Download The Circulatory System – The Heart

Lecture 15 part 2
The Circulatory System – The Heart
Make sure you know the pathway of blood flow through the chambers and valves of the
heart, as shown in figure 20.10 in your book, and as explained on page 548 (in fourth
edition Saladin textbook)
The Pulmonary and Systemic Circuits:
 The pulmonary circuit carries blood to the lungs for gas exchange and returns it to
the heart
 The systemic circuit carries blood to every organ of the body, including other
parts of the lungs and the wall of the heart itself
The Position, Size, and Shape of the Heart
 The heart is located in the thoracic cavity in the mediastinum, between the lungs
and deep to the sternum
o The mediastinum is the median portion of the thoracic cavity that
separates the lungs and contains the heart, great blood vessels, and thymus
 The broad superior portion is called the body, and the blunt point at the inferior
end is called the apex
 The heart is roughly the size of one’s fist, and it weighs about 300g
The Pericardium
 The heart is enclosed in a double-walled sac called the pericardium
 Between the layers is about 5 to 30 ml of pericardial fluid that lubricates the
membranes, so that the heart can beat almost without friction
The Heart Wall
 The epicardium is a serous membrane on the surface of the heart
 The myocardium is the thickest layer of the heart and consists of cardiac muscle
 The endocardium is the innermost layer of the heart, and lines the interior
chambers
More about the Chambers
 The atria are separated from one another by a wall called the interatrial septum
 The ventricles are separated from one another by an interventricular septum
 The atrioventricular valves (tricuspid and bicuspid valves) allow blood to flow in
only one direction
o They have chordae tendinae (tendinous cords) that look like the lines of a
parachute, extending down into the ventricles
o The chordae tendinae attach to dome-shaped, or conical, papillary muscles
 Both ventricles exhibit internal ridges called trabeculae carneae.
The Conduction System
 The cardiac conduction system insures that the four chambers of the heart are
coordinated with each other
o The SA node is in the right atrium, just under the epicardium
 It initiates each heartbeat and determines the heart rate
 It sends signals through the atria, causing them to contract
o The AV node is near the tricuspid valve in the interatrial septum
 It acts as an electrical gateway to the ventricles
o The AV bundle forks to the right and left sending signals through the
interventricular septum toward the apex
o Purkinje fibers distribute signals from the apex through the myocardium of
the ventricles