KNES 323 - Blood Pressure

lectures 20 + 21

Blood pressure is an important ___________ pressure measure in the body that is defined as:

hydrostatic, the force exerted by blood upon the walls of the blood vessels and on the chambers of the heart

Arterial blood pressure in the larger vessels consists of several distinct components. What are they? (4)

• systolic BP (top number)

• diastolic BP (bottom number)

• pulse pressure (difference between systolic + diastolic BP)

• mean arterial pressure

Systolic pressure reflects the ____________ resulting from a ________________.

Diastolic pressure reflects the ____________ during _______________.

peak arterial pressure, ventricular contraction

arterial pressure, ventricular relaxation

Pulse pressure should be at least ______ of the systolic pressure.

25%

What does a low pulse pressure indicate? What does a high pulse pressure indicate?

low (narrow): may indicate a low stroke volume as seen in congestive heart failure, stenosis of the aortic valve, etc. 

high (wide): common in healthy people during + following strenuous exercise

How is mean arterial pressure calculated? Why?

1) adding 1/3 of the pulse pressure to the diastolic pressure or

2) adding 1/3 of the systole pressure to 2/3 of the diastole pressure 

the cardiac cycle is roughly spent as 1/3 in systole and 2/3 in diastole

What are the 5 main variables that affect blood flow and blood pressure?

1. cardiac output

2. compliance

3. volume of the blood

4. viscosity of the blood

5. blood vessel length + diameter

Any factor increasing cardiac output will _____________ blood pressure. What are some of those factors?

Any factor decreasing cardiac output will ____________ blood pressure. What are some of those factors?

increase

• sympathetic stimulation, epinephrine, norepinephrine, thyroid hormones, increased Ca2+ 

decrease

• parasympathetic stimulation, altered K+ levels, decreased Ca2+ 

What is compliance? Are veins or arteries more compliant? When is compliance reduced and what are the effects on blood flow?

• ability of a blood vessel to expand to accommodate increased content → a compliant vessel is able to expand with surges in blood flow without increased resistance or blood pressure

• veins are more compliant than arteries + therefore can hold more blood

• compliance is reduced when arteries are stiffened (hypertension) and resistance to blood flow is increased

When does blood volume decrease? When does it increase?

decrease: dehydration, vomiting, severe burns, diarrhea, bleeding

increase: retention of water + Na+

What are the 2 main determinants of blood viscosity? The viscosity of blood is directly proportional to _____________ and inversely proportional to _____________. 

formed elements + plasma proteins, resistance, flow

Vessel length is ______________ to its resistance. Why?

directly proportional

• longer vessels have greater resistance causing lower flow

• shorter vessels have lower resistance causing greater flow

How does blood vessel diameter affect resistance and blood pressure?

smaller diameter increases the amount of blood contacting the vessel walls, increasing both friction and resistance → increasing BP

What is Poiseuille’s Law?

resistance is inversely proportional to the radius of the vessel raised to the fourth power (R = 1/r⁴)

How does the skeletal muscle pump work?

• increases pressure in the veins through contraction of surrounding skeletal muscles

• leg muscles contract, they exert pressure on nearby veins which causes blood to flow upwards

• upward blood flow opens valves superior to the contracting muscles so blood flows through

• simultaneously, valves inferior to contracting muscles close, preventing blood flow back to the feet

How does the respiratory pump aid blood flow through the veins of the thorax and abdomen?

• during inhalation, volume of the thorax increases + compresses the abdominal cavity, which decreases air pressure in the thorax

• thorax vein blood pressure decreases to below abdominal vein pressure 

• blood flows along its pressure gradient from abdominal veins to thoracic veins, then to the atria

• during exhalation, air pressure increases within the thoracic cavity, thoracic vein BP also increases, which increases blood flow velocity into the heart

Go through this flowchart.

