BIOL 1051H Chapter Notes - Chapter 19: Tricuspid Valve, Pulmonary Circulation, Heart Valve
Cardiovascular Physiology
The right side of the heart drives the pulmonary circulation. Deoxygenated blood from
the body enters the right atrium via the superior and inferior vena cava. Then the right ventricle
provides the pumping power to drive the deoxygenated blood out through the pulmonary
arteries and through the capillary system of the heart and return the newly oxygenated blood to
the left side of the heart. The oxygenated blood passes through the pulmonary veins and enters
the left atrium. The blood is sent to the rest of the body by the contraction of the left ventricle
which provides the great deal of force needed to move the blood into the aorta, and then all the
vasculature in the body, except the lungs. The figure to the right shows the two blood circulation
circuits.
The heart itself has 4 chambers. Each side has an atrium and a ventricle with a valve in
between. The left and right sides of the heart are separated by a muscular septum, such that
mixing of blood from the two sides is prevented. It is important to realize that the amount of
blood moving through the right pump (pulmonary circuit) exactly matches that of the left pump
(systemic circuit). However, the left side of the heart needs to produce considerably more force
to move the blood through the extensive vascular system of the body, compared to that of the
lungs. As a result, the work done by the left side of the heart is 5 – 7 times greater than that of
the right ventricle, and consequently, the left ventricle is 3 – 4 times thicker than the right
ventricle. Between atria and ventricles is layer of dense connective tissue called the fibrous
skeleton which structurally and functionally separates them. This results in three myocardia. The
upper myocardium is formed by the atria which attach to the top of the fibrous skeleton. The
lower two myocardia are formed by the ventricles which attach to the bottom of the fibrous
skeleton, and along the intraventricular septum. The three myocardia function independently.
The atria contract together sending blood into the ventricles at the same time, and the ventricles
contract together sending blood away from the heart at the same time. The fibrous skeleton
keeps this separation by physically dividing the atria from the ventricles, but it also prevents
action potentials from spreading directly between atrial and ventricular cells. The fibroskeleton
also forms the rings that hold the heart valves, the annuli fibrosi. Atrioventricular valves (AV
valves) are located between the atria and ventricles. Between the right atrium and right ventricle
is the tricuspid valve, and between the left atria and ventricle is the bicuspid valve (mitral valve).
The opening and closing of these valves is the result of pressure differences between the atria
and ventricles. As ventricles relax after pumping blood they create negative pressure which
pulls blood from the atria, through the AV valves and into the ventricles. This pressure
difference is accentuated by the contraction of the atria which contribute to the pressure
difference. When the ventricles contract the high pressure of the ventricle sends the blood out of
the heart. It is prevented from flowing back into the atrial by the papillary muscles and attached
chorda tendinae which pull against the pressure exerted on the valves. During ventricular
contraction, blood is pumped through the aortic (left) and pulmonary (right) semilunar valves.
These valves close during relaxation of the ventricle.
The cardiac cycle is the repeating pattern of contraction and relaxation of the heart.
Systole is the contraction phase while diastole is the relaxation phase. Both atria contract
simultaneously followed by the ventricles 0.1 - 0.2 seconds later.
Closing of the AV and semilunar valves produce sounds that can be heard through the
stethoscope. The first heart sound (Lub) is produced by the closing of the AV valves. The
second sound (Dub) is produced by the closing of the semilunar valves
Heart murmurs are heart sounds that are not normal. They result from turbulence in
blood flow. The most common cause of a heart murmur is blood leaking back through a heart
valve. The timing and loudest location of the murmur can help in the diagnosis of the cause.
Murmurs themselves are not dangerous. They become a problem when leaking through the
valve is sufficient to badly affect heart efficiency in which case they can lead to heart failure. The
other problem is that the turbulent blood tends to have areas of stagnant blood. This can lead to
the formation of blood clots in the heart or allow for the growth of bacteria, bacterial
endocarditis. For this last reason people with heart murmurs are often treated with antibiotics
before having their teeth cleaned. This prevents bacteria from colonizing the heart while
bacteria are shed into the blood from the dental procedure.
Cardiac Muscle and Muscle Contraction: Like skeletal muscle, the heart contractions are
due to the spread of action potentials through the cardiac muscle cell. Myocardial cells are
short, branched, and interconnected by gap junctions which result in the ability for action
potentials to easily spread from one cell to another. The entire muscle mass that is
interconnected in this way is called a myocardium (functional syncytium). What separates the
chambers is nonconductive tissue which prevents the uncontrolled spread of action potentials
from one chamber to another.
Cardiac muscle needs to be coordinated in its contractions in order to efficiently pump
blood. In a normal heart the sinoatrial node (SA node) functions as a pacemaker for the heart.
The cells in this region undergo spontaneous depolarizations (pacemaker potential) at around
40 per minute. Once the SA node depolarizes, and an action potential is achieved it spreads in
a controlled manner through the heart
Myocardial cells in general have a resting membrane potential of -85 mV. This is more
negative than the pacemaker cells, and even more negative than neurons. Action potentials in
neighboring cells depolarize myocardial cells to threshold. This is followed by a rapid influx of
Na+ (just like neuron) which causes the membrane potential to go up to +15. The membrane
potential then rapidly declines to about -15 and then very slowly continues the decline. This
plateau phase lasts about 200 – 300 msec. During this phase Ca2+ is entering the cell slowly,
causing the cells to contract. Repolarization then takes place quickly as K+ moves out of the
cell.
Document Summary
The right side of the heart drives the pulmonary circulation. Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cava. Then the right ventricle provides the pumping power to drive the deoxygenated blood out through the pulmonary arteries and through the capillary system of the heart and return the newly oxygenated blood to the left side of the heart. The oxygenated blood passes through the pulmonary veins and enters the left atrium. The figure to the right shows the two blood circulation circuits. Each side has an atrium and a ventricle with a valve in between. The left and right sides of the heart are separated by a muscular septum, such that mixing of blood from the two sides is prevented. It is important to realize that the amount of blood moving through the right pump (pulmonary circuit) exactly matches that of the left pump (systemic circuit).
