Midclavicular line: imaginary line parallel to the midline and passing through the midpoint of the clavicle on the anterior surface of the body.
Anterior axillary line: imaginary line parallel to the midclavicular line beginning at the anterior axillary fold.
Midaxillary line: imaginary line parallel to the anterior axillary line beginning at the midpoint of the axillary fold under the arm.
The Human Heart
The human heart is a four-chambered organ responsible for supplying oxygenated blood to the entire body.
The four chambers consist of the right atrium and left atrium, and the right and left ventricles.
The right and left sides of the heart are separated by a wall called septum.
The atriums receive blood which is then passed down to the ventricles.
The ventricles either pump blood towards the lungs for oxygenation or to the body for perfusion.
The pericardial sac completely surrounds the heart and provides protection and lubrication between the heart and other organs in the chest.
Right and Left Coronary Arteries:
The coronary arteries carry oxygenated blood to the atria and ventricles.
Right coronary artery provides blood to the right atrium and right ventricle.
Left coronary artery carries oxygenated blood to the myocardium and bifurcates into the left circumflex artery, and the left anterior descending artery.
The left circumflex artery supplies oxygenated blood to the posterior aspect of the left ventricle.
Pulmonary circulation
Carries blood from right ventricle of heart to lungs
Carbon dioxide is removed: oxygen is picked up
Oxygenated blood returns to the left atrium of the heart
Systemic circulation
Carries oxygenated blood and nutrients from left ventricle to body cells
Returns deoxygenated blood with carbon dioxide and wastes from cells to right atrium
A series of valves ensures that blood flows in the correct direction as it is pumped through the heart. The valves of the heart are made up of endocardium tissue.
Conduction System
Normal pathways
The synchronous, rhythmic contraction of the heart muscle is controlled by an electrical conduction system.
In the normal heart, electrical impulses are initiated in the sinoatrial node (SA node), conducted through both atria, and directed to the atrioventricular node (AV node).
The AV node delays transmission of the signal to the ventricles, allowing them to completely fill with blood.
Depolarization continues toward the apex of the heart through the bundle of His and the left and right bundle branches until it terminates in the Purkinje fibers.
As the electrical impulses reach the myocardium, the muscle cells depolarize and contract.
In addition to the organized conduction system, the heart muscle can initiate its own impulse. This property is called automaticity.
Occasionally, a small area in atrial or ventricular tissue becomes irritated and initiates an electrical impulse from outside the normal pathways.
These are called ectopic beats and can be seen on the EKG as premature atrial or ventricular complexes.
Sinoatrial node:
The SA node is found in the right atrium and functions as the primary pacemaker of the heart.
In adults, it fires approximately 60 to 100 times per minute.
Atrioventricular node:
The atrioventricular (AV) node is the only part of the conduction system that connects the atria to the ventricles.
Just below the AV node lies the AV junction, where the AV node and the bundle of His come together.
The junction can function as a backup pacemaker if the SA node fails, and fires at approximately 40 to 60/min.
The AV node "holds" the electrical signal received from the SA node for a short period of time to allow the ventricles to completely fill with blood.
Bundle of His:
The AV node conducts the impulse to the bundle of His located in the interventricular septum.
The bundle of His transmits impulses to the right and left bundle
Right bundle branch:
The right bundle branch carries the electrical impulse from the AV node to the Purkinje fibers of the right ventricle, causing depolarization.
Left bundle branch:
The left bundle branch, located in the interventricular septum, carries the electrical impulse from the AV node to the Purkinje fibers of the left
ventricle, causing depolarization.
Purkinje fibers:
The Purkinje fibers are a network of conduction pathways that
traverse the surface of the ventricles and depolarize them, initiating myocardial contraction.
In the absence of electrical stimulation from the SA and AV nodes, the Purkinje fibers will fire at an intrinsic rate of 20 to 40/min.
