RAK B.P. Lecture Notes PDF

The arterial blood pressure  Objectives  At the end of this lecture, the students will be able to:  Evaluate the very rapid and less rapid mechanisms that are involved in regulation of arterial blood pressure, including neural and hormonal mechanisms.  The arterial bl...

The arterial blood pressure  Objectives  At the end of this lecture, the students will be able to:  Evaluate the very rapid and less rapid mechanisms that are involved in regulation of arterial blood pressure, including neural and hormonal mechanisms.  The arterial blood pressure  = The force exerted by the blood on the wall of the blood vessel  Systolic blood pressure:  It is the maximum pressure in the blood vessel during the phase of systole.  It depends on the COP (pumping of blood) and the elasticity in the vessel.  Diastolic blood pressure  It is the minimum blood pressure during the phase of diastole.  It is not related to the pumping of blood.  It depends on the peripheral resistance  Systolic blood pressure  The normal systolic pressure is lower than 120 mmHg (up to 90 mmHg)  Diastolic blood pressure  The normal diastolic pressure is lower than 80 mmHg (up to 60 mmHg) The American Heart Association  Pulse pressure  It is the difference between systolic and diastolic pressures.  E.g., for a B.P. 110/70, Pulse pressure= 110 – 70 = 40 mmHg. Mean systemic arterial blood pressure  It is the average pressure in the arteries during the cardiac cycle.  Mean SAP = Diastolic pressure + 1/3 pulse pressure= 70 + 1/3 (40) =about 83 mmHg  Quiz  The B.P. of a 45-year-old worker is 150/60 mmHg.  The systolic= ……………………………………….  The diastolic= ……………………………………..  The pulse pressure= …………………………….  The mean arterial pressure= …………………  Mean systemic arterial blood pressure  The mean systemic arterial pressure (MSAP) drives blood to the tissues throughout the cardiac cycle. Physiological Variations of Arterial Blood Pressure 1- Age  2-Sex  Before menopause, females have lower B.P. than males  After Menopause: B.P. in females often increases  3-Effect of gravity  -/+ 0.77 mmHg (from mean B.P.) for every cm above or below the level of the heart.  4- Exercise. - Generally increases systolic and diastolic B.P. - Hard isotonic exercise decreases diastolic B.P.  5- Respiration.  Inspiration = Reduction ≈ 10 mmHg  Expiration = (the reverse)  Pulsus paradoxus= Reduction of systolic > 10 mmHg  6- Meals. Because of shifting of blood to GIT= some people develop postprandial hypotension Factors that determine the arterial blood pressure  Factors that determine the arterial blood pressure  From the equation: F= ∆P/R COP =Mean SAP/PR Mean SAP = COP x PR  Factors that determine the arterial blood pressure  i.e. the determinants: COP (stroke volume & heart rate) Determines the systolic > diastolic Totalperipheral resistance (radius of vessels & blood viscosity) Determines the diastolic > systolic MECHANISMS OF B.P. REGULATION Mechanisms of B.P. Regulation 1- Very rapid mechanisms: Act within seconds Neural Mechanisms of B.P. Regulation 2- Less rapid mechanisms Act within minutes Hormonal Mechanisms of B.P. Regulation 3- Long term mechanisms Act within hours- days Raise the blood volume & therefore the B.P.  Very Rapid Mechanisms  Baroreceptors reflex  Chemoreceptors reflex  CNS ischemic response  The Baroreceptors reflex  = Autonomic reflex  The Baroreceptors reflex  Receptors: Stretch receptors located in the carotid sinus and aortic sinus Stretch by high B.P.  The Baroreceptors reflex  Afferents: Glossopharyngeal & vagus nerves Conduct the inhibitory discharge of the baroreceptors to the medulla  The Baroreceptors reflex  Centers: Vasomotor center (VMC) Inhibited by the baroreceptors Cardio-stimulatory center (CC) Inhibited by the baroreceptors Cardio-inhibitory center (CIC) Stimulated by the baroreceptors (to inhibit the heart)  The Baroreceptors reflex  Efferents & effectors The sympathetic discharge to the heart & blood vessels is from CC & VMC is decreased The parasympathetic discharge to the heart from the CIC is increased  The Baroreceptors reflex  The overall effect in response to elevation of blood pressure: Decreased heart rate Decreased stroke volume Vasodilation = Decreased blood pressure back to normal  The Baroreceptors reflex  The opposite occurs when there is sudden drop in blood pressure  Quiz  Describe the immediate cardiovascular changes that occur following standing. Venous return is reduced, which leads to a decrease in cardiac stroke volume, a decline in arterial blood pressure, and an immediate decrease in blood ow to the brain.  The Chemoreceptors reflex  The peripheral chemoreceptors are found in the carotid and aortic bodies  The Chemoreceptors reflex  They are stimulated by :- Decrease in Pao2 Increase in Paco2 Increase H+ (Low pH) - e.g in Hypotension  They send excitatory impulses (through 9 &10) to the medullary centers.  They increase the sympathetic & decrease the parasympathetic discharge to the heart to increase the B.P.  The CNS ischemic response  A decrease in B.P.  = Decreases the blood supply to the brain  = Results in: Accumulation of CO2  The CNS ischemic response  The VMC is very sensitive to CO2  It increases the sympathetic discharge to the blood vessels: Vasoconstriction Increase in B.P  The CNS ischemic response  Mention the effect on the heart rate (Cushing’s sign)  Less Rapid Mechanisms 45 Hormonal vasoconstriction Vasopressin (antidiuretic hormone) Catecholamines Renin-angiotensin II Capillary fluid shift  Hormonal vasoconstriction  1- ADH (Vasopressin) Peptide hormone (9 aa) Synthesized in the hypothalamus & stored in the posterior pituitary gland Stimulated by: Hypotension Stress Others Causes vasoconstriction to increase the blood pressure Hormonal vasoconstriction 48 2- Catecholamines (norepinephrine and epinephrine) From the sympathetic neurons & the adrenal medulla Stimulus: - Hypotension (stress)  Catecholamines 49 Effects: - Vasoconstriction - Increased cardiac output  Hormonal vasoconstriction  3- Renin-Angiotensin II  Renin enzyme from Juxtaglomerular cells in the kidney  Stimulated by: Hyponatremia Renal ischemia Sympathetic stimulation  Converts angiotensinogen to angiotensin I  Angiotensin I is converted to angiotensin II by the converting enzyme  Angiotensin II is a potent vasoconstrictor= Increases B.P.  Capillary fluid shift  Less rapid mechanism (act within minutes to regulate the blood pressure)  Hypotension leads to movement of Fluids from the tissue spaces to the intravascular compartment  Capillary fluid shift  The fluid shift increases plasma volume= increases C.O.P. = increases arterial blood pressure.