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Flashcards in Introduction to Abnormal Blood Pressure Deck (39)
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Mean Arterial Pressure

MAP- determines blood flow through the organs; average pressure over time


Central venous pressure (CVP) = R atrial pressure (RAP); majority of time want it to be close to or at 0
If reduce CO by 50% but double SVR then the MAP remains the same

Systemic vascular resistance or peripheral vascular resistance = SVR


Factors Affecting Mean Arterial Pressure

MAP = (CO * SVR) + CVP

Local factors: factors produced by endothelium; affects SVR
Vascular Anatomy affects SVR

Neurohumoral Factors:
HR affects CO
Inotropy affects SV
Venous Compliance affects Preload
Renal Na+ and H2O Handling affects Blood Volume


MAP and Systolic/Diastolic Pressure Relationship

MAP = Pdias + 1/3(Psys-Pdias)
Thus, if normal 120/80:
MAP = 80 + 1/3(120-80) = 93mmHg

Closer to diastolic because during HR the heart cycle stays longer in diastole than systole
Not necessarily a normal value for MAP because changes throughout the system depending on where you measure it; at least 65mmHg to adequately perfuse the tissues
This is an average


Arterial (Aortic) Pulse Pressure

Calculated by Psys-Pdias

Determined by SV and compliance (acute vs. chronic)

Compliance – elasticity of vessel determines relationship between pressure and volume
As we age, the arteries harden and lose compliance
Long term increase in PP is caused by loss of compliance from disease or age; elderly has a higher PP than someone in their 20s

More elastin fibers – less resistance
More collagen fibers – more resistance


Vascular Anatomy and Function

Aorta is a distributing vessel; it does not have a lot of constriction and very compliant

Large arteries are for distribution and don’t have a role in regulating pressure on a regular basis

Small arteries and arterioles do regulate pressure and are controlled by sympathetic system and have receptors for hormones and tissue factors that affect resistance

Capillaries: exchange vessels and not constricting or dilation because no smooth muscle

Veins and venules: collection of capacitance and are blood reservoirs


Anatomic Determinants of Arterial Pressure: Equation

Poiseuille’s Law: F = (change in P * r^4) / (viscosity * length)
Length and viscosity tend to remain constant, therefore radius(diameter) is the most important factor
Small changes in vessel diameter can lead to large changes in resistance

This equation only applies to a single vessel at a time, but be applied to any vessel
0.4 to 0.8 radius (increasing) which decreases the resistance and flow increases by a factor of 16
If you reduce the radius of renal arterial in kidney by 50% the resistance of that vessel will go up 16 fold


Series vs. Parallel Arrangement of Vasculature

Series – Rtotal = R1+ R2 + R3 + R4…..

Parallel – 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + 1/R4....


Distribution of BP

High BP from aorta and then decreases through system
As we move from heart the pressure decreases
Pressure and MAP will differ depending on where the measurement is being taken

Greatest pressure change is along small arteries and arterioles

2/3 of SVR is arteriolar resistance


Relationship Between Vessel Diameter, Cross-Sectional Area, MAP, and Velocity

Order: Elastic Arteries (aorta), Muscular Arteries, Arterioles, Capillaries, Venules, Veins, and Vena Cava

Vessel Diameter: greatest at elastic arteries then decreases then greatest again at vena cava

Average BP: greatest at elastic arteries then decreases

Total Cross Sectional Area: bell curve where capillaries are greatest

Velocity of Blood Flow: greatest at elastic arteries then decreases and then rises slightly starting at veins


The Windkessel Effect

Elasticity or Distensibility of the arteries
Dampens the pulsatile nature of blood flow
Responsible for continuous blood flow

Refers to elasticity of the arteries
Ability of aorta to dampen the pulse that is coming out
Only about 50% of blood immediately shoots through the aorta, but the other 50% is stored in the “pouch” of aorta and then the ejection of it is slow and continuous (dampens pulses) until next rapid ejection


