Enrico Tiepolo
The heart as a pump
The heart can be considered as a pump, more precisely two pulsatile pumps: each made of 2 chambers,
the atrium and the ventricle; creating a right heart (pumps blood through the lungs) and a left heart
(pumps blood through the systemic circulation). The ventricles can be considered the main
pumping force. In the heart, differently from the skeletal muscle, the various muscles cells are not
fused together, but are connected through gap junctions. These connections are called functional
syncytia, which let current, and calcium go through. The cells of the myocardium that are closest to
the internal cavity of the heart (endocardium) are different from those in the external part of the muscle
wall (pericardium).
The heart works as a two-stages pump:
1. The atria help fill the ventricles,
2. The latter eject blood in the circulation.
The purposes of circulating blood are:
• to exchange the carbon dioxide produced by cellular respiration for oxygen, at the lungs;
• to exchange oxygen for carbon dioxide and nutrients for metabolites at the tissues;
• to distribute hormones, carry signaling molecules, balance out heat among tissues.
In circulating through the vessels, blood must be pushed by a pressure difference. We know that liquids
are incompressible, thus, in principle, if pressure is applied to a fluid in a point of a rigid conduct, the
same pressure should be observed elsewhere. This luckily does occur and allows us to press on the brake
pedal to generate a pressure that the brake fluid transmits, practically unaltered, onto the brake pads.
However, if a real, viscous fluid is moved in a pipe, shear forces will be generated within the fluid and
between the fluid and the walls; so, the force impressed on the fluid will be partly ‘wasted’ in fighting this
friction and a corresponding fall in pressure will be observed along the path.
All the exchanges at the tissues occur through the endothelial walls at the capillaries. The latter oppose
a relatively high resistance to the flow of blood, because of their very small diameter (few μm).
Since a capillary bed is needed at the lungs, for gas exchange, and another one, in series, is needed at
the organs and tissues, having two pumps is a particularly efficient arrangement:
1. One that receives blood from the lungs and sends it to the systemic circulation.
2. The other that collects systemic venous blood to send it to the pulmonary circle.
With the two pumps (the left and right sections of the heart) in series, the same amount of blood
that travels (flow) in the systemic circle must also travel in the pulmonary circle (same
flux); however, the overall resistance of the systemic circle is almost four times that in the pulmonary
circle, so the pressure generated by the left ventricle must be about 4 times bigger than the
one generated by the right ventricle.
For the system to work best, adequate filling of the ventricles must be granted before they contract.
This is helped by the contraction of the atria, which however are smaller than the ventricles and can
only account for about ¼ of the ventricular filling. The cardiac cycle can therefore be split into three
periods:
• Passive filling of both the atria and the ventricles during relaxation (diastole).
• Atrial contraction à active filling of the ventricles.
• Ventricular contraction (systole).
Since most of the filling of the ventricle is passive and occurs during the diastole, a sufficiently prolonged
period of diastole is essential for the pump to work; also, the heart can sustain adequate circulation even
if the atrium does not work (the main problem in that case is stasis of blood in the atrium, and the
resulting thrombotic risk).
The ventricles of the ‘standard’ adult are typically filled – at rest – to some 150 ml (end-diastolic
volume, EDL):
- They eject about 90 ml during the systole (stroke volume, SV, also at rest).
- At the end of the systole the ventricle contains some 60 ml (end-systolic volume, ESV).
- From there, about 65 ml enter passively during the diastole and some 25 ml are contributed by the
atrial systole.
47 Body At Work II
, Enrico Tiepolo
A typical heartbeat is about 70 impulses per minute: 70 x 90ml = 6 liters of blood per minute
passing through the heart (a human has more or less 5/6 liters of blood in its body).
6 liters/minute = cardiac output - CO
The heart is composed of three major types of cardiac muscles – the atrial muscle, ventricular muscle
and the excitatory and conductive muscle fibers -. The atrial and ventricular muscles contract much like
the skeletal muscles, but the AP is much longer. The fibers provide an excitatory system that controls
rhythmic beating of the heart through automatic rhythmical APs.
The pump function of the ventricle requires that when it contracts the blood does not flow back into the
atrium, and when it relaxes blood can fill it from the atrium but does not flow back from the aorta (or
pulmonary artery). This is made possible by the cardiac valves.
