Circulatory system I: The Heart
Heart sites in your chest cavity between your lungs
The four principle functions of the cardiovascular system are:
o It transports oxygen and nutrients to all the cells of the body
o It transports carbon dioxide and waste from all cells
o It helps maintain body temperature and pH
o It transports and distributed hormones and other substances within the body
Anatomy – The heart
The heart essentially consists of two side-by-side pumps:
o The right atrium and ventricle, which pumps blood to the lungs,
o The left atrium and ventricle, which pumps blood to the rest of the body
The wall of the left ventricle is much thicker than the wall of the right ventricle.
o The left ventricle, which delivers blood to the entire body, must contract more forcefully
to propel the blood through the entire systemic circulation.
o The right ventricle, on the other hand, only propels the blood to the nearby lungs and,
therefore, does not need to contract as forcefully.
The valves in the heart, which ensure the one-way flow of blood through the heart, may have
several different names.
o The right atrioventricular (AV) valve is also called the tricuspid valve
o The left atrioventricular (AV) valve is also known as the bicuspid or mitral valve.
Anterior View and Posterior view
Superior vena cava: delivers blood to the heart from the head and upper limbs
Pulmonary artery: blood leaving the right ventricle travels to the lungs through the pulmonary
Aorta: blood leaving the left ventricle travels through the aorta and is distributed to the entire
body Right atrium: receives blood from the entire body. This blood is low in oxygen and high in
carbon dioxide. The right atrium will then pump the blood into the right ventricle through the
right atrial-ventricular (AV or tricuspid) valve
Left atrium: receives blood coming from the lungs. This blood is rich in oxygen and low in carbon
dioxide. The left atrium will then pump the blood into the left ventricle through the left atrial-
ventricular (AV or bicuspid) valve
Right ventricle: pumps blood into the pulmonary artery. The pulmonary artery then delivers this
blood to the lungs for gas exchange
Left ventricle: pumps blood into the aorta which then distributed the blood to the entire body
Right AV (tricuspid) valve: ensure that the blood travels in only one direction – from the right
atrium to the right ventricle – and prevents blood from backing up into the atrium when the
Left AV (bicuspid) valve: ensures that the blood travels only in one direction – from left atrium
to left ventricle – and prevents blood from backing up into the atrium when ventricle contract
Pulmonary semilunar valve: ensures that the blood travels in one direction – from right
ventricle to pulmonary artery – and prevents the blood form backing up into the ventricle when
Aortic semilunar valve: ensures that blood travels only from the left ventricle to aorta and
prevents it from backing up into the left ventricle when it relaxes
Chordae tendineae: are cords of collagen that attach to the valves at one end to papillary
muscles at the other. These structures prevent AV valves from being pushed into the atria when
the pressure in ventricles is high
Papillary muscles: are extensions of the ventricular muscles and are attached to the chordae
tendineae. When the ventricles contract so do the papillary muscles and the AV valves are held
in place and don’t fold backward into the atria Circulation through the heart Myocardial cells
There are two principal types of myocardial cells
o Contractile cells (similar features to skeletal muscle cells)
o Nodal/conducting cells (similar features to nerve cells)
The contractile cells are considered to be the real muscle cells of the heart and form most of
the walls of the atria and ventricles. They have similar features and contract in almost the
same way as skeletal muscle fibers
The contractile cells of the heart contain the same contractile proteins actin and myosin
arranged in bundles of myofibrils surrounded by a sarcoplasmic reticulum.
They differ from skeletal muscle by having only one nucleus but far more mitochondria.
o One-third of their volume is taken up by mitochondria.
o These cells are extremely efficient at extracting oxygen; they extract roughly 80% of
the oxygen from the passing blood—about twice the amount of other cells.
The cells are much shorter, are branched, and are joined together by special structures called
The intercalated discs are structures that contain tight junctions that bind the cell together,
while the gap junctions allow for the movement of ions and ion currents between the
Because of the gap junctions, the myocardial cells of the heart can conduct action potentials
from cell to cell without the need of nerves (an important feature)
Myocardial cells – nodal/conducting cells The second type of cells found in the heart are nodal or conducting cells
These cells contract very weakly because they contain very few contractile elements
These special cells are able to spontaneously generate action potentials without the help of
nervous input like regular neurons.
Along with this special property of self-excitability, they can also rapidly conduct the action
potentials to atrial and ventricular muscle
Thus, these specialized cells provide a self-excitatory system for the heart to generate
impulses and a transmission system for rapid conduction of the impulses throughout the
Origin of Self-Excitability
Although nearly all of the cells in the heart can spontaneously generate action potentials, the
sinoatrial node (or SA node) is generally the site of origin
The SA node is located in the upper posterior wall of the right atrium, and it is the first area
to spontaneously depolarize, producing an action potential; this is why it is called the
pacemaker of the heart.
From here, the action potential travels through the atria to the atrial-ventricular node (AV
node) and then to the Bundle of His. From the Bundle of His, the action potential travels
through the Purkinje Fibers and then to the ventricular muscle.
Myocardial cells – remember!
Na+, Cl-, Ca++ and K+ are also responsible for the action potential in
the heart, however this action potential begins by itself
The important ions here will be Na+, K+ and Ca++
Ca++ are very important when it comes to the heartbeat in more than
one way SA Node action potential
Although the cause for spontaneous generation of the action potential is still
controversial, several characteristics of the SA node are generally considered to be
responsible for its self-excitability.
Recall that Na are moving into the cell, down their concentration gradient. In fact,
the Na permeability is slightly higher here than in other cells. This will make the
inside of the cell more positive (depolarized) over time.
Ca are similar to Na —they are also trying to move into the cell and will also
depolarize the cell.
