CARDIOVASCULAR SYSTEM

 The heart has a somewhat conical form and is enclosed by the pericardium.
 It is positioned posteriorly to the body of the sternum with one-third situated on
the right and two-thirds on the left of the midline.
 The heart measures 12 x 8.5 x 6 cm and weighs ~310 g (males) and ~255 g
(females)
 Anteriorly: the body of the sternum, and adjoining costal cartilages; left lung, and
pleura (apex)
 Posteriorly: oesophagus, descending thoracic aorta, azygos, hemiazygos veins,
and thoracic duct
 Superficially: bifurcation of the main pulmonary trunk
 Inferiorly: diaphragm
 Laterally: lungs, pleura

The heart wall consists of three layers enclosed in the pericardium:

1. Epicardium - the outer layer of the wall of the heart and is formed by the
visceral layer of the serous pericardium.
2. Myocardium - the muscular middle layer of the wall of the heart and has
excitable tissue and the conducting system.
3. Endocardium.

 A middle concentric layer

 A subendocardial layer.
The rest of the heart is composed mainly of the subepicardial and subendocardial layers.
Structure
The heart is subdivided by septa into right and left halves, and a constriction subdivides
each half of the organ into two cavities, the upper cavity being called the atrium, the
lower the ventricle. The heart, therefore, consists of four chambers:

 right atrium
 left atrium
 right ventricle
 left ventricle.

 Venous blood returning from the body drains into the right atrium via the SVC,
IVC and coronary sinus
 The right atrium pumps blood through the tricuspid valve into the right
ventricle
 The right ventricle pumps blood through the pulmonary semilunar valve into
the pulmonary trunk to be oxygenated in the lungs
 Blood returning from the lungs drains into the left atrium via the four
pulmonary veins
 The left atrium pumps blood through the bicuspid (mitral) valve into the left
ventricle
 The left ventricle pumps blood through the aortic semilunar valve into the
ascending aorta to supply the body.
Heart Valves

The heart has four valves. All four valves of the heart have a singular purpose: allowing
forward flow of blood but preventing backward flow. The outflow of each chamber is
guarded by a heart valve:
Atrioventricular valves between the atria and ventricles

1. tricuspid valve (R side of the heart)

2. mitral valve/bicuspid valve (left side of the heart)
Semilunar valves which are located in the outflow tracts of the ventricles

1. aortic valve (L side heart)

2. pulmonary valve (R side heart)
The heart is supplied by two coronary arteries:

1. Left main coronary artery carries 80% of the flow to the heart muscle. It is
a short artery that divides into two branches

 Left anterior descending artery that supplies anterior two-thirds of the inter-
ventricular septum and adjoining part of the left ventricular anterior wall
 Circumflex coronary artery that supplies blood to the lateral and posterior
portions of the left ventricle.
2. Right coronary artery: branches supply the right ventricle, right atrium, and left
ventricle's inferior wall.
Conduction System
 An electrical conduction system regulates the pumping of the heart and timing of
contraction of various chambers.
 Heart muscle contracts in response to the electrical stimulus received system
generates electrical impulses and conducts them throughout the muscle of the
heart.
 stimulating the heart to contract and pump blood. Among the major elements in
the cardiac conduction system are the sinus node, atrioventricular node, and the
autonomic nervous system.

1. The sinus node is the heart's natural pacemaker. The sinus node is a
cluster of cells situated in the upper part of the wall of the right atrium.
The electrical impulses are generated there. (The sinus node is also called
the sinoatrial node.)
2. The electrical signal generated by the sinus node moves from cell to cell
down through the heart until it reaches the atrioventricular node (the AV
node), a cluster of cells situated in the center of the heart between the atria
and ventricles.
3. The AV node serves as a gate that slows the electrical current before the
signal is permitted to pass down through to the ventricles. This delay
ensures that the atria have a chance to fully contract before the ventricles
are stimulated. After passing the AV node, the electrical current travels to
the ventricles along special fibers embedded in the walls of the lower part
of the heart.
 4. The autonomic nervous system controls the firing of the sinus node to
trigger the start of the cardiac cycle. The autonomic nervous system can
transmit a message quickly to the sinus node so it in turn can increase the
heart rate to twice normal within only 3 to 5 seconds. This quick response
is important during exercise when the heart has to increase its beating
speed to keep up with the body's increased demand for oxygen.

Regulation of heart
The main control of the heart resides with the medulla oblongata. There is an area called
the cardio acceleratory centre, or pressor centre, in the upper part of the medulla
oblongata, and an area called the cardioinhibitory centre, or depressor centre, in the
lower part. Together they are called the cardio regulatory centre, since they interact to
control heart rate, etc.
The nervous supply to the heart is autonomic, consisting of
both sympathetic and parasympathetic parts. The sympathetic fibres arise from the
pressor centre, while the parasympathetic fibres arise in the depressor centre.

 The sympathetic nervous system acts on the sinoatrial node, speeding up the
depolarisation rate, and therefore increasing the heart rate.
 The parasympathetic system works in reverse in order to slow the heart rate
down.
 The heart itself has a natural pacemaker, the sinoatrial node, which does not
need a nervous supply to function. If you sever all the nerves to the heart,
then it will continue to beat. In fact, it will beat faster than normal, since
there is normally a parasympathetic supply slowing the heart down.

Blood Vessels
The networks of hollow tubes like pipes, which carry blood to and from all parts of the body,
are called blood vessels. These vessels carry blood in both the directions, i.e. one from the
heart to all other parts and another from all body parts to the heart.

