Blood Pressure
Blood Pressure
Blood pressure is the force exerted by the blood against any unit area of the vessel wall. When one says that the pressure in a vessel is 50 mm Hg, one means that the force exerted is sufficient to push a column of mercury against gravity up to a level 50 mm high. If the pressure is 100 mm Hg, it will push the column of mercury up to 100 millimeters.
Two pressures are recorded when measuring blood pressure, systolic pressure is the peak arterial BP attained during ventricular systole, and diastolic pressure is the minimum arterial BP between heartbeats at which the heart get its own blood supply. The mean arterial pressure is the average of the arterial pressures (Table1.1). Tissue perfusion and organ perfusion depend on mean arterial pressure (MAP). MAP should exceed 70 to 80 mm Hg for cells to receive the oxygen and nutrients needed to metabolize energy in amounts sufficient to sustain life. It is not equal to the average of systolic and diastolic pressure because the arterial pressure remains nearer to diastolic pressure than to systolic pressure during the greater part of the cardiac cycle.
Several systems contribute to the regulation of arterial blood pressure. Blood pressure is determined mainly by cardiac output, blood volume, and peripheral resistance.
Mean arterial blood pressure = cardiac output × peripheral resistance
Cardiac output (the amount of blood discharged from the left or right ventricle per minute, it is about 5 liter for a 70 kg adult) is determined by stroke volume (the amount of blood ejected at systole or at each beat) and heart rate (the number of beats of the heart per unit of time, adult heart rate is 60 to 90 per minute).
Peripheral resistance is the resistance that the blood encounters in the vessels as it travels away from the heart. Peripheral resistance is determined by the diameter of the arterioles. It results from the friction of blood against the walls of the vessels and is proportional to three variables: blood viscosity, vessel length, and vessel radius. Blood viscosity ("thickness" of the blood) is due mainly to erythrocytes and plasma albumin. Vessel length: The farther a liquid travels through a tube, the more cumulative friction it encounters; thus, pressure and flow decline with distance therefore you would obtain a higher value in the arm than in the ankle. If perfusion is good at that distance from the heart, it is likely to be good elsewhere in the systemic circulation. Vessel radius, a change in vessel radius is called vasomotion. This includes vasoconstriction, the narrowing of a vessel, and vasodilation, the widening of a vessel. Vasoconstriction occurs when the smooth muscle of the blood vessel contracts. Vasodilation occurs when this muscle relaxes and allows the blood pressure within the vessel to push its walls outward.
Blood pressure is regulated by the baroreceptors (pressure receptors) located in the carotid sinus and aortic arch. The baroreceptor system is a simple and excellent example of a rapidly acting control mechanism. In the walls of the bifurcation region of the carotid arteries in the neck, and also in the arch of the aorta in the thorax, are many nerve receptors called baroreceptors, which are stimulated by stretch of the arterial wall. When the arterial pressure rises too high, the baroreceptors send barrages of nerve impulses to the medulla of the brain. Here these impulses inhibit the vasomotor center, which in turn decreases the number of impulses transmitted from the vasomotor center through the sympathetic nervous system to the heart and blood vessels. Lack of these impulses causes diminished pumping activity by the heart and also dilation of the peripheral blood vessels, allowing increased blood flow through the vessels. Both of these effects decrease the arterial pressure back toward normal. Conversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the vasomotor center to become more active than usual, thereby causing vasoconstriction and increased heart pumping, and raising arterial pressure back toward normal.
When blood pressure drops, catecholamines (epinephrine and norepinephrine) are released from the adrenal medulla of the adrenal glands. This increases heart rate and vasoconstriction, thus restoring blood pressure. Chemoreceptors, also located in the aortic arch and carotid arteries, regulate blood pressure and respiratory rate using much the same mechanism in response to changes in oxygen and carbon dioxide concentrations in the blood. These primary regulatory mechanisms can respond to changes in blood pressure on a moment-to-moment basis.
The kidneys also play an important role in blood pressure regulation. They regulate blood pressure by releasing renin, an enzyme needed for the conversion of angiotensin I to angiotensin II, a potent vasoconstrictor. This stimulation of the renin-angiotensin mechanism and resulting vasoconstriction indirectly lead to the release of aldosterone from the adrenal cortex, which promotes the retention of sodium and water. The increased concentration of sodium in the blood then stimulates the release of antidiuretic hormone (ADH) by the pituitary gland. ADH causes the kidneys to retain water further in an effort to raise blood volume and blood pressure.
In general any pathophysiologic alterations on the factors which regulate blood pressure; blood volume, peripheral resistance and cardiac output will alter the normal blood pressure. Management of blood pressure alteration e.g., hypertension is directed to correct and maintain those factors that regulate blood pressure.