The animation at right shows the movement of Na and Ca into an SA nodal cell,
producing an initial depolarization of the membrane. We have not yet created an
Although the movement of both Na and Ca into the cell causes a depolarization, the main
cause of the spontaneous action potential is the movement of K . +
Recall that K are trying to leave the inside of the cell down their concentration gradient. By
itself, this will make the inside more negative (a hyperpolarization). But you do not want this
to happen if you want to depolarize the cell.
Instead, the potassium permeability of the SA node cells decreases over time (that is, less
K leak out).
In addition, since the Na /K pump is always pumping K+ into the cell, both of these factors
will cause these cells to depolarize.
Because Na+ and Ca++ are flowing into the cell and K+ build up inside, the membrane
potential of the SA nodal cells depolarizes from -60 mV to -40mV (the threshold of these
cells) Consequently, the SA nodal cells do not have a stable “resting” membrane potential like
neurons or muscle cells
This slow depolarization is completely spontaneous and is called the pacemaker potential
The pacemaker potential is responsible for setting the pace of the heartbeat, and any
alterations to it will affect the heart
Once the membrane potential depolarizes to threshold (– 40 mV), special voltage-gated Ca ++
channels will open. Ca will rapidly flow in, producing the depolarization phase of the SA
node action potential.
These Ca channels will begin closing at roughly the same times as voltage-gated K
channels begin to open, allowing K out to repolarize the cell.
Once the cell has returned to its lowest value of roughly –60 mV, the pacemaker potential
will begin depolarizing the cell and the sequence will repeat itself. Later, we will see that this
influx of Ca is important during the contraction of the heart.
You should notice that the sequence of events is similar to the generation of a neuronal
action potential, yet there are some important differences in terms of ions and their
movements. You may wish to compare and contrast these events. Myocardial cells – conducting system of the heart
Once the action potential is generated at the SA node, it travels throughout the heart in a
highly coordinated manner.
From the SA node, the action potential spreads throughout the atrial muscle, causing it to
From the atria, the action potential travels to the ventricles. However, the atria are
electrically isolated from the ventricles by a fibrous tissue. Therefore, the action potential
cannot jump directly down to the ventricles.
The action potential must first travel through the atrio-ventricular (AV) node. Once through
the AV node, the action potential travels through each branch of the Bundle of His down to
the apex of the heart.
From here, the action potential propagates through the Purkinje Fibers, which rapidly
distribute the action potential to the ventricular muscle, which then contracts.
It is very important to have a well-coordinated contraction for the heart to function properly
as a pump. Therefore, the conduction speed of the action potential will vary as it moves
through the heart.
The SA node has one of the slowest conduction speeds (0.05 m/s).
The action potential speeds up through the atrial muscle (1 m/s) to ensure that this muscle
With the SA node at the top of the heart, the action potential and, consequently, the
contraction of the muscle moves from the top down. This ensures that the blood is forced
down into the ventricles.
The AV node slows the conduction speed (0.03 m/s) in order to ensure that the atria have
finished contracting before the ventricles contract.
The action potential must now reach the base of the heart rapidly. It does this through the
Bundle of His, which conducts the action potential at a very fast rate (1 m/s)
It is important for the action potential to reach the apex of the heart to contract first so the
blood can be forced up and out through the valves at the top of the ventricles.
The Purkinje fibers (5 m/s) then spread the action potential throughout the ventricular
muscle (1 m/s) so it contracts from the apex upward.
We have seen how the action potential is generated and how it spreads through the heart to
Since body fluids are good conductors of electricity and the heart sits in the middle of this
conducting fluid, when the action potential passes through various parts of the heart, the
electrical current can spread to the surface of the body
If electrodes are placed on the skin around the heart, electrical potentials generated by the
heart can be recorded. Such a recording during the cardiac cycle is called the
electrocardiogram (ECG) The P wave represents the electrical activity in the heart associated with the depolarization
of the atrial muscle leading to their contraction.
The large QRS complex is produced by the depolarization of the ventricular muscle just prior
to its contraction.
The T wave is a result of the repolarization of the ventricular muscle as it relaxes
Notice that there is no wave associated with the repolarization of the atrial muscle. This
event, which does occur, is obscured by the much larger QRS complex
Once complete heart contraction is called the cardiac cycle
The Cardiac Cycle (WATCH ANIMATION)
The cardiac cycle consists of all of the mechanical, electrical, and valvular events taking place in the
heart during a single contraction
An understanding of the relationship between all of these events is important in order to understand how
the heart functions as a pump.
The cardiac cycle has two primary phases (systole and diastole) that can be divided into several
As we work through the cycle, it will be important to pay attention to the ECG, the pressure changes
(remember that in order for blood to flow, there must be a pressure gradient from high to low between
two areas), the volume in the ventricle, and the activity of the valves. Each of the five steps of the cardiac cycle is summarized below, while the animation at right shows (WATCH
ANIMATION) how they fit together. Note that we are looking at events on the left side of the heart.
Step 1—Atrial systole.
This first phase of the cycle begins with the depolarization of the atria (P wave in the ECG). The
atria contract. Atrial pressure is greater than ventricular pressure. The AV (mitral) valve opens and blood
flows into the ventricle. Ventricular volume increases slightly (this is the end diastolic volume).
Step 2—Isovolumetric ventricular contraction (also called early ventricular systole).
This begins with the ventricles depolarizing (QRS complex) then contracting. Ventricular
pressure increases rapidly (above atrial but below aortic pressures). The mitral valve closes. No change
in ventricular volume.
Step 3—Ventricular systole (also called ejection period).
The ventricles are still contracting, but