Unit 2 - Blood Flow & Blood Pressure PART E Flashcards
How does the body homeostatically regulate BP?
by regulating mean arterial pressure (MAP)
Arterial pressure
is a balance b/t BF into the arteries & BF out of the arteries
If flow in exceeds flow out…
blood volume in the arteries ↑’s & MAP ↑’s
If flow out exceeds flow in…
volume ↓’s & MAP falls
What will happen if MAP is too low or too high?
If MAP is too LOW, blood does not perfuse organs fast enough to clear wastes and bring in nutrients/oxygen.
If MAP is too HIGH, perfusion is too fast to allow for adequate exchange between the blood and the tissues.
MAP =
CO x TPR
What can MAP be regulated by controlling?
- Cardiac Output
- TPR (Total Peripheral Resistance)
- Blood Volume
MAP can be regulated by controlling:
Cardiac output; how?
- By increasing or decreasing HEART RATE
- By increasing or decreasing the FORCE OF CONTRACTION TO ALTER STROKE VOLUME.
- By increasing or decreasing VENOUS RETURN.
MAP can be regulated by controlling:
TPR (Total Peripheral Resistance); how?
- By increasing or decreasing VASOCONSTRICTION/VASODILATION (arteriolar radius)
What is BF out of the arteries primarily influenced by?
TPR
TPR (Total Peripheral Resistance)
resistance to flow offered by the arterioles
MAP can be regulated by controlling:
Blood Volume; how?
- Increasing/decreasing blood volume increases/decreases venous return, increases/decreases stroke volume.
- Also increases MAP directly, by increasing the hydrostatic pressure of the blood pressing harder against the walls of the vessels).
(if BV ↑’s, BP ↑’s & if BV ↓’s, BP ↓’s)
- small ↑’s in BV occur throughout the day due to ingestion of food & liquids, but usually don’t create long-lasting changes in BP b/c of homeostatic compensations
- if BV ↓’s, the kidneys cannot restore the lost fluid (can only conserve BV & thereby prevent further ↓’s in BV)
What happens if CO ↑’s?
the heart pumps MORE blood into the arteries per unit time
What happens if resistance to BF out of the arteries doesn’t change?
flow into the arteries is GREATER than flow out, BV in the arteries ↑’s, & arterial BP ↑’s
What happens if CO remains unchanged but peripheral resistance ↑’s?
- flow into arteries is UNCHANGED but flow out is ↓’ed
- blood again accumulates in the arteries, & the arterial pressure again ↑’s
BF into the aorta is = to…
the CO of the LV
Most cases of hypertension are believed to be caused by…
↑’ed PR without changes in CO
What is MAP?
Mean arterial pressure is a function of CO & PR
- illustrates mass balance: the volume of blood in the arteries is determined by input (CO) & flow out (altered by changing PR)
- as arterial volume ↑’s, pressure ↑’s
Specific Control Mechanisms…
use negative feedback to keep MAP constant
What are the specific control mechanisms that use negative feedback to keep MAP constant?
- Baroreceptor Reflex Control (for short-term regulation of MAP)
- Blood Volume Control (for long-term regulation of MAP)
Baroreceptor Reflex Control (for short-term regulation of MAP)
An autonomic reflex that is the single most import mechanism for short term regulation of MAP. Causes changes in TPR and CO.
What is the Baroreceptor Reflex Control (for short-term regulation of MAP) stimulus?
Stimulus: changes in blood pressure (MAP) and pulse pressure
What is the Baroreceptor Reflex Control (for short-term regulation of MAP) receptors?
Receptors: Mechanical stretch receptors (BAROreceptors) in aortic arch and carotid sinuses (where they continuously monitor the pressure of blood flowing to the brain (carotid baroreceptors) & to the body (aortic baroreceptors))
Describe Baroreceptors discharge rate
Their discharge rate is directly proportional to MAP within physiological limits (so as MAP increases, so does the frequency of action potentials in the baroreceptor cells).
- when ↑’ed BP in the arteries stretches the baroreceptor membrane, the firing rate of the receptor ↑’s
- if BP falls, the firing rate of the receptor ↓’s
What do baroreceptors show?
