3 - Physiology of the heart Flashcards
- Electric activity of the heart
- There are 3 excitable tissues:
1. Working fibers: generate AP(plateau); prevents early secondary contraction
2. Pacemakers: has no permanent RMP, but turns into constant depolarization
3. Conductive system: provides rapid spreading of stimuli, hence providing synchronized contractions - Additional elements: anulus fibrosus (non conducting), and Aschoff-Tawara node(delayes the atrial signal)
- Working fibers:
- Constitute a syncitium, i.e. muscle cells are connected via gap junctions
- AP of working fibers:
- Average RMP is -90 mV.
- Fiber is stim by an electrical impulse so that the RMP shifts towards and reaches the threshold pot
- Reaching threshold pot: voltage sensitive fast Na chs open and a sudden influx from EC occur. With this change one enters the so-called 0. phase of the AP.
0. phase - depolarization: Influx of NA continues, membr.pot. reaches +25 mV. At this point the channels are inactivated and flow of Na stops.
1. phase – overshoot: Depolarization is stopped. Repolarization begins. Short Cl- influx and K+ efflux.
2. phase – plateau: Ca chs open, Ca influx. At the same time K+ chs open, K+ efflux. The balance of these processes causes the elongation of this phase.
3. phase – full repolarization: Late K+ chs open and K+ rapidly flows out of the cell according to its electrochem. gradient, while Ca chs close.
- Electromechanical coupling
•Structural unit is the DIAD (at skeletal muscle: Triad): T-tubuls and Sarcoplasmic Retic (SR) are in contact here.
-AP – huge Ca-transient (tubular L-type Ca channel(voltage gated) open + rianoid-Ca ch open + Ca-dependent Ca-channel (from SR) + membrane Ca-dependent Ca channel from EC space are open: huge amount of IC Ca is around the sarcomers – results in contraction.
•After (during) contraction: ATP-dependent Ca pump drives back the Ca to the SR, plus Na/Ca antiport pumps back the Ca to EC space – IC Ca conc. Drops - resulting in relaxation.
- Electrocardiogram
-“Electric Activity of the Heart”
-The sum of the activities of all fibers measured on the body-surface.
-“Einthoven’s Triangle”; electrodes placed around the heart, as dipole, detecting slight electric changes (mV)
-Potential differences (U) are measured betw. the pairs of electrodes
-During rest the integral vector is always zero.
•Einthoven’s standard bipolar leads:
Lead 1: reference electrode = RA, investigating electrode = LA
Lead 2: reference electrode = RA, investigating electrode = LL
Lead 3: reference electrode = LA, investigating electrode = LL
- Analysis of ECG
- Waves: Deflections on the ECG curve
- Segment: between the waves
- Intervals/complexes: Larger parts, may contain one or more waves and segments
- P-wave: A depolarized.
- PQ-segment: Total activation of A. Period of AV conduction.
- QRS-complex: depolarization of V, repolarization of A.
- Q-wave: transmission of the excitation from the bundle of His to the V muscles. Downward deflection.
- R-wave: V depolarized. Largest deflection.
- S-wave: RV depolarizes from endocardium towards epicardium.
- ST-segment: all V muscle fibers activated, causing a shift from the isoelectric line
- T-wave: V repolarization
- TP-segment: resting phase after full repolarization.
- Mechanical properties of the heart
- Elements of contraction
- CC (contractile elements): Actin and myosin
- SEC (serial elastic component): Attached to CC, relaxes during diastole and expandes during systole.
- PEC (parallel elastic component): Attached to CC and SEC. Stretched by the blood filling heart during diastole: energy is stored in these fibers.
- Col (collagen fiber system): prevents overexpansion and rupture of the tissue
- Isometric phase: At the beginning of the contraction the weight stretches the SEC elements only.
- Isotonic phase: When the stretch in the SEC gets into balance with the weight, the weight begins to move. Shortening occurs and the stretching force remains equal to the weight of the object lifted.
- Properties of the total working musculature:
- End diastolic volume (EDV): The amount of blood found in the heart by the end of diastole
- End systolic volume (ESV): The amount of blood remaining in the heart by the end of systole
- The difference is called stroke volume (SV), this volume fraction passes into the aorta at each cycle.
- Cardiac output = volume of blood forwarded by the left ventricle into the aorta per unit time.
