Human Anatomy and Physiology (AB_1197): Physiology Summary (VU Amsterdam)

Lecture 1 – Physiology of the Heart


 cardiomyocytes:
 the cells responsible for generating contractile force in the intact heart
 specialized cardiomyocytes form the cardiac conduction system, responsible for
control of rhythmic beating of the heart
 cardiomyocytes undergo enlargement (hypertrophy) in response to chronic demand
for increased contractile force, but an inability to meet these needs leads to
insufficient cardiac output for the demands of the whole organism (heart failure),
one of the most common causes of death in the Western world

 pulmonary circulation:
 moves blood between the heart and the lungs
 transports deoxygenated blood to the lungs to absorb oxygen and release carbon
dioxide and the oxygenated blood then flows back to the heart
 low pressure system
 systemic circulation:
 moves blood between the heart and the rest of the body
 sends oxygenated blood out to cells and returns deoxygenated blood to the heart
 high pressure system (left ventricle has a thicker muscular wall)

, functions of the heart:
 pumps deoxygenated blood to the lungs
 pumps oxygenated blood to all the organs in the body
 together with blood vessels provides adequate perfusion of all organs & tissues of
the body
 contraction and relaxation determine cardiac output

The conducting system of the heart

 a network of specialized cardiac muscle cells, called pacemaker and conducting cells, that
initiates and distributes electrical impulses
 automation of the heart:
 the heart can beat independent of hormonal or neuronal input (autorhythmicity)
 the heart is spontaneously active (due to pacemaker cells)




 pacemaker cells (nodal cells):
 essential for establishing the normal heart rate
 can be found in the sinoatrial (SA) node (cardiac pacemaker) and the
atrioventricular (AV) node
 conducting cells:
 interconnect the SA and AV nodes and distribute the contractile stimulus
throughout the myocardium
 in the atria, they can be found in internodal pathways
 in the ventricles, they can be found in the atrioventricular (AV) bundle (the
bundle of His) and the bundle branches, as well as the Purkinje fibers

, pacemaker potential – each time a pacemaker cells repolarizes, its membrane potential
shifts toward threshold (gradual depolarization)
 results from a slow inflow of Na + without a compensating outflow of K+
 the rate of spontaneous depolarization differs in various parts of the conducting system
 it is fastest at the SA node
 isolated cells of the AV node depolarize more slowly
 because the SA node reaches threshold first, it establishes the basic heart
rhythm, or sinus rhythm

, a number of clinical problems result from abnormal pacemaker function:
 bradycardia – a condition in which the heart rate is slower than normal
 tachycardia – a condition in which the heart rate is faster than normal


The action potential

,
,Heart rate

 determined by:
 resting membrane potential of SA node cells
 velocity of depolarization: slope of the prepotential
 the sympathetic nervous system (SNS) releases the hormones (catecholamines -
epinephrine and norepinephrine) to accelerate the heart rate
 the parasympathetic nervous system (PNS) releases the hormone acetylcholine to slow the
heart rate

,Excitation-contraction coupling

 contraction of the heart following electrical stimulation of cardiomyocytes




 excitation-contraction coupling links electrical excitation of the cardiomyocyte cell
membrane (via an action potential) to actin-myosin cross-bridge cycling that results is the
cell’s contraction
 excitation, contraction, and relaxation are all ATP-dependent processes
 contraction of cardiomyocytes is dependent on rapid release of Ca 2+ from the sarcoplasmic
reticulum via the ryanodine receptor – calcium induced calcium release
 relaxation of cardiomyocytes is dependent on the reuptake of Ca2+ back into the
sarcoplasmic reticulum via an ATP-dependent pump called SERCA
 the autonomic nervous system, as well as proteins troponin, tropomyosin, and
phospholamban play key regulatory roles

,ECG

 monitors the electrical events of the conducting system
 measures the depolarization of the muscle
 ECG signal reflects electrical differences between regions in the heart
 performed according to Einthoven (I, II, III)




 P wave: atria depolarize
 QRS complex: ventricles depolarize, and atria repolarize
 T wave: ventricles repolarize

 the amount of depolarization during the P wave and the QRS complex is particularly
important in making a diagnosis
 for example, an excessively large QRS complex often indicates that the heart has
become enlarged
 a smaller-than-normal electrical signal may mean that the mass of the heart muscle
has decreased (although monitoring problems are more often responsible)
 the size and shape of the T wave may also be affected by any condition that slows
ventricular repolarization
 for example, starvation and low cardiac energy reserves, coronary ischemia
(inadequate blood flow to cardiac cells), or abnormal ion concentrations reduce the
size of the T wave
 arrythmia – an irregularity in the normal rhythm or force of the heartbeat
 serious arrhythmias may indicate damage to the myocardium, injuries to the
pacemaker cells or conducting cell pathways, exposure to drugs, or abnormalities in
the electrolyte composition of extracellular fluids

,The cardiac cycle

,  blood flow through the heart is completely determined by pressure differences




1) passive filling

 AV valves: open
 aorta/pulmonary valve: closed




2) atrial kick

 AV valves: open
 aorta/pulmonary valves: closed




3) isovolumetric contraction

 AV valves: closed
 aorta/pulmonary valves: closed