These receptors show adaptation. (if BP changes, the freq. of AP’s traveling from the baroreceptors to the medullary CVCC changes - the response of the baroreceptor reflex is quite rapid) Rate of firing will initially increase, but prolonged exposure to higher/lower MAP causes them to adapt and return their discharge rate to normal levels. This is one reason why the baroreceptor reflex works best for short-term regulation.
_________ project from the baroreceptors to the cardiovascular CONTROL CENTER IN THE MEDULLA (CVCC, integration center).
Sensory nerves (input signal)
_______ initiates the the appropriate CV adjustments by…
MEDULLARY CVCC
sending signals (output) through PARASYMPATHETIC (VAGUS) and SYMPATHETIC nerves (ANS), to the HEART and BLOOD VESSELS (effectors).
_____ bring about a response that is opposite to initial stimulus.
Effectors
At any given MAP, if pulse pressure increases, firing rate…
ALSO INCREASES
Normal resting value of MAP =
~93 mmHg
Medullary Cardiovascular Control Center (CVCC) functions:
primary function: is to ensure adequate BF to the brain & heart by maintaining sufficient MAP
- also, receives input from other parts of the brain & has the ability to alter function in 1 or 2 organs or tissues while leaving others unaffected
therefore, CVCC is constantly monitoring MAP & adjusting its output as req. to maintain homeostasis
Is the baroreceptor reflex functioning all the time & is it an all-or-none response?
functioning all the time & is NOT an all-or-none response
- b/c a change in BP can result in a change in both CO & R or a change in only 1 of the 2 variables
Heart function is regulated by antagonistic control. Explain
- ↑’ed sym. activity, ↑’s HR, shortens conduction time through the AV node, & enhances the force of myocardial contraction
- ↑’ed para. activity, slows HR but has only a small effect on ventricular contraction
Baroreceptors ↑ their firing rate as BP…
↑’s, activating the medullary CVCC
- in response, the CVCC ↑’s para. activity & ↓’s sym. activity to slow down the heart & dilate arterioles
When HR falls, CO…
falls
- in the vasculature, ↓’ed sym. activity causes dilation of arterioles, lowering their R & allowing more blood to flow out of the arteries
MAP ∝ CO x R
therefore, combo of ↓CO & ↓R; ____ the MAP
LOWERS
What is an ex of Baroreceptor Reflex Control?
Example: Hemorrhage which causes a decrease in MAP and blood volume (notes page 29)
During hemorrhage, the immediate baroreceptor reflex only…
In order to restore normal MAP,
…brings MAP back towards normal.
…the lost blood volume needs to be replaced by a shift in fluid balance (or a blood infusion if there is substantial loss of volume).
Orthostatic hypotension is the
decrease in BP upon standing
Explain how the Orthostatic hypotension triggers the baroreceptor reflex:
result of this: ↑’ed CO & ↑’ed R, which together ↑ MAP & bring it back to normal within 2 heartbeats
- skeletal muscle pump also contributes to the recovery by enhancing VR when ab & leg muscles contract to maintain an upright position
Describe when the baroreceptor reflex isn’t always effective
ex: bed rest or in 0 gravity conditions of space flights
- blood from lower extremities is distributed evenly throughout the body rather than pooled in the lower extremities
- when they get out of bed or return to earth: orthostatic hypotension occurs, & the baroreceptors attempt to compensate
- but the CV is unable to restore normal pressure b/c of the loss of BV
- therefore, the individual become light-headed or even faint from reduced delivery of O2 to the brain
What is an excellent ex of the integration of organ system function?
together, the heart & kidneys play a major role in maintaining homeostasis of body fluids
Specific Control Mechanisms: (use negative feedback to keep MAP constant)
2. Blood Volume Control (for long-term regulation of MAP)
- Changes in blood volume increase/decrease MAP by influencing venous pressure, venous return, EDV and SV, all of which affect cardiac output.
- Since MAP = CO x TPR, any change in CO due to a change in volume will change MAP.
- Blood volume also influences MAP directly by increasing the hydrostatic pressure on the walls of
the arteries. - Blood volume is regulated by hormones secreted by the brain, kidneys and heart. These hormones influence fluid intake and urine output.
What are the 3 kinds of blood volume control (for long-term regulation of MAP)?
a. Vasopressin (antidiuretichormone, ADH)– from posterior pituitary.
b. Renin – from kidney
c. Atrial Natriuretic Peptide (ANP) – from heart.