- CO = SV x FR (frequency)
- Since SV=EDV-ESV -> CO = (EDV-ESV) x FR
- Measuring cardiac output: Fick’s principle: Cardiac output is equal to the total oxygen consumption divided by the arterio-venous oxygen concentration difference.
- Starlings’ law
- The heart muscle can adapt itself to higher requirements automatically, without the intervention of the nervous system. This is known as the “law of the heart”, by Starling and Frank.
- Starlings experiment: First he increased the venous return, then he changed the peripheral resistance. Proved that the heart can adapt to the incr load. By incr venous return, the CO and SV incr. By changing the peripheral resistance, the SV and CO remained unchanged.
- Physiological importance of Starlings law: The heart can increase its diastolic reserves so that it can perform better:
- Posture: Mediated by the change in the venous return. Due to gravitational effects
- Left-right symmetry: Heterometric autoregulation, a continuous phenomenon
- Parameters of the cardiac cycle: volume fractions
- Cardiac output (CO): the volume of blood pumped into the circulation by the heart in one minute. (heart rate times stroke volume)
- End diastolic volume (EDV): The ventricles are maximally filled at a time just before the heart contracts.
- End systolic volume (ESV): When the ventricles are maximally emptied, there is still some blood remaining in them.
- Stroke Volume (SV)=EDV-ESV
- Cardiac Output = (EDV – ESV) x Frequency, CO = SV x Frequency
- Parameters of the cardiac cycle: factors influencing cardiac output
- Diastolic filling time
- V compliance (decr. w. age)
- V filling (preload): maintained by CVP
- Symphatetic effects:
* Artificial incr. of Fr., i.e. by pacemaker - result: duration of diastole decr. (Starling doesn´t work, CO decr.)
* Natural incr. by symp. stimuli - result: red. systolic time, remaining diastolic time (Starling works, SV incr., CO incr.) - Aortic P (afterload): when the semilun. valvaes are still open.
- Contractility: depends on isometric max tension (Sm) and max contraction speed (Vmax).
- Parasymp. effect: normally heart is under parasymp. control. Constant firing of n.vagus decr. frequency and contractility
- The cardiac cycle
-The basic unit of the hearts functioning, repeated uninterruptedly.
-Consists of coordinated el. activity and mech. properties of the heart.
-Composed of systole (contraction) and diastole (relaxation).
•Total length in dog: 800 msec
-Ventricular systole (270 msec):
*During the former diastole the Vs were filled with blood.
*Tension in the Vs closes the cuspidal valves (to the A).
*The V continues to incr its tension while all valves are closed.
*In this phase the contractile components shorten and stretch the serial elastic elements.
*There is no volume change, while the tension increases: therefore the name of this contraction is “isovolumetric contraction”
*The incr tension deforms the heart
-Including in the ventricular systole:
*Isovolumetric contraction: When the internal pressure of the Vs goes just above the pressure found in the aorta and the pulmonary artery, the semilunar valves open. 50 msec.
*Auxotonic contraction: Blood enters the large arteries while the tension further increases. 220 msec. Fast ejection: 80% of the stroke volume. Lasts for 90 msec. Slow ejection phase: Tension drops, but ejection continues. Lasts for 130 msec.
-Ventricular diastole (530 msec)
*Including in it: Isovolumetric relaxation (120 msec) and Isotonic relaxation: Isotonic filling (410 msec, fast/reduced) and atrial systole (110 msec)
- The cardiac cycle: parameters
1) Pressure: The minute to minute changes of pressure values in the pulmonary veins, LA, LV and in the aorta determine the position of the valves and accordingly the flow of blood in these compartments.
2) Volume: is constant in the isovolumetric stages of cardiac cycle. In the early ejection phase of systole the volume of the V decr rapidly and then the rate of emptying slows down
3) Valves: Semilunar valves (aorta and pulmonary artery) are closed during diastole and open during systole, while the cuspid valves (from atria) are open during diastole and closed during systole.
4) Heart sounds:
-1st sound: During and just after the closure of the cuspidal valves (systolic heart sound). Three components:
o Vibration of muscle contraction (weak)
o Turbulence of blood due to closure of the valves (most pronounces)
o Turbulence of blood caused by fast ejection (weak)
-2nd sound: Generating during and just after the closure of the semilunar valves (diastolic heart sound). Very much pronounced in large animals.
o First the aortic valve closed, followed by the semilunar valves of the pulmonary artery.
