Systolic blood pressure refers to the pressure in the arterial system during ventricular contraction or “systole”. Since the arteries have highly elastic walls, the pressure in the aorta is reverberated throughout the major arteries of the body and can be measured using a blood pressure cuff. Also, the “pulse” can be felt as a rebounding of the elastic walls of the arteries in many superficial locations in the body, such as a carotid artery and radial artery.
The systolic blood pressure therefore tells us about “how hard the heart is working” during the contraction phase. A high systolic blood pressure generally means there is a greater resistance that the heart has to work against during systole.
Diastolic blood pressure is the pressure in the arterial system during the ventricular relaxation phase (i.e. between contractions of the heart). During this phase, the pressure that remains in the arteries reflects the peripheral resistance to blood flow during ventricular relaxation. Therefore, a high diastolic blood pressure is a reflection of the “stiffness” of the blood vessels.
A person is considered to be “hypertensive” (or to have high blood pressure) if either their systolic blood pressure is measured to be 140mmHg or greater OR their diastolic blood pressure is measured to be 90 mmHg or greater on 3 occasions. Some individuals may have both a high systolic and high diastolic blood pressure.
Mean arterial pressure (MAP) is calculated from both the systolic and diastolic blood pressures. Since more time is spent in diastole (ventricular relaxation), it has a greater effect on MAP:
The resistance to peripheral blood is called total peripheral resistance (TPR) and has an inverse relationship with cardiac output:
Regulation of Blood Flow
Blood flow is highly regulated in order for adequate oxygenated blood to be supplied to working organs.
There are 3 factors that determine the resistance to blood flow:
- viscosity of the blood (blood thickness),
- length of the vessel and
- diameter of the blood vessel.
This is shown by Poiseuille’s Law which expresses the relationship of factors affecting flow in a cylindrical vessel:
Vessel radius is the most powerful regulator of the blood flow, since it is raised to the 4th power. Therefore if the vessel radius is reduced by half, flow is decreased by a factor of 16.
Relationship Among Vascular Area, Blood Flow Velocity and Blood Pressure
Large blood vessels have faster rates of blood flow and higher pressure, but small total areas. On the other hand, capillaries have a large total area, slow rate of flow and low pressure. This allows time for nutrient exchange between the capillaries and tissue.
Figure 5. The relationships among total vessel cross-sectional area (top panel), velocity of blood flow (middle panel) and blood pressure in the vessel (lower panel). The large arteries have smaller areas and higher velocity and pressure, while the capillaries have a larger total area, and low velocity and flow rates to enhance the time and area for nutrient exchange to occur.
The major control mechanisms of vessel radius are: local factors, neural factors and hormonal factors.
- Local factors: local reductions in oxygen level, increases in temperature, accumulation of metabolic byproducts (CO2, hydrogen ion, adenosine etc.), nitric oxide and certain ions (magnesium, potassium) can cause the smooth muscle around capillaries to relax resulting in vasodilation. There is an increases in blood flow and/or opening of dormant capillaries. This is also known as an “autoregulatory mechanism” to increase blood flow.
- Neural factors: the sympathetic nervous system can override vasoregulation by local factors. Sympathetic nerve fibers innervate the smooth muscles of small arteries, arterioles and precapillary sphincters. Sympathetic fibers that supply the vessels of organ beds release of norepinephrine (noradrenalin) and cause vasoconstriction. Sympathetic fibers that innervate the vessels supplying skeletal muscle and cardiac muscle release acetylcholine and cause vasodilation.
- Hormonal factors: the medullary portion of the adrenal glands (which sit on top of the kidneys) receives input from the sympathetic nervous system. When stimulated, it releases epinephrine and norepinephrine into the circulation. These hormones cause vasoconstriction of the blood vessels supplying the organs but not those supplying the skeletal muscle or heart.