Location and orientation with the thorax Structure of the heart Structure of the Heart Wall Chambers of the Heart Valves of the Heart Pathway of blood through the heart Cardiac Muscle Tissue Conducting System and Innervation Four Steps of Cardiac Conduction Blood Supply to the Heart

1. The heart
BY : Ahmed A. Abdulwahab

2. Location and orientation with the thorax
 The heart is located in the chest between the lungs behind the sternum and above
the diaphragm.
 It is surrounded by the pericardium.
 Its size is about that of a fist, and its weight is about 250-300 g.
 Its center is located about 1.5 cm to the left of the midsagittal plane. Located
above the heart are the great vessels: the superior and inferior vena cava, the
pulmonary artery and vein, as well as the aorta.
 The aortic arch lies behind the heart.
 The esophagus and the spine lie further behind the heart.

3. Structure of the heart
coverings
 Pericardium
 The heart sits within a fluid-filled cavity called the pericardial cavity.
 The walls and lining of the pericardial cavity are a special membrane known as the
pericardium.
 Pericardium is a type of serous membrane that produces serous fluid to lubricate the
heart and prevent friction between the ever beating heart and its surrounding organs.
 Besides lubrication, the pericardium serves to hold the heart in position and maintain a
hollow space for the heart to expand into when it is full.
 The pericardium has 2 layers—a visceral layer that covers the outside of the heart and a
parietal layer that forms a sac around the outside of the pericardial cavity.

4. Structure of the Heart Wall
The heart wall is made of 3 layers: epicardium, myocardium and endocardium.
 Epicardium. The epicardium is the outermost layer of the heart wall and is just another
name for the visceral layer of the pericardium. Thus, the epicardium is a thin layer of
serous membrane that helps to lubricate and protect the outside of the heart. Below the
epicardium is the second, thicker layer of the heart wall: the myocardium.
 Myocardium. The myocardium is the muscular middle layer of the heart wall that
contains the cardiac muscle tissue. Myocardium makes up the majority of the thickness
and mass of the heart wall and is the part of the heart responsible for pumping blood.
Below the myocardium is the thin endocardium layer.
 Endocardium. Endocardium is the simple squamous endothelium layer that lines the
inside of the heart. The endocardium is very smooth and is responsible for keeping
blood from sticking to the inside of the heart and forming potentially deadly blood clots.
The thickness of the heart wall varies in different parts of the heart. The atria of the heart
have a very thin myocardium because they do not need to pump blood very far—only to the
nearby ventricles. The ventricles, on the other hand, have a very thick myocardium to pump
blood to the lungs or throughout the entire body. The right side of the heart has less
myocardium in its walls than the left side because the left side has to pump blood through
the entire body while the right side only has to pump to the lungs.

6. Chambers of the Heart
 The heart contains 4 chambers: the right atrium, left atrium, right
ventricle, and left ventricle. The atria are smaller than the ventricles and
have thinner, less muscular walls than the ventricles. The atria act as receiving
chambers for blood, so they are connected to the veins that carry blood to the
heart. The ventricles are the larger, stronger pumping chambers that send
blood out of the heart. The ventricles are connected to the arteries that carry
blood away from the heart.
 The chambers on the right side of the heart are smaller and have less
myocardium in their heart wall when compared to the left side of the heart.
This difference in size between the sides of the heart is related to their
functions and the size of the 2 circulatory loops. The right side of the heart
maintains pulmonary circulation to the nearby lungs while the left side of the
heart pumps blood all the way to the extremities of the body in the systemic
circulatory loop.

7. Valves of the Heart
 The heart functions by pumping blood both to the lungs and to the systems of the body.
To prevent blood from flowing backwards or “regurgitating” back into the heart, a
system of one-way valves are present in the heart. The heart valves can be broken
down into two types: atrioventricular and semilunar valves.
 Atrioventricular valves. The atrioventricular (AV) valves are located in the middle of
the heart between the atria and ventricles and only allow blood to flow from the atria
into the ventricles. The AV valve on the right side of the heart is called the tricuspid
valve because it is made of three cusps (flaps) that separate to allow blood to pass
through and connect to block regurgitation of blood. The AV valve on the left side of the
heart is called the mitral valve or the bicuspid valve because it has two cusps. The AV
valves are attached on the ventricular side to tough strings called chordae tendineae.
The chordae tendineae pull on the AV valves to keep them from folding backwards and
allowing blood to regurgitate past them. During the contraction of the ventricles, the AV
valves look like domed parachutes with the chordae tendineae acting as the ropes
holding the parachutes taut.
 Semilunar valves. The semilunar valves, so named for the crescent moon shape of
their cusps, are located between the ventricles and the arteries that carry blood away
from the heart. The semilunar valve on the right side of the heart is the pulmonary
valve, so named because it prevents the backflow of blood from the pulmonary trunk
into the right ventricle. The semilunar valve on the left side of the heart is the aortic
valve, named for the fact that it prevents the aorta from regurgitating blood back into
the left ventricle. The semilunar valves are smaller than the AV valves and do not have
chordae tendineae to hold them in place. Instead, the cusps of the semilunar valves are
cup shaped to catch regurgitating blood and use the blood’s pressure to snap shut.