Cardiac Valves
The cardiac valves are holes that can be reversibly occluded by membranous leaflets. They are purely
passive (remember: it’s not the valves that decide the flow of blood, it is the flow of blood that decides
the state of the valves), in that they are open and closed passively by the pressure on the two
sides of the valve.
The atrio-ventricular (AV) valves are constituted by two (mitral/ bicuspid) or three (tricuspid)
flaps anchored to the circumference of the valve and protruding in the ventricles.
- When the pressure in the atrium is higher than in the ventricle, the flaps are pushed into the
ventricle and blood freely flows across the hole.
- As soon as the ventricle starts contracting and the ventricular pressure exceeds that in the atria,
the flaps are pushed back; however, together with the rest of the myocardium, also the
papillary muscles contract; they are attached to the internal wall of the ventricles and pull
through tendinous cords on the margins of the valvular flaps, avoiding that they fold back into
the atrium and therefore stopping the flow of blood in
both directions.
- The presence of papillary muscles attached to the valves
through chordae tendineae is needed, because these valves
have to sustain a big pressure. Also, they allow for the
valves to have a bigger opening à even if the atrium
doesn’t contract much a lot of blood will flow in the
ventricle.
The semilunar valves (aortic and pulmonary valves) are
formed by three cup-shaped flaps with fibrous margins: while
the ventricular pressure, acting on the convex sides, can open the
valve; the pressure on the arterial, concave surface, cannot fold
back the valve open. These valves are strong structures, that’s why
they don’t require anchoring ligaments to avoid folding back.
All four valves lie in the same plane: they are inserted and anchored
to a fibrous diaphragm, the atrio-ventricular septum, that
gives them the necessary mechanical support and stability. The
skeleton of the valves separates the muscles of the atrium from the muscles
of the ventricle. This is important because the atrium and the ventricle
need to contract at different times, otherwise we would have a 1
stage pump. The atrium contracts first and after (only when the
ventricle is filled with blood), the ventricle contracts.
The two, separated functional syncytia (atrial and ventricular
syncytia) allow the atria to contract a short time ahead the ventricles
which is important for the effectiveness of the heart.
48 Body At Work II
The heart as a pump
The heart can be considered as a pump, more precisely two pulsatile pumps: each made of 2 chambers,
the atrium and the ventricle; creating a right heart (pumps blood through the lungs) and a left heart
(pumps blood through the systemic circulation). The ventricles can be considered the main
pumping force. In the heart, differently from the skeletal muscle, the various muscles cells are not
fused together, but are connected through gap junctions. These connections are called functional
syncytia, which let current, and calcium go through. The cells of the myocardium that are closest to
the internal cavity of the heart (endocardium) are different from those in the external part of the muscle
wall (pericardium).
The heart works as a two-stages pump:
1. The atria help fill the ventricles,
2. The latter eject blood in the circulation.
The purposes of circulating blood are:
• to exchange the carbon dioxide produced by cellular respiration for oxygen, at the lungs;
• to exchange oxygen for carbon dioxide and nutrients for metabolites at the tissues;
• to distribute hormones, carry signaling molecules, balance out heat among tissues.
In circulating through the vessels, blood must be pushed by a pressure difference. We know that liquids
are incompressible, thus, in principle, if pressure is applied to a fluid in a point of a rigid conduct, the
same pressure should be observed elsewhere. This luckily does occur and allows us to press on the brake
pedal to generate a pressure that the brake fluid transmits, practically unaltered, onto the brake pads.
However, if a real, viscous fluid is moved in a pipe, shear forces will be generated within the fluid and
between the fluid and the walls; so, the force impressed on the fluid will be partly ‘wasted’ in fighting this
friction and a corresponding fall in pressure will be observed along the path.
All the exchanges at the tissues occur through the endothelial walls at the capillaries. The latter oppose
a relatively high resistance to the flow of blood, because of their very small diameter (few μm).
Since a capillary bed is needed at the lungs, for gas exchange, and another one, in series, is needed at
the organs and tissues, having two pumps is a particularly efficient arrangement:
1. One that receives blood from the lungs and sends it to the systemic circulation.
2. The other that collects systemic venous blood to send it to the pulmonary circle.
With the two pumps (the left and right sections of the heart) in series, the same amount of blood
that travels (flow) in the systemic circle must also travel in the pulmonary circle (same
flux); however, the overall resistance of the systemic circle is almost four times that in the pulmonary
circle, so the pressure generated by the left ventricle must be about 4 times bigger than the
one generated by the right ventricle.