______________ are specialized stretch receptors located primarily in the aorta and carotid sinuses that send impulses to the _________________ in the medulla oblongata to regulate blood pressure.

baroreceptors, cardiovascular center

If blood pressure falls, stretch receptors __________ their firing rates which initiates the ____________ to accelerate HR, ___________ force of contraction and promote vasoconstriction.

decrease, sympathetic reflexes, increase

If blood pressure rises, stretch receptors __________ their firing rates which initiates the _______________ to decelerate HR, ___________ force of contraction, promote vasodilation.

increase, parasympathetic reflexes, decrease

The chemoreceptors respond to increasing CO₂ and H⁺ (more acidic) levels by _____________ and ______________ peripheral vessels, which ______________ blood return to lungs so that CO₂ can be exhaled.

increasing ventilation, constricting, increases

The chemoreceptors respond to decreasing CO₂ and H⁺ levels by _____________ and ______________ peripheral vessels, which ______________ return to lungs.

decreasing ventilation, dilating, decreases

ADH (vasopressin) is produced in the ___________ and released from the ________________.

hypothalamus, posterior pituitary

What does ADH do?

signals the kidneys to reabsorb more water → decreases urine output

results in an increase in overall fluid levels + restores blood volume + pressure

What is the function of the renin-angiotensin system?

to raise blood pressure when it is too low by constricting blood vessels and increasing Na⁺/water retention

Explain the steps of the renin-angiotensin system.

low BP (low blood flow) → kidneys secrete renin → renin activates angiotensinogen (produced in liver) into → angiotensin I → converted to angiotensin II in the lungs (stimulates thirst in the hypothalamus) → angiotensin II causes vasoconstriction + increases aldosterone + ADH → increases water reabsorption → BP increases

Atrial natriuretic hormone (ANH) is secreted by cells in the _________ when blood volume is high enough to cause stretching of the _______ cells. What does ANH do?

atria, cardiac

• promotes loss of Na+ and water from kidneys

• suppresses renin, aldosterone + ADH production + release

• decreases BP

What are autoregulation mechanisms?

self-regulatory mechanisms that don’t require specialized nervous system stimulation or endocrine control, allow each region of tissue to adjust its own blood flow + perfusion through chemical + physical alterations

What chemical signals control blood vessel dilation/constriction? How does the dilation/constriction occur?

dilation:

• decreased O2

• increased CO2

• histamine release

these conditions stimulate endothelial cells to release NO which dilates blood vessels

constriction:

• increased O2

• decreased CO2

• antihistamine release

these conditions stimulate endothelial cells to release endothelins which constrict blood vessels

What is ambrisentan?

a strong vasoconstrictor

What is the Bayliss effect?

myogenic response by which blood vessels regulate their own diameter in response to stretching of their walls (activation of Na+ and Ca2+ channels in the smooth layer)

high pressure → more stretch → vessels constrict to prevent flooding

low pressure → less stretch → vessels dilate to maintain flow

____________ and _____________ are the short-term controls for BP, ____________ is the long-term control for BP.

Baroreceptors, cardiovascular system, renal/blood volumes

The Valsalva maneuver is _____________ against a closed glottis. Performing it causes an __________ in intrathoracic pressure, leading to a reduction in ___________ to the heart.

forced expiration, increase, preload

What is phase I of the Valsalva maneuver?

onset of strain, rise in BP because of the emptying of some blood from the large veins + pulmonary circulation (not breathing)

What is phase II of the Valsalva maneuver?

positive intrathoracic pressure leads to a reduced venous return to the heart → reduced stroke volume → fall in BP → baroreceptors activated → sympathetic system triggered → tachycardia → increased CO₂ → vasoconstriction → recovery of BP to normal values (not breathing)

What is phase III of the Valsalva maneuver?

release of strain which leads to a sudden dip in BP → release of positive pressure leads to expansion of pulmonary vascular bed + reduces left ventricular CSA → fall in BP

What is phase IV of the Valsalva maneuver?

overshoot of BP above the baseline because of resumed normal venous return to the heart stimulated by sympathetic nervous system in phase II — overshoot of BP leads to baroreflex stimulation → bradycardia → return to baseline BP