Clinical Relevance of the Windkessel Effect

Increasing the stiffness of the aorta (reducing compliance)
Increases systolic pressure
Increases diastolic pressure, pulse pressure, and systolic velocity
Increases left ventricular afterload
Reduces subendocardial blood supply during diastole

Increase stiffness of aorta by decreasing compliance you increase pressure; heart must work harder to overcome the pressure
This graph looks at pulse and age


Vessels of Greatest Resistance

Resistance is greatest in the small arteries and arterioles making them primarily responsible for controlling arterial blood pressure and blood flow to organs


Distribution of Blood Volume

60-80% is normally within the venous vessels
Altered by changes in venous tone

Can affect MAP
Can lose about 20% of blood volume, but because of storage in veins it helps reduce the effects that result from the loss of volume since veins keep 60-65% of blood
Increase vasocontriction then decrease amount of blood in veins but increase in arterial system and vice versa
Gravity: has effect of where greatest blood volume is


Vascular Tone

Degree of vasoconstriction under a given physiologic state
Differs among organs

Factors that Detemine Constriction and Dilation:
Extrinsic: come from outside the blood system; neural and humoral
Intrinsic: come from endothelial or tissue directly surrounding the vessel; tissue metabolites, local hormones, myogenic, and endothelial factors


Tissue and Vascular Factors Affecting SVR and Arterial Pressure

The vasodilator theory: ↑ metabolism or ↓ O2/nutrients causes ↑ production of vasodilators

Adenosine is usually very low, but if hypoxic then ATP is being used and adenosine is increased
Increased metabolism increases CO2
Depolarization causes K+ is being lost and if very fast then can cause the pumps not to keep up and excess K+ causes hyperpolarization
Paracrine hormone: released from one cell and acts on nearby cells
Histamine and bradykinin (both cause vasodilation); bradykinin causes NO and prostacyclin release for this; ACE inhibition causes decrease in breakdown of bradykinin to allow for more vasodilation


Endothelial Factors

Nitric oxide (NO): vasodilator; released in response to shear stress created by blood flow and paracrine hormones
Chronic hypertension or atherosclerosis
Nitrate containing drugs

Endothelin: vasoconstrictor; increased in injured vessels; can be a vasodilator or vasoconstrictor depending on which receptor it interacts with; regulatory mechanism

Prostacyclin (PGI2): vasodilator; synthesis stimulated by adenosine and NO; inhibits platelet aggregation


Myogenic Mechanisms

Stretch induced contraction
High arterial pressure stretches vessel – reactive vasoconstriction
Low arterial pressure reduces stretch – reactive vasodilation
Stretch induced vascular depolarization


Brain Regions and Autonomic Control

Cerebral Cortex: higher center influences; has effect on BP; stress, anxiety, anger changes BP through hypothalamus by modulating control

Hypothalamus: integrative control

Medulla: location of sympathetic and parasympathetic (vagal) neurons and receives sensory input; cell bodies in sympathetic and para are located; unconscious control center; receiving sensory input from baroreceptors

Spinal cord: sympathetic efferents

*All are working together


Autonomic Nerves

Increases resistance to flow
Increases heart rate and contractility
Vasoconstrictor tone
Effects mediated by norepinephrine

Decreases heart rate
Effects mediated by acetylcholine


Effect of Sympathetic Activation on Arterial Pressure

↑ SVR (vasoconstriction): α-adrenergic receptors coupled to Gq

↑ CO (HR x SV):
β1-adrenergic receptors coupled to Gs
↑ blood volume (RAA mediated)
↑ cardiac preload

Activation of sympathetic nerves:
↑ pressure during physical activity
Heart failure - reflex response via baroreceptors
Shock (circulatory and cardiogenic): decrease in BP to stimulate reflexes to increase pressure


Effects of Parasympathetic Activation on Arterial Pressure

Direct vasodilation:
Only some tissues (erectile tissue)
Ach binds to muscarinic receptors
Results in NO production

Indirect actions:
↓ HR leading to ↓CO
Stimulates production of vasodilator substances, eg. Bradykinin

