In this article we will discuss about:- 1. Meaning of Blood Pressure 2. Factors Influencing Blood Pressure 3. Measurement 4. Regulation.
Meaning of Blood Pressure:
Blood pressure is defined as the lateral pressure exerted on the vessel wall by column of flowing blood or it is also termed as end arterial pressure.
Systolic blood pressure is defined as maximum pressure that can be recorded in arteries during ventricular systole. In a normal adult of 20 years it ranges from 100 to 140 mm Hg and the mean pressure is 120 mm Hg.
Diastolic blood pressure is the minimum pressure that can be recorded in arteries during ventricular diastole. In a normal adult of 20 years, it ranges from 60 to 90 mm Hg and the mean will be 80 mm Hg.
Mean arterial pressure is the mean pressure in the arteries during a cardiac cycle. It is diastolic pressure plus one-third of pulse pressure. So it will be 80 + 40/3 = about 94 mm Hg.
Pulse pressure is the difference between systolic and diastolic pressure. So it is 120 – 80 = 40 mm Hg.
Factors Influencing Blood Pressure:
Factors influencing blood pressure are:
i. Age
ii. Sex
iii. Body build
iv. Posture
v. Exercise
vi. Emotional aspects
Factors that maintain the normal blood pressure are:
i. Cardiac output
ii. Peripheral resistance
iii. Blood volume
iv. Viscosity of the blood
v. Elasticity of the blood vessel
Cardiac output:
Systolic blood pressure depends entirely on the cardiac output. An increase in the cardiac output increases the systolic pressure and a decrease in the output will have the opposite effect.
Peripheral resistance affects the diastolic pressure and has a direct relationship. The seat of resistance is the arterioles. Arterioles contain large number of smooth muscle fibers that are supplied by sympathetic vasoconstrictor fibers. Vasoconstriction increases the resistance offered to blood flow and, therefore, the diastolic blood pressure is increased.
Blood volume:
If blood volume is reduced, it decreases the blood pressure. Decreased blood volume decreases the systemic filling pressure. This in turn will decrease the venous pressure, decrease the venous return and cardiac out put and, therefore, the blood pressure.
Blood pressure:
i. Later pressure
ii. End arterial pressure
iii. Normal BP 120/80 mm Hg
120 mm Hg—systolic BP
80 mm Hg—diastolic BP
Pulse pressure = Systolic-Diastolic
= 120-80
= 40 mm Hg
Mean arterial BP = Diastolic + rd of pulse pressure
= 80 + (13 or 14)
= 94 mm Hg
Measurement of Blood Pressure:
1. Direct:
Inserting a needle into an artery
2. Indirect:
i. Sphygmomanometer
a. Palpatory method
b. Auscultatory method
Viscosity of the blood:
Increased viscosity offers increased resistance to blood flows and, therefore, increases the blood pressure. This is usually seen in cases of polycythemia vera.
Elasticity of the blood vessels:
As age advances, the amount of elastic fiber decreases in the vessel wall. The blood vessel becomes more rigid tubes; the distensibility of the blood vessel is reduced. As a result, the systolic blood pressure is increased and the diastolic pressure is decreased.
However, in practice, what is seen is an increase in the diastolic pressure. This is because of atherosclerotic changes, the tube (blood vessel) diameter decreases and, therefore, offers greater resistance to blood flow.
Measurement of blood pressure can be done both by direct and indirect methods. But the direct method is not done during routine measurement of blood pressure as the technique is invasive and needs insertion of needle into an artery to determine blood pressure. Hence the indirect method is preferred.
Indirect methods are:
i. By palpatory method
ii. By auscultatory method.
By palpatory method, only the approximate systolic pressure can be measured. Diastolic blood pressure cannot be measured by this method.
Auscultatory method is the most accurate method of measuring the blood pressure. Both systolic and diastolic pressures can be measured by this method. Instrument used to measure blood pressure is known as sphygmomanometer.
In palpatory method, an approximate systolic pressure is obtained whereas in auscultatory method an accurate systolic and diastolic pressure can be obtained.
While determining blood pressure by auscultatory method, the sounds heard using stethoscope is known as Korotkoff sound. Korotkoff sounds are produced due to turbulence created by flow of blood through the partially obstructed blood vessel.
In the normal course, the blood flow through the blood vessel is laminar or silent as there is no obstruction. In laminar flow, the central most layer of blood will be flowing at maximal velocity and the velocity of flow of the layer of blood nearer to the wall of vessel will be least.
