Mean Arterial Pressure Equation
MAP = CO x TPR = HR x SV x TPR
normally = 93.3 mmHg
MAP = 2/3 (DBP) + 1/3 (SBP)
What three systems regulate Mean Arterial Pressure?
- Baroreceptor Reflex (rapid)
- Renin-Angiotensin-Aldosterone System (RAAS)
- slower
- ADH (vasopressin) and ANP (slower)
Baroreceptor Classic Reflex Arc
Detector –> Afferent Neural Pathway –> Brain Stem –> Efferent Neural Pathway –> Effectors
- effectors correct deviation from set point
What are the two major baroreceptors and their nerves? What do they feed into?
- Carotid Sinus (Glossopharyngeal N./Sinus N. of Hering)
- Aortic Sinus (Vagus N./Aortic N.)
- lead to nucleus tractus solitarius (NTS)
- afferent neurons use GLUTAMATE
How do baroreceptors respond to changes in pressure?
Mechanoreceptors: respond to arterial stretch/relaxation
Chemoreceptors: respond to oxygen, carbon dioxide, and hydrogen
- inc. firing rate from inc. stretch
- dec. firing rate from dec. stretch
What are the 3 major control centers in the Brain for Arterial Pressure?
- Nucleus of Tractus Solitarius (NTS)
- receive/coordinate peripheral signals
- Dorsal Motor Nucleus and Nucleus Ambiguus
- parasympathetic activity
- Rostral Ventrolateral Medulla
- sympathetic activity
Which baroreceptor, and why, has a higher threshold for activation?
- Aortic Arch
- brain has lower threshold to protect it from brain damage if pressure becomes to high
- carotid sinus more sensitive to rate
Sympathetic Response to BP and 3 effects
- triggered by DECREASED baroreceptor firing
- influences heart muscles and SA node directly
Constrict arterioles/veins (alpha receptors)
Inc. HR and Contractility (beta1 receptors)
Kidney fluid retention and renin secretion
Parasympathetic Response to BP and 2 effects
- triggered by INCREASED baroreceptor firing
Decreased HR (Vagus N. to SA node: muscarinic) Indirect vasodilation of BVs (stimulate NO release)
Baroreceptor Adaptation
- baroreceptors able to adjust set-point to different conditions, such as Hypertension
EX: hypertension resets receptors to regulate pressure at a higher set point
“Getting Used to It”
Renin Role in RAAS (Renin-Angiotensin-Aldosterone System)
- renin (from kidney) released in response to LOW BP
- stimulated by B1 adrenergic receptors/SNS
- dec. salt at macula densa
- causes angiotensinogen –> angiotensin I
- angiotensin I –> angiotensin II (in lungs)
- secreted from Juxtaglomerular Cells in walls of renal afferent arterioles
Angiotensin II Role in RAAS (Renin-Angiotensin-Aldosterone System)
- causes aldosterone secretion from adrenal cortex
- sodium/water retention
- inc. blood vol., preload, stroke volume, CO, and BP
- stimulates secretion of antidiuretic hormone
- VASOPRESSIN (reduced urine production)
- global arteriole vasoconstriction
Antidiuretic Hormone (ADH)
aka “Vasopressin”
V1 = BV constriction, V2 = kidney fluid resorption
- increases TPR and water retention
Natriuretic Peptides (3) and their function
ANP (atrial), BNP (brain), CNP (C-type)
- increased secretion by excessive preload of atria and ventricles (too much blood volume)
- caused by arteriolar dilation; increases fluid loss and inhibits renin
- protects against overdilation or overstretching of cardiac chambers
Response to Hemorrhage (Dec. Blood Volume)
- dec. venous return, preload, stroke, volume, CO, MAP
- vascular function curve shifts left
Compensatory Response:
- dec. carotid firing = inc. HR, contractility, CO
- inc. TPR
- inc. epinephrine, ADH, renin, angiotensin II, aldost.
Posture Changes
- upright movement initiates muscle pumps that push blood upwards towards heart past valves in the limb
When no movement is occuring:
- venous return accumulates in lower limbs
- inc. venous/capillary hydrostatic pressure
- potential edema/hypotension
Reflexes will attempt to bring BP back to normal