Types of Blood Vessels

The three types of blood vessels are:
Arteries
Arteries are the main blood vessels that carry and transport oxygenated blood or oxygen-rich
blood from the heart to other parts of the body. They are the strongest blood vessels with
thicker walls and are muscular in nature. It consists of three distinct layers, which are rigid,
thicker and highly muscular. Arteries are located deep within the body and are red in colour.
These blood vessels are with high pressure and move in a downward direction from the heart
to the body tissues.
Veins
Veins are thin, tube-like elastic blood vessels, present closer to the surface of the skin. These
translucent blood vessels are blue in colour and carry impure or deoxygenated blood from all
parts of the body to heart. These blood vessels are with low pressure and move in an upward
direction, i.e. from the cells, tissues and other organs to the heart. Compared to the arteries,
veins are thin-walled.
Capillaries
Short and tiny blood vessels, found within the tissues are called capillaries. Capillaries bring
about the exchange of substances between blood and tissues. These blood vessels also
function by connecting arterial systems to the venous system and help in exchange of
substances across cells.

Layers of Blood Vessels

Both arteries and veins consist of three layers.

 Tunica Intima: It is the innermost and thinnest layer of arteries and veins, which
have a direct contact with the blood flow.
 Tunica Media: It is the middle layer of an artery or vein, which is made up of
smooth-muscle cells.
 Tunica Externa: It is present adjacent to the tunica media and is composed of
collagen and functions by supporting the elastic lamina in arteries

Functions of Blood Vessels

 Blood vessels play a vital role in the gaseous exchange.

 Blood vessels help in maintaining a constant internal temperature of an organism.
 The vital function of the blood vessels is protecting against the loss of blood during
injuries.
 Blood vessels are also involved in circulating both oxygenated (poor) and
deoxygenated (impure) blood from and to the heart.
 Blood vessels help in the transportation of nutrients, water, minerals, hormones and
all other essential components required for the different body metabolism

cardiac output

The measure of the volume of the blood the heart pumps every minute is the cardiac
output (CO). It can be measured by multiplying the heart rate by the stroke volume.
Afterload, preload and contractility are used to ding out the stroke volume. The
standard range of cardiac output is 4-8 L/min.
However, this can differ based on the metabolic requirements of the body. This
measure of cardiac output is significant as it could estimate the delivery of oxygen to
cells. For instance, with each contraction, if the stroke volume of a person is 75mL,
and heart rate is 60 beats every minute, then his cardiac output is 4.5L every minute.

cardiac cycle
The cardiac cycle involves events, patterns of contraction and relaxation of the heart to
complete one complete heartbeat. The cardiac output is the measure of the rate of flow of
blood through the heart involving blood vessels. The change in pressure enables the flow of
blood through the cardiac cycle. This is regulated by the cardiac conduction system and
controlled by the medulla through the autonomic nervous system.
The cardiac cycle is the time period comprising all the events from one heart contraction to
the start of the next heart contraction. Each of these cycles starts with the depolarization of
the SA node which is then followed by the atrial systole [0.1 seconds], then ventricular
systole [0.3 seconds], followed by the diastole of the complete heart [0.4 seconds].

Mechanism of Cardiac Cycle

Physiologically, the contraction of both the atria precedes that of both the ventricles. This
contraction sequence enables the separation of the right and the left heart (functionally at
least) as two distinct circuits. The cardiac cycle events can be split into diastole and systole.
 The diastole indicates ventricular filling, while the systole indicates ventricular ejection or
contraction. Though with varying pressures, the systole and diastole occur in both the right
and left heart.
The diastole starts with the closing of the aortic valve and terminates with the closing of the
tricuspid or mitral valve. This period includes ventricular filling and relaxation. The diastole
indicates when blood vessels revert blood to the heart, preparing for the following ventricular
contraction.
On the other hand, systole starts when the tricuspid or mitral valve closes and ends with the
closing of the aortic valve. This phase of the cardiac cycle indicates ventricular contraction,
which forces blood into the arteries. During the contraction of the ventricles, the pressure in
the ventricles starts becoming greater compared to that of the adjacent blood vessels, valves
allow the blood to flow out.

Phases of Cardiac Cycle

The cardiac cycle occurs through these stages –
 The Atrial and Ventricular diastole – relaxed chambers filling with blood,
 The Atrial systole – contraction of atria, remaining blood is pushed into ventricles,
 The Ventricular systole – contraction of ventricles forcing the blood out through the
aorta and pulmonary artery.
 The different phases hence are –
 Stage 1 – Early joint diastole, late joint diastole
 Stage 2 – Atrial systole
 Stage 3 – Early ventricular systole, late ventricular systole

Electrocardiogram

An electrocardiogram is a graphic record produced by an electrocardiograph that provides details

about one’s heart rate and rhythm and depicts if the heart has enlarged due to hypertension (high blood
pressure) or evidence of a myocardial infarction previously (heart attack if any). Electrocardiogram
(ECG) is one of the most common and effective tests for all drugs. It is easy to perform, non-invasive,
yields outcomes instantly and is useful to identify hundreds of heart conditions.

The Electrocardiogram Wave

An ECG has three main components: the P wave, which denotes depolarising atria; the QRS
complex, denotes the depolarization of the ventricles; and the T wave represents repolarising
ventricles.
During each pulse, a healthy heart has an ordered process of depolarization that starts with
pacemaker cells in the sinoatrial node, extends throughout the atrium, and moves through the
atrioventricular node into its bundle and into the fibres of Purkinje, spreading throughout the
ventricles and to the left.
The electrical activity occurs in a small patch of pacemaker cells called the sinus node during
a regular heartbeat. This produces a small blip called the P wave when the impulse stimulates
the atria. It then activates the main pumping chambers, the ventricles, and produces the large
up-and-down in the middle, the QRS complex. The last T wave is a time of regeneration as
the impulse reverses over the ventricles and travels back.