Mean Arterial Pressure

Sheet Subject # 14 Physiology Done by Corrected by Doctor Mean Arterial Pressure - Recall, mean arterial pressure (MAP) is the average pressure in a patient's arteries during one cardiac cycle.  MAP = 1/3 Systolic pressure + 2/3 Diastolic pressure  Pulse pressure = Systolic pressure – Diastolic pressure  MAP = Diastolic pressure + 1/3 Pulse pressure - Factors mainly affecting the pressure are cardiac output and total peripheral resistance according to the formula: MAP = CO x TPR.  Arterial Pressure can be increased by: 1- Constricting almost all arterioles of the body which increases total peripheral resistance. 2- Constricting large vessels of the circulation thereby increasing venous return and cardiac output. 3- Directly increasing cardiac output by increasing heart rate and contractility. Measuring Blood Pressure (Auscultatory method): - A sphygmomanometer (mercury/electronic manometer) is a device used to measure blood pressure, composed of an inflatable cuff to collapse and then release the artery under the cuff in a controlled manner. 1|Page 1- The cuff is placed around the upper arm above the cubital fossa, with the stethoscope on the antecubital artery (not the vein because veins have a very weak pulsation). 2- Pressure in the cuff is raised above the systolic pressure stopping blood flow, thus no pulsation will be heard. 3- We start decreasing the pressure of the cuff. As the pressure decreases, a pounding sound is heard when the blood flow starts again in the artery. The pressure at which this sound began is noted and recorded as the systolic blood pressure.  The sound indicates that there is blood flow during systole; because the manometer pressure now is equal to or less than the systolic pressure → The sound heard is caused by the turbulent flow of blood → The turbulent flow is caused by arterial constriction which is caused by the manometer cuff. 4- The cuff pressure is further released until the sound can no longer be heard; because the flow becomes laminar, not turbulent. The pressure at which the sound disappears is equal to the diastolic pressure. - In conclusion: the beginning of sound marks the systolic pressure and the disappearance of the sound marks the diastolic pressure. Note: Auscultatory method is the most commonly used method for measuring both diastolic and systolic pressure. In the palpatory method, only systolic pressure is measured. Blood Vessels and the Autonomic Nervous System - The sympathetic nervous system is important in controlling the circulation, while the parasympathetic is mainly important in regulating heart function via the vagus nerve. - Parasympathetic nerves do not supply blood vessels. Almost all blood vessels are only innervated by the sympathetic nervous system except capillaries (since they lack smooth muscles), precapillary sphincters, and some metarterioles. - Normally without any stimulation, there is a basal tone of the vessels; in which there is basal sympathetic outflow causing the vessels to be semi-constricted. Therefore, increasing the sympathetic outflow leads to vasoconstriction, and reducing the sympathetic outflow leads to vasodilation. 2|Page Blood Pressure Regulation Blood pressure can be regulated through: 1- Short-term regulators: these regulate pressure through neural mechanisms acutely; in minutes or seconds. Example: a- High-pressure baroreceptors b- Low-pressure baroreceptors c- Chemoreceptors 2- Intermediate regulators: work in terms of hours to days through chemicals. 3- Long-term regulators: they regulate pressure in long periods (chronic) through chemicals; in terms of days, weeks, or months. They usually work through the renal system and body fluid. Example: Renin-Angiotensin Aldosterone system. ‘In this sheet, we will discuss short-term regulation’ 1- High-pressure Baroreceptors - They are called high-pressure baroreceptors since they are sensors located in high-pressure areas: 1- The carotid sinus; at the bifurcation of external and internal carotids. 2- The aortic arch. - Baroreceptors are stretch-regulated receptors (mechanoreceptors / pressoreceptorts). They sense changes in pressure according to change in the tension (stretching) of the arterial wall; increased pressure causes more stretching and vice versa. Signals are then relayed to the brain so that proper blood pressure can be maintained. This baroreflex mechanism is a fast response to changes in blood pressure.  More stretch → Increased firing rate  Less stretch → Decreased firing rate 3|Page - Afferent nerves carry impulses to relay the information of blood pressure changes from the baroreceptors to the brain: 1- Aortic Baroreceptors: supplied by a branch from the vagus nerve (cranial nerve X). 