Ch 13/14 Cardiovascular Physiology (Day 4) Flashcards
What is the mean BP in systemic circulation?
ranges from just under 100 mm Hg in the aorta to a low of just a few mm Hg in the venae cavae.
Pressure Differentials –> Blood Flow
-Pressure created by contracting muscles (friction) is transferred to blood
–>Driving pressure is created by the ventricles
BP affected by:
- If blood vessels dilate, blood pressure decreases
- If blood vessels constrict, blood pressure increases
- Volume changes affect blood pressure in cardiovascular system
Gradient
change or difference in the magnitude of a given parameter
–> e.g. pressure, in one location with respect to another location
Driving force for blood flow
FLOW through a tube is directly proportional to the PRESSURE GRADIENT
- Flow ΔP
- 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)
Resistance Opposes Flow
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
What determines resistance?
- Resistance is proportional to LENGTH (L) of the tube (blood vessel)
- Resistance INCREASES as length increases - Resistance is proportional to VISCOSITY (), or thickness, of the fluid (blood)
- Resistance INCREASES as viscosity increases - Resistance is inversely proportional to tube RADIUS to the fourth power
- Resistance DECREASES as radius increases
Poiseuille’s Law
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]
Inverse relationship between flow and resistance
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)
Under normal conditions the most important factor in determining resistance is…
vessel diameter
What is the equation for resistance?
R = 8Ln/pi(r4)
L: length of vessel (constant)
n: viscosity of blood (constant)
r: radius (changes greatly)
Flow rate is ____ regardless of where you are in the system.
the same
Cardiac Output (CO)
–> 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]
Cardiac Output: RV vs. LV
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
Regulation of Cardiac Rate
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
What is stroke volume regulated by?
SV: how much blood you are pumping out
- 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] - Total peripheral resistance: Frictional resistance in the systemic arteries
- –stroke volume DECREASES w/resistance
- –Called afterload - Contractility: strength of ventricular contraction
- –Stroke volume INCREASES with contractility.
How much of EDV is ejected?
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
Preload
EDV
Afterload
total peripheral resistance
Frank-Starling Law of the Heart
increased EDV –> increased contractility –> increased SV
Intrinsic Control of Contraction Strength
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
Extrinsic Control of Contractility
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
Venous Return
EDV controlled by factors that affect venous return:
- Total blood volume
- 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.
Factors in Venous Return
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