Vasopressin (antidiuretic hormone, ADH) – from posterior pituitary, effect on Blood Volume Control
Increases H2O REABSORPTION by the kidneys (diuretics increase the volume of urine produced and decrease blood volume, so ADH does the opposite)
Renin – from kidney (effect on Blood Volume Control)
i. Triggers the conversion of plasma angiotensinogen into angiotensin II
ii. Stimulates ADH release, so increases H2O reabsorption by the kidneys.
Atrial Natriuretic Peptide (ANP) – from heart. (effect on Blood Volume Control)
i. Increases Na+ and H2O excretion by the kidneys
ii. Inhibits vasopressin secretion
iii. Inhibits renin secretion (and production of angiotensinII).
iv. Result: all together these responses increase urine out put and reduce thirst
Why do people develop hypertension (or hypotension)?
- Hypertension/Hypotension are failures of pressure homeostasis.
- Slow changes in MAP are accompanied by ‘resetting’ of the
baroreceptor (receptor adaptation) and CV center responses - still opposes minute-to-minute changes in pressure, but
baroreceptors are now activated at a higher (or lower) pressure
level. (i.e. the set point for MAP has changed, so receptors
respond only to values that are higher or lower than the new set point).
Hypertension
typically caused by a chronic increase in TPR
Primary Hypertension
- Causes: obesity, stress, cholesterol, smoking, genetics, Na+ retention, and immune system responses
- Initially leads to ventricular HYPERTROPHY, however, myocardial contractile function diminishes over time – eventually leads to congestive heart failure and edema.
- Also increases risk of atherosclerosis, heart attack, kidney damage, and stroke
Secondary Hypertension
increase in MAP arising from other conditions (e.g. pregnancy)
Treatments for Primary Hypertension
a. Exercise, ↓’d Na+ intake, and weight loss lower resting MAP
b. Ca2+ CHANNEL BLOCKERS
c. Diuretics
d. b-adrenergic receptor blockers
e. Angiotensin-converting enzyme (ACE) inhibitors
How do Ca2+ channel blockers treat Primary Hypertension?
promote VASODILATION of vascular smooth muscle; decreases heart contractility and the rate of SA node depolarization
How do Diuretics treat Primary Hypertension?
increase urinary excretion of Na+ and H2O; decreases CO with little effect on TPR
How do b-adrenergic receptor blockers treat Primary Hypertension?
target b1 receptors; decreases NE and E stimulation of CO
How do Angiotensin-converting enzyme (ACE) inhibitors treat Primary Hypertension?
blocks production of angiotensin II; decreases TPR
Normal resting value (set point) MAP = ~93 mmHg.
Hypertension: Set point changed to MAP of…
~125 mmHg
5 key points about movement of material across the capillary wall:
- Protein channels (intracellular channels) in endothelial cell membranes and intercellular pores between adjacent cells (paracellular pathway)
- Serves two purposes
- Starling forces
- Net Filtration Pressure
- Net colloid osmotic pressure
Movement of material across the capillary wall.
- Protein channels (_____) in endothelial cell membranes and (______) between adjacent cells (paracellular pathway) are:
INTRACELLULAR CHANNELS
INTERCELLULAR PORES
Movement of material across the capillary wall.
- Protein channels (intracellular channels) in endothelial cell membranes and intercellular pores between adjacent cells (paracellular pathway) are:
a. EXCHANGE SITES FOR H2O, Na+, K+, GLUCOSE, AMINO ACIDS
b. mostly impermeable to macromolecules:
i. E.g. PLASMA PROTEINS (e.g. ALBUMIN) are so large they remain in capillary
ii. Any larger proteins that can be exchanged do so using transcytosis
Transcytosis =
endocytosis into endothelial cell from plasma/ISF, transport across cell, ending with exocytosis from cell into the ISF/plasma.
Tissues with a ↑ metabolic rate req. MORE O2 & nutrients, therefore…
have MORE capillaries per unit area
- subcutaneous tissue & cartilage have the LOWEST capillary density
- muscles & glands have the HIGHEST
Movement of material across the capillary wall.
- Serves two purposes
a. Exchange of material between the blood and the cells.
b. Maintain fluid balance between plasma and interstitial fluid.
Movement of material across the capillary wall.
