressure upstream in the arteries
• Nitric oxide (NO) is a major inducer of vasodilation, while endothelin is the most potent inducer of vasoconstriction
• Cues from the nervous and endocrine systems regulate the production of NO and endothelin in blood vessels

Blood Pressure and Gravity

• Blood pressure is generally measured for an artery in the arm at the same height as the heart
• For a healthy 20-year-old human at rest, arterial blood pressure is typically about 120 mm Hg at systole and 70 mm Hg at diastole
• Gravity has a significant effect on blood pressure, with blood pressure decreasing with increasing height
• The relationship between blood pressure and gravity is key to understanding fainting
• Fainting is triggered when the nervous system detects that the blood pressure in the brain is below the level needed to provide adequate blood flow

Adaptations to Gravity

• Animals with long necks require high blood pressure to overcome gravity and get blood to their brains
• A giraffe requires a systolic pressure of more than 250 mm Hg to get blood to its head
• Dinosaurs with long necks would have required even greater systolic pressure, but calculations suggest they did not have a heart powerful enough to generate such high pressure
• Some biologists believe that long-necked dinosaurs fed close to the ground rather than on high foliage

Blood Flow in Veins

• Gravity draws blood downward to the feet and impedes its upward return to the heart
• Valves inside the veins have an important function in maintaining the unidirectional flow of blood within these vessels
• The return of blood to the heart is further enhanced by rhythmic contractions of smooth muscles in the walls of venules