2- Carotid Baroreceptors: supplied by Hering’s nerve; a branch from the glossopharyngeal nerve (cranial nerve IX). - Impulses carried through the afferent nerves specifically reach the ‘Solitary Tract’ in the medulla oblongata of the brain stem. The Solitary tract is a sensory nucleus that receives input from the baroreceptors, it regulates: 1- Cardio-acceleratory center: sends impulses to the heart through sympathetic fibers. 2- Cardio-inhibitory center: sends impulses to the heart through parasympathetic fibers. 3- Vasomotor center (VMC): transmits impulses downward through the spinal cord to almost all blood vessels. It is located bilaterally in the reticular substance of the medulla and the lower third of the pons. The VMC is composed of 3 areas: a- Sensory area: impulses go to the sensory area, then either to the vasoconstrictor area or the vasodilator area. b- Vasoconstrictor area sends impulses to the vessels through sympathetic fibers. c- Vasodilator area: sends its impulses upward inhibiting the vasoconstrictor area to cause vasodilation rather than sending impulses directly to vessels.  Solitary Tract upon stimulation (increased firing rates in case of increased BP) causes: 1- Activation of the Cardo-inhibitory center and the vasodilator area. 2- Inhibition of the Cardio-acceleratory center and vasoconstrictor area.  The opposite occurs in a reduced firing rate due to decreased BP. 4|Page Baroreflex Mechanism: Recall: factors affecting the pressure mainly are cardiac output and total peripheral resistance according to the formula: MAP = CO x TPR A- In the case of increased Blood Pressure: Arterial stretching increase due to increased BP → increased afferent firing rates from baroreceptors → stimulates the ‘Solitary Tract’ leading to: 1- Activating cardio-inhibitory center while inhibiting the cardio-acceleratory center: In other words: increased PNS with decreased SNS effects.  ↓ Heart rate and Stroke volume → ↓ Cardiac output → ↓ BP 2- Inhibiting vasoconstrictor area while activating the vasodilator area:  Suppression of the vasoconstrictor area → vasodilation → ↓ TPR → ↓ BP B- In the case of decreased Blood Pressure: ‘the opposite happens’ Arterial stretching decrease due to decreased BP → decreased afferent firing rates from baroreceptors → inhibits the Solitary Tract leading to: 1- Inhibiting cardio-inhibitory center while activating the cardio-acceleratory center:  ↑ Heart rate and Stroke volume → ↑ Cardiac output → ↑ BP 2- Activating vasoconstrictor area while inhibiting the vasodilator area:  No suppression of the vasoconstrictor area → vasoconstriction → ↑TPR → ↑BP 5|Page Response of the Baroreceptors to Arterial Pressure Curve: - This figure plots arterial pressure vs. the number of impulses from the carotid sinus baroreceptors. ΔI = change in impulse ΔP = change in pressure - Baroreceptors respond to pressure changes in the range from 60-180mmHg. They work best (most sensitive) around the normal MAP (almost 100mmHg).  Any small change above or below 100mmHg leads to a large change in the number of impulses to effectively maintain normal BP.  High-pressure baroreceptors are not sensitive to very low BP (<60 mmHg). Other receptors (low-pressure baroreceptors) can work at <60mmHg which will be discussed later.  Baroreflex response to very high BP (>180 mmHg) is diminished; i.e. there isn't that much change in the number of impulses. Side Notes: 1- The previous curve looks like a buffer curve. A buffer resists any change in pH and brings it back to normal. Any buffer works best around its pKa. We consider baroreceptors buffers for blood pressure; they resist any change in pressure and work best around ≈100mmHg. 2- Baroreceptors in the aortic arch are less sensitive than in the Carotid sinus. Carotid sinus baroreceptors are responsive to both increased or decreased in pressure, while aortic arch baroreceptors are mostly responsive to increased pressure only. Functions of the Baroreceptors: 1- They oppose either increases or decreases in arterial pressure thereby reducing daily variations in arterial pressure maintaining the BP around 100mmHg most of the time. When we denervate baroreceptors (i.e. cut their afferent nerves), variations in pressure will be more exaggerated, a lot less or a lot more than normal, since the baroreflex responses are lost. They are unimportant in long term control of arterial pressure because the baroreceptors adapt and have a low gain (gain corresponds to the effectiveness with which the baroreflex is able to maintain constant conditions; 𝐺𝑎𝑖𝑛 = 𝐶𝑜𝑟𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝐸𝑟𝑟𝑜𝑟 ). 