Lecture 6: Regulation of Blood Pressure by Other Systems & Exercise

RAS

renin aldosterone system

RAS responds to

decreased renal perfusion pressure

Renin secreted by

kidneys (juxtaglomerular cells)

renin converts

angiotensinogen (from liver) to angiotensin I

AC (in lungs) converts angiotensin I →

angiotensin II (AT II)

angiotensin II stimulates

aldosterone → ↑ Na⁺/H₂O reabsorption → ↑ blood volume

angiotensin II Causes

vasoconstriction → ↑ TPR → ↑ BP

Angiotensin II directly enhances

Na⁺ reabsorption in the kidney

aldosterone

steroid hormone released from the adrenal cortex in response to angiotensin II

aldosterone acts on

distal tubule and collecting duct of the nephron

aldosterone promotes

Na⁺ reabsorption and K⁺ secretion

Water follows

sodium → ↑ ECF volume → ↑ blood pressure

Peripheral chemoreceptors

in carotid & aortic bodies

peripheral chemoreceptors detect

↓ O₂ (hypoxia)

↑ CO₂ (hypercapnia)

↓ pH (acidosis)

peripheral chemoreceptors stimulate

sympathetic outflow → vasoconstriction in kidneys, gut, muscle

central chemoreceptors

in medulla

central chemoreceptors respond to

↑ CO₂ and ↓ pH

central chemoreceptors promote

systemic vasoconstriction to prioritize cerebral perfusion

↑ Blood volume stretches atrial baroreceptors →

release of ANP

Atrial natriuretic peptide (ANP)

vasodilation, ↑ renal Na⁺ and water excretion,

↓ ADH and aldosterone secretion

net effect of ANP

↓ blood volume and pressure

Gravity causes blood pooling in the legs →

↓ venous return → ↓ CO → ↓ BP

blood pressure after blood pooling in legs normally corrected by

baroreceptor reflex

baroreceptor reflex

↑ HR, ↑ vasoconstriction

Orthostatic hypotension occurs when

Baroreflex is impaired & ↓ brain perfusion → dizziness, lightheadedness, syncope

In space flight, volume is

lost (no gravity pooling), and orthostatic response is compromised on return to Earth

hyperemia

Increased blood flow to a tissue due to increased metabolic activity or chemical signals

active hyperemia

in response to exercise

reactive hyperemia

following ischemia

Autoregulation of blood flow

local mechanisms maintain constant blood flow despite changes in systemic pressure

autoregulation found in

renal, cerebral, coronary, skeletal muscle, and pulmonary circulations

local vasodilators

CO₂, adenosine, H⁺, lactate, K⁺

effects of local regulation of blood flow

essential during exercise, ischemia, & hypoxia

local vasodilators relax

arterioles → ↑ perfusion to meet tissue demands

myogenic autoregulation

Vessels constrict in response to ↑ pressure; dilate with ↓ pressure

myogenic autoregulation maintains

constant flow despite pressure fluctuations

metabolic autoregulation triggered by

tissue metabolites: ↓ O₂, ↑ CO₂, H⁺, K⁺, adenosine

metabolic autoregulation promotes

vasodilation to increase blood supply during high metabolic activity

systemic regulation of systemic Blood flow maintains

MAP

systemic regulation of systemic Blood flow involves

baroreceptors, RAS, SNS

systemic regulation of systemic Blood flow affects

all organs simultaneously

local control (autoregulation) maintains

tissue perfusion

local control involves

local metabolites and myogenic tone

local control is

organ/tissue specific adjustments

Increased HR and SV→

↑ CO

Vasoconstriction (during exercise) in

gut, kidney, skin (initially)

vasodilation (during exercise) in

skeletal muscle (local metabolites override SNS)

exercise may cause

slight rise in MAP, TPR drops

during exercise, venous return is

enhanced via venoconstriction & muscle pump

Cerebral circulation supplied by

2 carotid & 2 vertebral arteries

cerebral circulation merges into

circle of Willis

cerebral circulation autoregulated via

myogenic, metabolic, & neurogenic

myogenic

pressure/ICP

metabolic

↑ CO₂, ↓ O₂, ↑ H⁺ → vasodilation

neurogenic

minor; protects small vessels from high pressure

blood brain barrier limits

entry of many systemic substances into brain tissue

blood brain barrier prevents

systemic vasoconstrictors/vasodilators from affecting cerebral vessels

blood brain barrier ensures

tight local control of cerebral perfusion

diastolic pressure drops with

decrease in total peripheral resistance