-3rd sound: Originates from the rapid filling of the V
-4th sound: Result of the turbulent flow caused by the atrial contraction (may lack).
1) Jugular pressure: With the sudden onset of cardiac relaxation at the beginning of the diastole the basis of the heart moves cranially and this increases the pressure in the jugular vein.
- Factors determining the blood pressure
•Blood pressure maintains the flow of blood in the circulatory bed.
Physiological effects:
-Cardiac output:
o If the C.O. is suddenly incr experimentally the mean arterial pressure incr to a value that is able to forward the incr volume of blood appearing in the arterial system.
o After some cardiac cycles the incr C.O. incr the BP, which then forwards the incr amount of blood to the venous side.
-Increase of heart rate: Incr the BP because blood is forwarded from the venous reservoir system into the arterial resistance system.
-Total peripheral resistance (TPR):
o A sudden incr in TPR, while other parameters remain constant, decreases the runoff.
o The arterial blood volume grows and a consequent incr of the mean arterial pressure helps to forward the SV towards the incr resistance.
Physical effects:
-Arterial blood volume (Va):
o From the physical point of view the major determinant of BP is Va
o Under constant compliance and peripheral resistance the incr of Va causes incr of both pulse pressure and mean arterial pressure.
-Arterial distensibility and compliance:
o Vessels are capable to take up more blood when pressure incr because their wall is distensible.
o Distensibility = the capacity to expand/stretch under pressure, D = ∆V/∆P x Va
o Compliance = flexibility, C = ∆V/∆P
- Pressure on different parts of the circulatory bed
Hgmm - kPa
- Left ventr.: 120/0-5 - 16/0-1
- Aorta: 120/80 - 16/10
- Arteries: 100/70 - 13/9
- Arterial side of capillaries: 30 - 4,2
- Venous side of capillaries: 10 - 1,3
- Right atrium: 3 - 0,4
- Right ventr.: 25/0 - 3,3/0
- Pulm. artery: 25/10 - 3,1/3,2
- Lung capill.: 7 - 1
- Left atrium 4/1 - 0,8
- Measuring the blood pressure
- Direct method: It can be used on fully anesthetized animal inserting a fluid-filled catheter into the carotis, which is connected to a pressure transducer that converts the oscillation of the arterial pressure into recordable electrical signals.
- Indirect method:
•Palpation
•Ausculation
- Microcirculation
- Exchange of materials between blood and ECF is made possible by permeable capillaries:
- The continuous type of capillaries is most common.
- Tissues taking part in secretion and resorption display fenestrated capillaries.
- Kidneys: porous capillary
- Liver: capillaries form sinusoids
- Arteries -> arterioles -> metarterioles -> capillaries
- At the point of branching of metarterioles into capillaries, precapillary sphincters are found. The majority of the sphincters are closed during rest.
- Bw arterioles and venules a shunt may be present
- Diffusion is the most important factor in the exchange of materials
- Rate of diffusion depends on the concentration gradient, permeability, and the surface area.
- Gases and small molecular substances are mostly exchanged by diffusion.
- Two types of transport by diffusion:
1. Flow limited: For small molecules it is only the rate of blood flow that limits the transport.
2. Diffusion limited: For large molecules (e.g. polypeptides) it is the rate of diffusion that limits the transport.
- Exchange of substances
- Hydrostatic pressure and oncotic pressure of the blood and of the tissue determine the pressure gradient for the fluids.
- Phcap = hydrostatic pressure in the capillary
- Point = oncotic pressure in the interstitium
- Pocap = oncotic pressure in the capillary
- Phint= hydrostatic pressure in interstitium
- Effective pressures: differences of the blood and tissue pressures
- Pheffective = Phcap - Phint
- Poeffective = Pocap - Point
- The final effective filtration is the difference of he effective hydrostatic and effective oncotic pressures
- Peffective = Pheffective - Poeffective
- The effective filtration pressure shows towards the tissue at the arterial side of the capillary: filtration may occur.
- On the venous end of the capillary the effective filtration pressure is negative, i.e. it shows towards the lumen of the capillary: resorption may occur.
- Volume flow, Q:
- The flow of fluid depends on the effective pressure and on the permeability
- In rest Qfiltrated is greater than Qresorbed