In a completely occluded vessel, there is no flow beyond the area of occlusion. When the vessel is partially opened (occlusion is removed partially), now blood has to pass through the narrow area. This creates turbulence beyond the area of partial obstruction.
This turbulence is responsible for production of Korotkoff sound. When the occlusion is completely removed, the turbulent flow is replaced by laminar flow. Hence there will be no more sound production.
Regulation of Blood Pressure:
This can be discussed as under:
i. Local mechanisms:
ii. Spinal cord in the regulation
iii. Medulla oblongata in the regulation
iv. Higher centers in the regulation
Or
i. Immediate mechanisms
ii. Intermediate mechanisms
iii. Long-term mechanisms
Local mechanisms include the production of vasodilator or vasoconstrictor substances. The vasoconstrictor substances are noradrenaline, angiotensin II, 5-hydroxytryptamine and others. The vasodilator substances are bradykinin, histamine, and adrenaline in certain regions. Hypoxia, hypercapnea, warmth, acidosis, etc. also bring about vasodilatation. These substances mainly alter the peripheral resistance and, therefore, the diastolic blood pressure.
Spinal Cord in the Regulation of Blood Pressure (Fig. 3.33):
Complete transverse section of the spinal cord (at the level of T1 segment) is followed by a marked fall in blood pressure. During the recovery phase, the lateral horn cells recover, send vasoconstrictor impulses to the blood vessels, blood vessels constrict, the blood pressure improves though it may not come back to normal.
During the state of spinal shock, carbon dioxide breathing will stimulate the lateral horn cells directly and increases the blood pressure. Influence of VMC is mediated through the lateral horn cells and the impulses from VMC on the lateral horn cells are always excitatory.
Role of Medulla Oblongata in the Regulation of Blood Pressure:
Medulla oblongata is the most important region of the central nervous system that is involved in the regulation of blood pressure. Collectively, these neurons are called as the vasomotor center (VMC). Stimulation of these neurons will give rise to vasoconstriction, which increases the peripheral resistance and increases the blood pressure.
On the other hand, when this neuronal activity is decreased it leads to vasodilation and, therefore, decreases the peripheral resistance and hence decreases the blood pressure.
Factors controlling the activity of the vasomotor center are (Fig. 3.34):
i. Impulses coming from the baroreceptors
ii. Impulses from the chemoreceptors
iii. Impulses from the higher centers
iv. Impulses from the pain receptors and from joint receptors
v. Impulses from the visceral receptors.
Baroreceptor Mechanism in the Regulation of Blood Pressure:
i. Baroreceptor mechanism is the most important mechanism in the regulation of blood pressure.
ii. Carotid sinus and arch of aorta contain the baroreceptors (Fig. 3.35).
iii. Carotid sinus is located at the bifurcation of the carotid artery and at the commencement of the internal carotid artery.
iv. Aortic arch receptors are located in the arch of the aorta.
v. A branch of the glassopharyngeal nerve supplies carotid sinus and the aortic nerve a branch from the vagus nerve supplies the aortic arch (Fig. 3.36).
vi. These receptors are stretch receptors. An increase in the blood pressure further stretches the receptors area resulting in production more number of impulses (Fig. 3.37).
vii. More number of impulses are produced and these impulses reach the following centers (Fig. 3.37):
a. Vasomotor center
b. Cardioinhibitory center
c. Respiratory center
Role of Vasomotor Center (Fig. 3.38):
i. Impulses going to the vasomotor center from the baroreceptors are inhibitory in nature and therefore, the activity of the vasomotor center is inhibited. This in turn decreases the number of impulses going to the lateral horn cells in the spinal cord. The activity of the lateral horn cells is decreased.
Vasoconstrictor impulses going to the arterioles are reduced, leading to vasodilatation, decreased peripheral resistance and a decrease in diastolic blood pressure.
ii. Inhibition of vasomotor center also decreases the venomotor tone; blood gets pooled in the venous compartment. This decreases the venous return, decreases the cardiac output and, therefore, the blood pressure.
iii. Inhibition of vasomotor center decreases the amount of catecholamine secretion from the adrenal medulla which in turn decreases the peripheral resistance and cardiac output.
iv. Due to decreased sympathetic activity, the heart rate and force of contraction of the heart are also reduced, reducing the systolic blood pressure.
The loss of function of baroreceptor (Fig. 3.39, when baroreceptor area is denervated or when baroreceptor area is not prefused—Figs 3.40 and 3.41) on BP variations has been depicted.
