Summary USMLE step 1 Physiology of Cardiovascular system Study guide - Study smarter not harder - your guide to pass the exam
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USMLE step 1 Physiology of Cardiovascular system Study guide - Study smarter not harder - your guide to pass the exam - Homeostasis
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,Concise medical physiology Cardiac
Introduction
The cardiovascular system is composed of the heart & a closed system of blood vessels.
The heart:
0 Consists of 4 chambers (the Rt. atrium & ventricle- the Lt. atrium & ventricle}
0 Has 3 layers; endocardium , myocardium & epicardium (fibrous cover)
0 Is surrounded by pericardia I sac that contains 5 - 30 ml clear fluid lubricates the heart
& allows it to contract with minimal friction .
The 2 main pumps are:
(1) The left ventricle: (pressure pump)
Pumps blood at a high resistance into the greater (systemic) circulation.
(2) The right ventricle: (volume pump)
Pumps the same volume of blood (at a lower resistance) to the pulmonary circulation
The valves: (allow the passage of blood one direction only & not the reverse)
(1) Atrioventricular (A- V) valves : (2 valves)
Mitral valve (2 cusps): prevents backflow of blood from the left ventricle to left atrium
Tricuspid valve (3 cusps) : prevents backflow of blood from the right ventricle to right atrium .
(2) Semilunar valves: (2 valves)
Aortic valve: prevents backflow of blood from aorta to the left ventricle.
Pulmonary valve: prevents backflow of blood from pulmonary artery to the right ventricle .
The papillary muscles:
These are ventricular muscles flaps attached to the cusps of the A- V valves by the cordae tendineae.
They contract with ventricular walls & pull the cusps of the valves toward the ventricles
~-----Ligamentum arteriosum
A s cend i ng aorta - - - - --+-- -.........---- Left pulmonary artery
Superior vena cava - - - --+ .... ,_ _ _ _ Pulmonary trunk
Right pulmona ry artery -~-r--- Left pu lm onary veins
Right pulmonary vei ns
~-r--- Left atrium
Right au r icle -----....:;.~ ~11---- Aortic semilunar valve
Interat rial sept um _ _ _ _....., -=l~r---- Left atrioventricular valve
~;: ~onary semilunar
Right atrium - - - - - -- -
Opening for Inferior - - - - 1
vena cava ~1:-r,....;.;~- Trabeculae carneae
Right atriovent r icular valv e
._....,.&~~......:11-- Interventricular septum
Chordae t e n d l neae ----...,w 1:':":":-'-t- L eft ventricle
Papillary muscle -----~
Right ven tricle - - - - - - -
I nf erior vena cava - - - --..,
89
,Cardiac Concise medical physiology
Functional histology of the cardiac muscle
The heart is composed of: Intwiateddist
1- Cells full of contractile muscle proteins (e .g. ventricular muscle)
2- Cells with few contractile elements: (e .g. sinoatrial nodal "SAN ")
The cardiac muscle fibers:
• Similar to skeletal muscle in striated structure
• Similar to smooth ms in being involuntary & act as a syncytium
The cardiac muscle cells (myocytes): Nm
• Represent 75% of the total myocardium & enveloped
within extracellular matrix (strengthened by collagen)
• Contain contractile proteins (60% of the ce ll volume) , mature sarcoplasm ic reticulum
& rich in mitochondria (30% of the cell volume)
The intercalated discs: connect the ends of adjacent muscle fibers at Z lines
Functions:
1) Provide a strong union between fibers to act as one unit
2) Contain gap junctions ~ electrical continuity between card iac cells
Gap junctions: low resistance intercellular junctions composed of
many channels ~ direct flow of ionic current between 2 adjacent cells
Gap junctions are numerous in Purkinje f ibers
& ventricular muscle fibers ~ rapid conduction
& few in AV node ~ slow conduction
Gop function
T Action
potentlot
lntorcollled cloc
Types cardiac muscle proteins:
As skeletal muscle (myosin , actin , tropon in & tropomyosin) in addition to titin & dystroph in
IT itinl A very large elongated protein
Binds myosin to the Z line
& keeps myosin filaments centered in the sarcomere
Congenital anomalies in titin ¢ abnormal dilated heart.
z
IDystrophinl A large protein JfiiiiiiiiiiliOiiiiiiiiiiUillOi'i'iim~
Connects actin to extracellular matrix i! z!=~~~~~~~==
¢ structural support to the muscle fiber.
Congenital defects in dystrophin ¢ muscle weakness. : ~~:.;.;.c" : : )I;;~~
hr===~~~~~~~~~
90
, Concise medical physiology Cardiac
Properties of the cardiac muscle
8 e
Contractility
e Rhy1:hmicity Conductivity
!-~-~L~I 1- Excitability
Definition: Ability to respond to an adequate stimulus by generating a propagating AP.
1- R.M.P. of cardiac muscle is(- 85 mV):
Mainly due to K• outflow (along its cone. gradient) through inward rectifying K+ channels (IRK)
Rectifying channels: favor ion movement in one direction than the other.
!Inward rectifying K+ channels! (IRK):
};> Responsible for the R.M .P. of cardiac cells & does not inactivate with time
};> At M.P.<- 85 mV (e .g. - 90 mV) conduct inward current better
than outward current (hence the name)
};> At M.P.- 85 mV (under normal conditions) the current is always outward
};> Inward current: inward movement of (+ve) ions c:> depolarization
};> Outward current: outward movement of (+ve) ions J . .
Or inward movement of (-ve) ions repolanzat1on
Equilibrium potential for K+ is at- 98 mV (K+ influx & efflux are balanced)
Equilibrium potential for Na• is at+ 64 mV
Electrogenic Na•- K• pump creates- 4 mV difference
Changes in extracellular K. conc.(K.)o affect the R.M.P of the cardiac muscle:
At both low & high (K.)o cone. the membrane is partially depolarized (more excitable)
0 At high (K.lo cone. depolarization is explained by H cone . gradient c:> H diffusion c:> H M.P
0 At very low (K.lo cone. depolarization is explained by slowing of Na• - K+ pump & so M.P. H
2- Action potential:
Cardiac myocytes action potential is formed of depolarization & repolarization
Monophasic action potential:
Recorded by placing 2 electrodes (one inside the cell & the other outside)
Has longer duration (200- 300 msec) than in nerve or muscle (1 - 5 msec)
Phases of the ventricular action potential:
Rapid depolarization & overshoot (Piiase 0
Due to opening of voltage gated fast Na• channels
The voltage gated fast Na• channels:
• Have 2 gates: a- outer (activation) gate: opens at- 65 mV "the firing level"
b- inner (inactivation) gate: closes & prevents further influx till action potential ends
• Conduct rapid inward current (INa) c:> rapid depolarization (upstroke) & reversal of polarity that
terminates at +20mV
• De polarizes the membrane to the level of activation of both inward Ca • 2 currents
& outward K• currents.
Repolarization: (slow & triphasic)
Phase 1: (rapid repolarization from +20 to +lOmV)
Due to:
1- Rapid inactivation of INa
2- Activation of transient outward currents caused by:
• Efflux of K+ • Influx of Cl'.
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