For the system to work best, adequate filling of the ventricles must be granted before they contract.
This is helped by the contraction of the atria, which however are smaller than the ventricles and can
only account for about ¼ of the ventricular filling. The cardiac cycle can therefore be split into three
periods:
• Passive filling of both the atria and the ventricles during relaxation (diastole).
• Atrial contraction à active filling of the ventricles.
• Ventricular contraction (systole).
Since most of the filling of the ventricle is passive and occurs during the diastole, a sufficiently prolonged
period of diastole is essential for the pump to work; also, the heart can sustain adequate circulation even
if the atrium does not work (the main problem in that case is stasis of blood in the atrium, and the
resulting thrombotic risk).
The ventricles of the ‘standard’ adult are typically filled – at rest – to some 150 ml (end-diastolic
volume, EDL):
- They eject about 90 ml during the systole (stroke volume, SV, also at rest).
- At the end of the systole the ventricle contains some 60 ml (end-systolic volume, ESV).
- From there, about 65 ml enter passively during the diastole and some 25 ml are contributed by the
atrial systole.
47 Body At Work II
, Enrico Tiepolo
A typical heartbeat is about 70 impulses per minute: 70 x 90ml = 6 liters of blood per minute
passing through the heart (a human has more or less 5/6 liters of blood in its body).
6 liters/minute = cardiac output - CO
The heart is composed of three major types of cardiac muscles – the atrial muscle, ventricular muscle
and the excitatory and conductive muscle fibers -. The atrial and ventricular muscles contract much like
the skeletal muscles, but the AP is much longer. The fibers provide an excitatory system that controls
rhythmic beating of the heart through automatic rhythmical APs.
The pump function of the ventricle requires that when it contracts the blood does not flow back into the
atrium, and when it relaxes blood can fill it from the atrium but does not flow back from the aorta (or
pulmonary artery). This is made possible by the cardiac valves.
Cardiac Valves
The cardiac valves are holes that can be reversibly occluded by membranous leaflets. They are purely
passive (remember: it’s not the valves that decide the flow of blood, it is the flow of blood that decides
the state of the valves), in that they are open and closed passively by the pressure on the two
sides of the valve.
The atrio-ventricular (AV) valves are constituted by two (mitral/ bicuspid) or three (tricuspid)
flaps anchored to the circumference of the valve and protruding in the ventricles.
- When the pressure in the atrium is higher than in the ventricle, the flaps are pushed into the
ventricle and blood freely flows across the hole.
- As soon as the ventricle starts contracting and the ventricular pressure exceeds that in the atria,
the flaps are pushed back; however, together with the rest of the myocardium, also the
papillary muscles contract; they are attached to the internal wall of the ventricles and pull
through tendinous cords on the margins of the valvular flaps, avoiding that they fold back into
the atrium and therefore stopping the flow of blood in
both directions.
- The presence of papillary muscles attached to the valves
through chordae tendineae is needed, because these valves
have to sustain a big pressure. Also, they allow for the
valves to have a bigger opening à even if the atrium
doesn’t contract much a lot of blood will flow in the
ventricle.
The semilunar valves (aortic and pulmonary valves) are
formed by three cup-shaped flaps with fibrous margins: while
the ventricular pressure, acting on the convex sides, can open the
valve; the pressure on the arterial, concave surface, cannot fold
back the valve open. These valves are strong structures, that’s why
they don’t require anchoring ligaments to avoid folding back.
All four valves lie in the same plane: they are inserted and anchored
to a fibrous diaphragm, the atrio-ventricular septum, that
gives them the necessary mechanical support and stability. The
skeleton of the valves separates the muscles of the atrium from the muscles
of the ventricle. This is important because the atrium and the ventricle
need to contract at different times, otherwise we would have a 1
stage pump. The atrium contracts first and after (only when the
ventricle is filled with blood), the ventricle contracts.
The two, separated functional syncytia (atrial and ventricular
syncytia) allow the atria to contract a short time ahead the ventricles
which is important for the effectiveness of the heart.
48 Body At Work II