Does not play a significant role in regulation of SVR and arterial pressure!
Vasovagal syncope: fear or emotion causes activation to get bradycardia and decreased sympathetic tone and fainting


Summary of Sympathetic vs. Parasympathetic Activation

Chronotropy = rate; sympathetic +, parasympathetic -
Ionotropy = contractility; sympathetic +, parasympathetic -
Dromotropy = conduction velocity; sympathetic +, parasympathetic -

Vessels = vasoconstriction:
Resistance = arteries, arterioles; sympathetic +, parasympathetic only certain organs
Capacitance = veins, venules; sympathetic +, parasympathetic none


Circulating Catecholamines

Primary: Adrenal medulla
Epinephrine – 80%
Norepinephrine – 20 %
Secondary: Sympathetic nerves

Cardiac effects:
Increased cardiac output
Cardiac β-adrenergic receptors
Increased blood volume (renal β1-adrenergic receptors) and cardiac preload

E has higher affinity for beta receptors so more function in the heart
NE has higher affinity for alpha so majority of actions are in blood vessels
Have affinity for beta and alpha for both, but just has one that it favors


Catecholamines: Vascular Effects and Increased Activation

Vascular effects:
Vasodilation (β2 receptors)
Vasoconstriction (α1 & α2 receptors)

Circulating catecholamines are increased by:
Increased physical activity
Heart failure
Shock (circulatory and cardiogenic)
Pheochromocytoma: benign tumor of adrenal gland
NOTE: These are the same events that increase SYMPATHETIC activity!


Baroreceptor Regulation

Provide feedback regulation of autonomic nerves

Carotid Sinus: innervated by sinus (Hering’s) nerve – joins the Glossopharyngeal nerve (CN IX)

Aortic Arch Receptors: innervated by the Vagus nerve (CN X)


Baroreceptor Firing

Baroreceptor firing is increased by:
Increased mean arterial pressure
Increased arterial pulse pressure

Increase in pressure in aorta causes aorta to stretch and cause firing or baroreceptor afferents to medulla; baroreceptors are constantly giving feedback
Firing only during increases
As rate of pressure changes get greater firing

Threshold for carotid sinus receptor activation is ~60 mm Hg. Maximal firing at 180 mm Hg. Maximal sensitivity – normal MAP (not fixed! Can change) = 95mmHg so at 50% firing rate, but remember MAP changes and is not fixed; aortic arch has a higher threshold for activation and less sensitive to changes


Medullary Connections

Afferents synapse in the nucleus tractus solitarius (NTS) in medulla, which have interneurons

Interneurons project to cardiovascular centers:
Enhances vagal efferent activity
Decreases sympathetic efferent activity
Say increase in pressure you activate inhibitory interneurons for sympathetic NS and decrease sympathetic efferent activity to overall decrease the pressure
Enhance vagal activity to by decreasing HR for example to decrease pressure

Hypothalamus modulates the medullary centers


Baroreceptors: Decrease in MAP

Fall in BP you decrease firing rate of baroreceptors and inhibiting parasympathetic NS and increasing sympathetic NS

A drop in aortic pressure initiates a baroreceptor reflex. Resistance vessels constrict to limit outflow from the arterial system.
Venoconstriction/ venous system capacity is reduced and blood is squeezed out.
LV preload increases: venous blood arrives back at the heart and EDP increases.
Inotropy increases: myocardium contracts with increased force
HR increases to move blood returning from veins back into arterial system
MAP is restored by increasing CO and SVR


Baroreceptor Resetting and Adaptation

Rapidly adapting; 1-2 days: short term regulation

In hypertension, baroreceptor firing curve shifts to the right (less sensitive)

If increase MAP for a couple of days, this will reset the baroreceptors
Only useful for short term regulation
As we increase BP to HTN and maintain the HTN it will increase threshold for baroreceptor firing


Baroreceptor Function Can be Altered By

Exercise: medullary and hypothalamic centers modulate autonomic responses; reset arterial pressure to higher level
Chronic hypertension
Chronic heart failure
Arterial vascular disease: carotid arteries become less compliant (stretch less)