Basic Cardiac Anatomy and Physiology

Basic Cardiac Anatomy

The Heart

A basic understanding of cardiac anatomy allows for correlation of physical exam finding with the unseen anatomy of the heart.  The adult heart is about the size of a closed fist and sits in the thorax on the left side of the chest in front of the lungs.  The heart is designed as a pump with four chambers - right atrium (RA), right ventricle (RV), left atrium (LA), and left ventricle (LV).  The two atria are the smaller, upper chambers of the heart and the two ventricles are the larger, lower chambers of the heart.  The hear is oriented in the chest rotated about 30 degrees to the left lateral side such the right ventricle is the most anterior structure of the heart.  The left ventricle is generally about twice as thick as the right ventricle because it needs to generate enough force to push blood through the entire body while the right ventricle only needs to generate enough force to push blood through the lungs.

The heart also has four valves.  The tricuspid valve is between the right  atrium and right ventricles.  The pulmonary valve is between the right ventricle and the pulmonary artery.  The mitral valve is between the left atrium and the left ventricle and the aortic valve is between the left ventricle and the aorta.  The valves, under normal conditions, insure that blood only flows in one direction in the heart.
 

The Cardiovascular System

In order to pump blood through the body, the heart is connect to the vascular system of the body.  This cardiovascular system is designed to transport oxygen and nutrients to the cells of the body and remove carbon dioxide and metabolic waste products from the body.  The cardiovascular system is actually made up of two major circulatory systems, acting together.  The right side of the heart pumps blood to the lungs through the pulmonary artery (PA), pulmonary capillaries, and then returns blood to the left atrium through the pulmonary veins (PV).  The left side of the heart pumps blood to the rest of the body through the aorta, arteries, arterioles, systemic capillaries, and then returns blood to the right atrium through the venules and great veins.

 

Basic Cardiac Physiology


A basic understanding of cardiac physiology is also essential to interpreting the physical finding during a cardiac exam.  Each pump or beat of the heart consists of two parts or phases - diastole and systole. During diastole the ventricles are filling and the atria contract.  Then during systole, the ventricles contract while the atria are relaxed and filling.  A more detailed understanding of the of cardiac physiology can be obtained by examining in detail the simultaneous pressure characteristics in the aorta, left atrium (atrium) and left ventricle (ventricle) through one cardiac cycle.

For the purposes for this discussion of cardiac physiology, we will focus on the physiology associated with the heart sounds S1, S2, S3, and S4.  S1 occurs near the beginning of (ventricular) systole with the closing of the tricuspid and mitral valves.  The closing of these two valves with increasing pressure in the ventricles as they begin to contract should be simultaneous.  Any splitting in which the closing of the two valves are heard separately should be considered pathological.  S2 occurs near the end of (ventricular) systole with the closing of the pulmonary and aortic valves.  The closing of these two valves occurs with beginning of backward flow in the pulmonary artery and aorta respectively as the ventricles relax.  The two valves can occur simultaneously or with slight gap between them under normal physiologic circumstances.  S3 occurs at the end of the rapid filling period of the ventricle during the beginning of (ventricular) diastole.  An S3, if heard should occur 120-170 msec after S2.  S4 occurs, if heard coincides with atrial contraction at the end of (ventricular) diastole.


Be sure you can correlate what you feel and what you hear in the cardiac exam with the underlying physiology.  The top tracing is the standard EKG (electrocardiogram) with underlying blue line indicating diastole and the red line indicating systole.  The second tracing indicates what you should hear in with your stethoscope, with the underlying colors indicating the cardiac physiology associated with each sound (mitral valve closure = purple (associated with S1), tricuspid valve closure = green (associated with S1), aortic valve closure = orange (associated with S2), pulmonic closure = blue (associated with S2), and if present ventricular wall tensing = yellow (associated with S3)).

The bottom tracing indicates the peripheral pulse wave felt when taking the pulse while auscultating, which is the recommended method.  Notice that the pressure wave begins near the end of systole and continues through the beginning and middle of diastole.  The green tabs underneath this tracing indicates when the pulse should be felt by the examiner.  This technique of simultaneously listening and feeling the pulse (generally radial) is especially helpful to avoid confusion between S1 and S2.  Palpation of the peripheral pulse should correspond with S2.

To view an excellant shockwave coordinate animations of the heart pumping blood, and the corresponding volume and pressure charts for both the atria and ventricles, ECG and heart sounds at each phase check out the University of Utah WWW site at http://www-medlib.med.utah.edu/kw/pharm/hyper_heart1.html.

More information about basic cardiac anatomy and physiology can be obtained by reviewing the relevant sections in the Year 1 CWRU School of Medicine syllabus, especially the lectures entitled The Cardiac Cycle and Cardiac Performance.

(10 October 2006)
david.kaelber@case.edu-- Copyright 1999-2006-- Unauthorized use prohibited