Role of Cardioinhibitory Center:
Impulses coming from the baroreceptors are excitatory to the cardioinhibitory center. Stimulation of cardioinhibitory center increases the vagal tone, which in turn decreases the heart rate and force of contraction of the heart. This will lead to decreased cardiac output.
Marey’s law states that heart rate is inversely proportionate to blood pressure. Whenever the blood pressure is increased, acting through the cardioinhibitory center, it reflexly lowers the heart rate and blood pressure.
Role of Respiratory Center:
Impulses coming from the baroreceptors are inhibitory to the respiratory center. This in turn decreases the rate and depth of respiration. Because of this, the changes in the intrapleural pressure become less. This will lead to decreased venous return. Decreased venous return decreases the cardiac output and, therefore, blood pressure.
Chemoreceptor Mechanism:
i. Chemoreceptors are the carotid bodies and aortic bodies.
ii. Decreased blood pressure decreases the blood flow through the chemoreceptors decreasing the oxygen supply.
iii. Hypoxia, hypercapnia and acidosis stimulate these chemoreceptors.
The impulses from the chemoreceptors in general are going to stimulate the vasomotor center, respiratory center and inhibit the cardioinhibitory center during the course of regulation of blood pressure. Stimulation of VMC increases the peripheral resistance and hence the blood pressure.
CNS Ischemic Response:
If the blood pressure falls to a greater extent, the blood flow to the brain is markedly reduced, metabolic waste products accumulate, the resulting hypercapnea and acidosis stimulate the vasomotor center directly and more powerfully. Peripheral blood vessels under go marked vasoconstriction and increases the blood pressure to a greater extent.
Stretch receptors present in the low pressure areas of the cardiovascular system:
i. Stretch receptors are present in the walls of great veins, right atrium.
ii. An increase in the blood volume, distension of the venous compartment. This leads to the receptors getting stretched and stimulated.
iii. Impulses travel to the higher centers and reflexly bring about the following changes:
a. Peripheral arteriolar dilation decreased peripheral resistance, therefore, a fall in the blood pressure.
b. Afferent arteriolar dilation leads to increased hydrostatic pressure in the glomerular capillary network leading to increased glomerular filtration rate and increase the fluid loss.
c. Decreased release of ADH will increase the urinary output. This in turn decreases the blood volume and blood pressure.
Atrial natriuretic factor:
A chemical substance released from the atrial muscle fibers due to distension of the atrium. This can occur whenever the blood volume is increased or whenever the venous return is increased. Any time this hormone is released it brings about peripheral vasodilation, increased excretion of water and salt. This in turn decreases the blood pressure.
Factors Regulating Blood Pressure- Intermediate Mechanisms:
Fluid shift mechanism:
Any time the blood pressure falls, the pre-capillary sphincter contracts, this decreases the hydrostatic pressure in the capillaries. All along the capillaries, the colloidal osmotic pressure remains high and, therefore, the fluid shifts from the extravascular compartment to the intravascular compartment. This increases the blood volume and blood pressure.
Renin-angiotensin mechanism (Fig. 3.42):
Decreased blood flow to the kidney due to a fall in the blood pressure will bring about the release of renin from the juxtaglomerular cells. This converts angiotensinogen to angiotensin I which is converted to angiotensin II.
This is a powerful vasoconstrictor substance which brings about constriction of the arterioles, increasing the peripheral resistance and blood pressure. This mechanism also increases the production of aldosterone. This hormone acts on the renal tubules, increasing the reabsorption of salt and water, increasing the blood volume and blood pressure.
ADH mechanism:
Any time when the blood volume is increased, the blood pressure is increased. The volume receptors are stimulated. Impulses arising from these receptors reach the hypothalamus and inhibit the secretion of ADH. More amount of water is lost from the kidneys, lowering the blood volume and blood pressure.
Long-term Regulation of Blood Pressure:
Kidneys play a very important role in the long-term regulation of blood pressure. Thus the fluid volume is maintained and, therefore, the blood volume and blood pressure is maintained.
Increased blood pressure in person with normal kidney excretes more of salt and water known as pressure natriuresis and pressure diuresis. The efficiency of the kidney in this respect is infinite (Fig. 3.43).
Baroreceptors of carotid sinus and aortic arch show the property of adaptation. Sustained increase in blood pressure occurs in essential hypertension. Sustained increase of blood pressure will lead to resetting of the baroreceptors due to the property of adaptation. Therefore, the baroreceptors fail to lower the blood pressure.