Download Heart Physiology

Heart Physiology
I.
Electrical Events
a. Intrinsic conduction
b. Does not depend on the nervous system itself
c. Sequence of excitation
i. Sinoatrial node (SA node)
1. Located in the right atrial wall
2. Sets the pace for the heart as a whole
ii. Atrioventricular node (AV node)
1. The depolarization wave spreads through gap junctions throughout the atria to
the AV node
2. Located in the inferior portion of the interatrial septum above the tricuspid
valve
3. Here the impulse in delayed momentarily – allowing the atria to respond
iii. Atrioventricular bundle (AV bundle)
1. A.k.a. the Bundle of His
2. Inferior part of the interatrial septum
3. The only electrical connection between the atria and ventricles
a. No gap junctions between cells
iv. Bundle branches
1. Right & left
2. Run along the interventricular septum
v. Purkinje fibers
1. Complete the pathway through the interventricluar septum and turn up the
ventricular walls
2. Stimulate the bulk of ventricular depolarization
3. Purkinjie network is more extensive on the left side of the heart
d. The time from initial SA impulse to the depolarization of the last of the ventricular
cells – 0.22 seconds
e. Electrocardiography
i. Electrocardiogram - graphic recording of electrical changes during heart activity
ii. Typically use 12 leads
iii. Waves
1. P wave – movement of the depolarization wave from the SA node through the
atria
a. Atrial contraction
2. QRS complex – ventricular depolarization
a. Ventricular contraction
3. T wave – ventricular repolarization
II.
Mechanical Events
a. Cardiac cycle
i. All events associated with the flow of blood through the heart during one complete
heartbeat
ii. A succession of pressure and blood volume changes within the heart
iii. Systole – contraction period of the heart
iv. Diastole – relaxation period of the heart
v. Period of ventricular filling: mid-to-late diastole
1. Pressure is low in the heart
2. Blood flows passively through the atria and into the ventricles
3. Semilunar valves are closed
4. The atria contract forcing the blood into the ventricles
vi. Ventricular Systole
1. The atria relax and the ventricles begin to contract
2. The AV valves close
3. Pressure increases until the pressure is greater then the arteries
4. The semilunar valves are forced open
5. Blood is forced into either the aorta or the pulmonary arteries
6. The atria are in diastole – filling with blood
vii. Isovolumetric relaxation: early diastole
1. Ventricular pressure drops
2. Semilunar valves close to prevent back flow
b. Key points
i. Blood flow through the heart is controlled by pressure changes
ii. Blood flows along a pressure gradient from high to low
1. Through any available opening
c. Heart Sounds
i. Two sounds distinguishable when listening with a stethoscope
ii. Lub-dup, pause, lub-dup, pause, lub-dup, etc.
iii. First sound – when the AV valves close
1. Onset of systole – ventricular pressure rises above atrial pressure
2. Louder, stronger and more resonant than the second sound
iv. Second sound – when the semilunar valves close
1. Short, sharp sound
2. Beginning of ventricular diastole
v. Can hear each valve close at different parts of the thorax
vi. Abnormal or unusual sounds - murmurs
d. Cardiac Output
i. The amount of blood pumped out by each ventricle in 1 minute
ii. The product of heart rate and stroke volume
1. Stroke volume - the volume of blood pumped out by a ventricle with each beat
iii. Normal adult volume – 5 liters (little more than a gallon)
1. Entire blood supply passes through each side of the heart once each minute
iv. Cardiac output is highly variable, changes with special demands
v. Cardiac reserve - difference between resting and maximal cardiac output