- Serves two purposes
a. Exchange of material between the blood and the cells.
- Rapid DIFFUSION of small solutes across capillary walls –> i.e. down PARTIAL PRESSURE, electrochemical, and concentration gradients
- Certain proteins are selectively transported across endothelial cells by a slow, energy-requiring process (TRANSCYTOSIS)
Movement of material across the capillary wall.
2. Serves two purposes
b. Maintain fluid balance between plasma and interstitial fluid.
- Distribution of ECF between plasma and ISF in a state of dynamic equilibrium due to FILTRATION and ABSORPTION of fluid by capillaries
- Via the BULK FLOW of protein-free plasma through channels and pores driven by HYDROSTATIC and COLLOID OSMOTIC PRESSURE gradients (termed ‘STARLING FORCES’) that augment diffusion processes
Filtration
the fluid movement, if the direction of flow is OUT of the capillaries
- caused by hydrostatic pressure that forces fluid out of the capillary through leaky cell junctions
Analogy: think of garden “soaker” hoses whose perforated walls allow water to ooze out
Absorption
the fluid movement, if the direction of bulk flow it INTO the capillaries
Bulk Flow
mass movement of fluid as the result of hydrostatic or osmotic pressure gradients
Hydrostatic Pressure Gradients
lateral pressure component of BF that pushes fluid out through the capillary pores
Colloid Osmotic Pressure Gradients
a measure of the osmotic pressure created by proteins
- higher in the plasma (πcap = 25 mm Hg) than in the interstitial fluid (πIF = mm Hg)
- therefore, the osmotic gradient favours water movement by osmosis from the interstitial fluid into the plasma
- constant along the length of the capillary, π = 25 mm Hg
Osmotic pressure
is determined by solute concentration of a compartment
- the main solute diff. b/t plasma & interstitial fluid is due to proteins
- the osmotic pressure created by the presence of these proteins is known as colloid osmotic pressure (π)
Movement of material across the capillary wall.
3. Starling forces
a. PC = capillary hydrostatic pressure (HP)
b. PI = interstitial fluid hydrostatic pressure
c. πC = plasma colloid-osmotic pressure
d. πI = colloid-osmotic pressure of interstitial fluid
PC = capillary hydrostatic pressure (HP)
- HP inside capillary favouring FILTRATION of plasma
- declines as blood moves from arteriole (~37 mm Hg) to venule side (~17 mm Hg) of capillary (as energy is lost to friction)
PI = interstitial fluid hydrostatic pressure
- HP outside capillary favouring ABSORPTION of ISF
- ~1 mm Hg
πC = plasma colloid-osmotic pressure
- osmotic pressure within capillary due to NONPENETRATING solutes that favours ABSORPTION of interstitial fluid
- ~25 mm Hg
πI = colloid-osmotic pressure of interstitial fluid
- osmotic pressure due to nonpenetrating solutes within the
ISF that favours plasma filtration - ~0 mm Hg –> means that water movement due to hydrostatic pressure is directed out of the capillary, with the pressure gradient decreasing from the arterial end to the venous end
Movement of material across the capillary wall.
4. Net Filtration Pressure
a. PC - PI = AT ANY GIVEN POINT ALONG THE CAPILLARY.
- Force favouring plasma FILTRATION along entire length of
capillary
- = 37 – 1 = 36 mmHg
Movement of material across the capillary wall.
- Net colloid osmotic pressure
a. πC – πI.
- force favouring ABSORPTION of ISF along entire length of capillary
- = 25 – 0 = 25 mmHg
Net fluid movement at any given point in capillary:
= Net P – Net π
= 36 mmHg – 25 mmHg = +11 mmHg (filtration)
= 16 mmHg – 25 mmHg = -9 mmHg (absorption)
Positive values indicate filtration, negative values indicate absorption
Positive values indicate _____, negative values indicate ______
FILTRATION
ABSORPTION
If the point at which filtration =’s absorption occurred in the middle of the capillary…
there would be NO net movement of fluid
- all volume that was filtered at the arterial end would be absorbed at the venous end
- however, filtration is usually greater than absorption, resulting in bulk flow of fluid out of the interstitial space
If filtered fluid couldn’t be returned to the plasma, the blood would…
turn into a sludge of blood cells & proteins