9. Pathway of blood through the heart
 . Blood enters the right atrium from the superior and inferior venae cavae,
and the coronary sinus.
 . From right atrium, it goes through the tricuspid valve to the right ventricle.
 . From the right ventricle, it goes through the pulmonary semilunar valves to
the pulmonary trunk
 . From the pulmonary trunk it moves into the right and left pulmonary
arteries to the lungs.
 . From the lungs, oxygenated blood is returned to the heart through the
pulmonary veins.
 . From the pulmonary veins, blood flows into the left atrium.
 . From the left atrium, blood flows through the bicuspid (mitral) valve into
the left ventricle.
 . From the left ventricle, it goes through the aortic semilunar valves into the
ascending aorta.
 . Blood is distributed to the rest of the body (systemic circulation) from the
aorta.

11. Cardiac Muscle Tissue
 This is a unique tissue found only in the walls of the heart.
 Cardiac (Heart) Muscle Tissue shows some of the characteristics of smooth muscle and
some of skeletal muscle tissue.
 Its fibres , like those of skeletal muscle, have cross-striations and contain numerous
nuclei.
 However, like smooth muscle tissue, it is involuntary.
 Cardiac muscle differ from striated muscle in the following aspects: they are shorter, the
striations are not so obvious, the sarcolemma is thinner and not clearly discernible, there is
only one nucleus present in the centre of each cardiac fibre and adjacent fibres branch but
are linked to each other by so-called muscle bridges.
 The spaces between different fibres are filled with areolar connective tissue which contains
blood capillaries to supply the tissue with the oxygen and nutrients.
Functions of Cardiac (Heart) Muscle Tissue :
 Cardiac muscle tissue plays the most important role in the contraction of the atria and
ventricles of the heart.
 It causes the rhythmical beating of the heart, circulating the blood and its contents throughout
the body as a consequence.

12. Conducting System and Innervation
our heart's electrical system includes three important part:
 1. S-A node (sinoatrial node) — known as the heart's natural pacemaker, the S-A
node has special cells that create the electricity that makes your heart beat.
 2. A-V node (atrioventricular node) — the A-V node is the bridge between the atria
and ventricles.
Electrical signals pass from the atria down
to the ventricles through the A-V node.
 3. His-Purkinje system — the His-Purkinje system carries the electrical signals
throughout the ventricles to make them contract. The parts of the His-Purkinje system
include:
o His Bundle (the start of the system)
o Right bundle branch
o Left bundle branch
o Purkinje fibers (the end of the system)

13.  The S-A node normally produces 60-100 electrical signals per minute —
this is your heart rate, or pulse. With each pulse, signals from the S-A
node follow a natural electrical pathway through your heart walls.
 The movement of the electrical signals causes your heart's chambers to
contract and relax.
 In a healthy heart, the chambers contract and relax in a coordinated way,
or in rhythm.
 When your heart beats in rhythm at a normal rate, it is called sinus
rhythm.

14. Four Steps of Cardiac Conduction
 Step 1: Pacemaker Impulse Generation
The sinoatrial (SA) node contracts generating nerve impulses that travel
throughout the heart wall. This causes both atria to contract.
 Step 2: AV Node Impulse Conduction
The atrioventricular (AV) node lies on the right side of the partition that
divides the atria, near the bottom of the right atrium.
When the impulses from the SA node reach the AV node they are delayed
for about a tenth of a second. This delay allows the atria to contract and
empty their contents first.
 Step 3: AV Bundle Impulse Conduction
The impulses are then sent down the atrioventricular bundle. This bundle of
fibers branches off into two bundles and the impulses are carried down the
center of the heart to the left and right ventricles.
 Step 4: Purkinje Fibers Impulse Conduction
At the base of the heart the atrioventricular bundles start to divide further
into Purkinje fibers. When the impulses reach these fibers they trigger the
muscle fibers in the ventricles to contract.

16. Blood Supply to the Heart
 The heart receives its own supply of blood from the coronary arteries. Two major coronary
arteries branch off from the aorta near the point where the aorta and the left ventricle meet.
These arteries and their branches supply all parts of the heart muscle with blood.
Right Coronary Artery (RCA)
The right coronary artery branches into:
 Right marginal artery
 Posterior descending artery
The right coronary artery supplies:
 right atrium
 right ventricle
 bottom portion of both ventricles and back of the septum
The main portion of the right coronary artery provides blood to the right side of the heart, which
pumps blood to the lungs. The rest of the right coronary artery and its main branch, the posterior
descending artery, together with the branches of the circumflex artery, run across the surface of
the heart's underside, supplying the bottom portion of the left ventricle and back of the septum.
Left Main Coronary Artery (also called the left main trunk)
The left main coronary artery branches into:
 Circumflex artery
 Left Anterior Descending artery (LAD)
The left coronary arteries supply:
 Circumflex artery - supplies blood to the left atrium, side and back of the left ventricle
 Left Anterior Descending artery (LAD) - supplies the front and bottom of the left ventricle and
the front of the septum