The Heart is a hollow muscular organ situated in the mediastinum of the thoracic cavity, enclosed in the pericardium. It’s somewhat pyramidal in shape and placed obliquely behind the sternum and adjoining parts of costal cartilages so that 1/3rd of the heart is to the right of median plane and 2/3rd of the heart is to the left of the median plane.
The heart is composed of 4 chambers – right atrium and right ventricle, and left atrium and left ventricle. On the surface the atria are divided from the ventricles by the atrioventricular groove (also named coronary sulcus) and ventricles from every other by interventricular grooves.
Shape And Measurements
Shape: Pyramidal or conical.
Length = 12 cm.
Width = 9 cm.
Weight = 300 g in males; 250 g in females.
The heart is somewhat larger than one’s own clenched fist.
The heart presents the following external features:
C. 3 surfaces (sternocostal, diaphragmatic, and left).
D. 4 borders (right, left, upper, and inferior).
Apex Of The Heart
The apex of the heart is a conical area created by left ventricle. It’s directed downwards and forwards, and to the left. It is located at the level of the 5th left intercostal space, 3.5 inches (9 cm) from the midline and just medial to the midclavicular line.
It’s the outermost and lowermost thrust of the cardiac contraction (during ventricular systole) felt on the very front of the chest or it’s the Point of Maximum Cardiac Impulse (PMCI). Normally the apex beat is felt as a light tap in left 5th intercostal space in the midclavicular line.
In infants, the heart is positioned more horizontally so the apex of the heart is located in third or fourth left intercostal space and consequently the apex beat in children up to 7 years of age is felt in the third or fourth intercostal space just lateral to the midclavicular line.
Normally, the apex of the heart is on the left side and apex beat is felt on the left side (left 5th intercostal space) but sometimes the heart is malpositioned with apex on the right side. This condition is named dextrocardia. It might be related to complete reversal of thoracic and abdominal viscera, a condition named situsinversus.
Base Of The Heart
The base (or posterior surface) of the heart is composed by 2 atria, primarily by the left atrium. Strictly speaking 2/3rd of the base is composed by the posterior surface of the left atrium and 1 -third by the posterior surface of the right atrium. It’s directed backwards and to the right (i.e., opposite to the apex).
Characteristic features of the base are as follows:
A. It is located opposite to the apex.
B. It is located in front of the middle 4 thoracic vertebrae (i.e, T5-T8) in the lying -down position and descends 1 vertebra in the erect posture (T6-T9).
Surfaces Of The Heart
The heart has the following 3 surfaces:
A. Sternocostal (anterior).
B. Diaphragmatic (inferior).
C. Left surface
It is created primarily by the right atrium and right ventricle, that are divided from every other by the anterior part of atrioventricular groove. The sternocostal surface is also partly created by the left auricle and left ventricle. The right ventricle is divided from left ventricle by the anterior interventricular groove.
The part of sternocostal surface is uncovered by the left lung (cardiac notch) creating an area of superficial cardiac dullness.
This surface is flat and rests on the central tendon of the diaphragm. It’s created by the right and left ventricles that are divided from every other by the posterior interventricular groove. The left ventricles create left 2/3rd of this surface and right ventricle creates only right 1/3rd of this surface.
Borders Of The Heart
The heart has the following 4 borders:
A. Right border
B. Left border
C. Inferior border
D. Upper border
It’s more or less vertical and is composed by the right atrium. It extends from the right side of the opening of SVC to that of IVC and separates the base from the sternocostal surface.
It’s curved and oblique. It’s created primarily by the left ventricle and partly by the left auricle. It extends from left auricle to the apex of the heart and separates sternocostal and left surfaces.
It is almost horizontal and extends from the opening of IVC to the apex of the heart. It is created by the right ventricle. The right atrium also creates a part of the border. The inferior border separates the sternocostal surface from the diaphragmatic surface. Near the apex it presents a notch termed incisura apiciscordis.
It is somewhat oblique and is composed by the left and right atria, primarily by the latter. The upper border is obscured from the view on the sternocostal surface because ascending aorta and pulmonary trunk is located in front of it. On the surface of the body it can be marked by a line joining a point on the lower border of the 2nd left costal cartilage, 1.5 in from the median plane to a point on the upper border of 3rd right costal cartilage, 1 inch far from the median plane.
Cardiac shadow in chest radiograph: In X-ray of chest, PA view, the term cardiac-shadow is utilized for mediastinal shadow. The left border of cardiac shadow, from above downwards is composed by: aortic arch, pulmonary trunk, left auricle and left ventricle. The right border from above downwards is composed by SVC and right atrium.