6|Page 2- Maintain relatively constant pressure despite changes in body posture. Upon changing positions from lying/sitting down to standing up, blood pools in the lower limb due to gravity causing a drop in BP. This sudden drop in blood pressure can cause dizziness or lightheadedness, and maybe even fainting. If baroreceptors are working normally, they sense this change within seconds and raise the pressure through sympathetic responses to prevent sudden hypotension. This fast baroreflex response can be diminished with medications that block sympathetic responses (e.g. β-blockers) and in older people. Note: elders feel dizzier after getting up than young people can be because of atherosclerosis of arteries. Atherosclerotic arteries are less stretchable (less compliant) reducing baroreceptors reflex to changes in pressure. 2- Low-Pressure Baroreceptors - Low-pressure receptors are baroreceptors located in low-pressure areas: large systemic veins, in the pulmonary arteries, in the walls of the Rt. atrium, and Rt. ventricles. - They are also called volume receptors; because these receptors respond to changes in the wall tension (stretch), which is proportional to the filling state of the low-pressure side of circulation (<60mmHg). Thus, they respond mostly to changes in volume rather than pressure and are involved with the regulation of blood volume. Low-Pressure Baroreflexes: Atrial volume receptors are baroreceptors found in the atria that are sensitive to low pressure. Increased blood volume stretches the atria causing activation of Atrial volume receptors which causes: 1- Decreased ADH (vasopressin) production. 2- Decreased sympathetic drive to the kidneys. Note: ADH causes vasoconstriction and increased water reabsorption. 7|Page A- Atrial-hypothalamic reflex In the case of increased volume: Atrial volume receptors are activated when detecting increased blood volume (increased pressure) in the atria. Signals are transmitted from the receptors to the hypothalamus inhibiting ADH production causing: a- Decreased renal water reabsorption. b- Peripheral vasodilation.  This decreases the blood volume and blood pressure back to normal. In the case of decreased volume: The hypothalamus, in turn, increases the production of ADH (vasopressin) causing: a- Increased renal water reabsorption. b- Peripheral vasoconstriction.  This increases the blood volume and blood pressure back to normal. B- Atrial-renal reflex In the case of increased blood volume: Atrial volume receptors upon stimulation can also cause decreased sympathetic drive to the kidneys:  Afferent renal arterioles vasodilation → increased glomerular filtration rate → increased urine formation → decreased blood volume (towards normal). In the case of decreased blood volume: Increased sympathetic drive to the kidneys:  Afferent renal arterioles vasoconstriction → decreased glomerular filtration rate → decreased urine formation → increased blood volume (towards normal). 8|Page 3- Chemoreceptors - Chemoreceptors are sensors that detect CO2, O2, and pH concentration changes in the blood. They are important for maintaining arterial blood pO2, pCO2, and pH within appropriate physiological ranges. - They are highly vascularized (they have very high blood flow), and sensitive to O2, CO2 and H+ concentrations in the interstitial fluid around them, but because they are highly vascularized, we can say that they are sensitive to arterial concentrations as well. - There are two types of Chemoreceptors according to their location: 1- Central (medulla): sensitive mainly to changes in pH and pCO2. 2- Peripheral (in the carotid body ear the carotid bifurcation and aortic arch): sensitive mainly to changes in pO2. ‘which we will discuss mainly’ 𝚫 𝐏𝐫𝐞𝐬𝐬𝐮𝐫𝐞 - From the formula: 𝐅𝐥𝐨𝐰 = constant resistance. - Peripheral chemoreceptors are sensitive to oxygen lack, CO2 excess, or H+ ion excess which usually happens due to decreased blood flow (decreased BP). Activation of chemoreceptors receptors results in stimulating the vasoconstrictor area; however, chemoreceptors are not stimulated until the pressure falls below 80mmHg. - Decreased BP (decreased blood flow) causes decreased O2 concentration and increased CO2 and H+ concentrations (↓ pH). This stimulates chemoreceptors causing: 𝚫 𝐑𝐞𝐬𝐢𝐬𝐭𝐚𝐧𝐜𝐞 , flow is directly proportional to the pressure with a- Stimulated cardio-acceleratory center and inhibited cardio-inhibitory center. b- Stimulated vasoconstrictor area.  Increased cardiac output (↑ HR and contractility) and blood pressure, restoring the blood’s flow and chemicals’ concentration back to normal. - Increased BP (increased blood flow) causes increased O2 concentration and decreased CO2 and H+ concentrations (↑ pH). This causes the opposite in the cardiovascular centers and vasoconstrictor area leading to:  Decreased cardiac output and pressure. 9|Page Additional picture from the slides which summarizes baroreceptors reflexes: 10 | P a g e