Ch 13/14 Cardiovascular Physiology (Day 4)

Question 1

What is the mean BP in systemic circulation?

Answer 1

ranges from just under 100 mm Hg in the aorta to a low of just a few mm Hg in the venae cavae.

Question 2

Pressure Differentials –> Blood Flow

Answer 2

-Pressure created by contracting muscles (friction) is transferred to blood

–>Driving pressure is created by the ventricles

BP affected by:

1. If blood vessels dilate, blood pressure decreases
2. If blood vessels constrict, blood pressure increases
3. Volume changes affect blood pressure in cardiovascular system

Question 3

Gradient

Answer 3

change or difference in the magnitude of a given parameter

–> e.g. pressure, in one location with respect to another location

Question 4

Driving force for blood flow

Answer 4

FLOW through a tube is directly proportional to the PRESSURE GRADIENT

• Flow ΔP
• The higher the pressure gradient, the greater the fluid flow
• Fluid flows only if there is a positive pressure gradient (ΔP)

–>absolute pressures do NOT matter, just the differences (gradient)

Question 5

Resistance Opposes Flow

Answer 5

Flow through a tube is inversely proportional to resistance

• Flow (Q)  1/R, where R = resistance
• If resistance increases, flow decreases
• If resistance decreases, flow increases

Question 6

What determines resistance?

Answer 6

1. Resistance is proportional to LENGTH (L) of the tube (blood vessel)
   - Resistance INCREASES as length increases
2. Resistance is proportional to VISCOSITY (), or thickness, of the fluid (blood)
   - Resistance INCREASES as viscosity increases
3. Resistance is inversely proportional to tube RADIUS to the fourth power
   - Resistance DECREASES as radius increases

Question 7

Poiseuille’s Law

Answer 7

Combine Resistance (R by  L/r4) with the effect
of pressure difference (ΔP) on fluid flow rate
(Q by Δ P):

ΔP = Q * R where Q = flow rate, R = resistance,

so, Q = ΔP / R = ΔPr4(π)/ηL(8) [Poiseulle’s Law]

Question 8

Inverse relationship between flow and resistance

Answer 8

Small change in radius has a large effect on resistance to blood flow

• –Vasoconstriction is a decrease in blood vessel diameter/radius and decreases blood flow
• –Vasodilation is an increase in blood vessel diameter/radius and increases blood flow

Flow = P/R
—Flow of blood in the cardiovascular system is directly proportional to the pressure gradient and inversely proportional to the resistance to flow (OPPOSITE EFFECTS)

Question 9

Under normal conditions the most important factor in determining resistance is…

Answer 9

vessel diameter

Question 10

What is the equation for resistance?

Answer 10

R = 8Ln/pi(r4)

L: length of vessel (constant)

n: viscosity of blood (constant)
r: radius (changes greatly)

Question 11

Flow rate is ____ regardless of where you are in the system.

Answer 11

the same

Question 12

Cardiac Output (CO)

Answer 12

–> volume of blood pumped each minute by each ventricle

KNOW THIS: 
cardiac output (ml/min) = stroke volume (ml/beat) X heart rate (beats/min)

• Average heart rate = 70 bpm
• Average stroke volume = 70−80 ml/beat
• Average cardiac output = 5,500 ml/minute [equivalent to total blood volume]

Question 13

Cardiac Output: RV vs. LV

Answer 13

CO (in RV) = CO (in LV): in each cycle lungs get 100% of CO from RV while all other organs share output of LV

a. Therefore, pulmonary circulation has low R, low P, and high Q.
b. MAPpulmonary = 10-20 mm Hg
c. MAPsystemic = 70-105 mm Hg

Question 14

Regulation of Cardiac Rate

Answer 14

Spontaneous depolarization occurs at SA node when HCN channels open, allowing Na+ in. [recall slide # 42]

• Open due to hyperpolarization at the end of the preceding action potential (based upon slope of pacemaker potential)
• Sympathetic norepinephrine and adrenal epinephrine keep HCN channels open, increasing heart rate.
• Parasympathetic acetylcholine opens K+ channels, slowing heart rate.
• Controlled by cardiac center of medulla oblongata that is affected by higher brain centers
• Actual pace comes from the net affect of these antagonistic influences
  1. Positive chronotropic effect – increases HR
  2. Negative chronotropic effect – decreases HR

Question 15

What is stroke volume regulated by?

Answer 15

SV: how much blood you are pumping out

1. End diastolic volume (EDV): volume of blood in the ventricles at the end of diastole
   - –Sometimes called preload
   - –Stroke volume INCREASES with increased EDV. [Frank-Starling]
2. Total peripheral resistance: Frictional resistance in the systemic arteries
   - –stroke volume DECREASES w/resistance
   - –Called afterload
3. Contractility: strength of ventricular contraction
   - –Stroke volume INCREASES with contractility.

Question 16

How much of EDV is ejected?

Answer 16

60% (ejection fraction)

1) Ejection fraction remains constant over a range of EDVs, such that SV ↑’s as EDV ↑’s
2) Thus strength of ventricular contraction increases

Question 17

Preload

Answer 17

EDV

Question 18

Afterload

Answer 18

total peripheral resistance

Question 19

Frank-Starling Law of the Heart

Answer 19

increased EDV –> increased contractility –> increased SV

Question 20

Intrinsic Control of Contraction Strength

Answer 20

Due to myocardial stretch

• –Increased EDV stretches the myocardium, which increases contraction strength.
• –Due to increased myosin and actin overlap and increased sensitivity to Ca2+ in cardiac muscle cells

Adjustment for rise in peripheral resistance

• –Increased peripheral resistance will decrease stroke volume
• –More blood remains in the ventricles, so End Systolic Volume (what’s left in ventricles) increases
• –Ventricles are stretched more, so they contract more strongly

Question 21

Extrinsic Control of Contractility

Answer 21

Contractility – strength of contraction at any given fiber length

• SYMPATHETIC norepinephrine and ADRENAL epinephrine (positive inotropic effect) increase contractility by making more Ca2+ available to sarcomeres. Also increases HR.
• PARASYMPATHETIC acetylcholine (negative chronotropic effect) will decrease HR which will increase EDV  increases contraction strength  increases SV –> but not enough to compensate for slower rate, so CO decreases

Question 22

Venous Return

Answer 22

EDV controlled by factors that affect venous return:

1. Total blood volume
2. Venous pressure (driving force for blood return)

Veins have high compliance - stretch more at a given pressure than arteries (veins have thinner walls).

Veins are capacitance vessels – 2/3 of the total blood volume is in veins

They hold more blood than arteries but maintain lower pressure.

Question 23

Factors in Venous Return

Answer 23

Δ P between arteries and veins (about 10mm Hg)

Δ P in venous system - highest P in venules vs. lowest P in venae cavae into the right atrium (0mm Hg)

Sympathetic nerve activity to stimulate smooth muscle contraction and lower compliance

Skeletal muscle pumps

Δ P between abdominal and thoracic cavities (respiration)

Blood volume