1.2.2 The endocardium
The chambers of the heart are lined with endocardium which is responsible for maintaining a smooth surface, free from thrombi which would have a deleterious effect on the circulation. The endocardium may also have a number of local biochemical regulatory roles, but these are poorly understood. In addition, the endocardium is developed to form the cardiac valves, which maintain movement of blood through the cardiac chambers in series, and prevent back-flow. Abnormalities of the valves may result in regurgitation of blood, which reduces the efficiency of the heart. Valvular regurgitation is a very common abnormality in the horse. Valves can also be stenosed, restricting outflow and increasing the work load on the chambers. However, valvular stenosis is a very rare abnormality in the horse (see section 6.1).
Two types of valve are present in the heart. The atrioventricular (AV) valves divide the atria from the ventricles. They are supported by connective tissue fibres called the chordae tendineae and by the papillary muscles. The tricuspid valve (right AV valve) divides the right side of the heart, has three principal leaflets and is supported by three or more papillary muscles. The mitral valve (left AV valve) divides the left side of the heart and consists of two main leaflets and a number of smaller leaflets. The septal (anterior) mitral leaflet is the largest and divides the inflow and the outflow tracts. The other main leaflet is the caudal (posterior) mitral leaflet. The left and right commissural leaflets are situated between the main leaflets and are variable in number and configuration. The mitral valve is supported by chordae tendineae which branch from two large papillary muscles. Both the right and left papillary muscles support chordae tendineae to the septal and caudal valve leaflets. The chordae tendineae consist of major and minor chordae, with the larger structures supporting the tips of the valves, and chordae of gradually decreasing thickness inserting nearer the valve base. The valves normally have a thicker edge than body, but nodule foron these valves is a common disease process of variable significance (see section 2.4.3). Together, the papillary muscles, chordae and valve leaflets make up a functional unit - the valve apparatus.
The semilunar valves are three leaflet structures which are positioned between the ventricles and the great arteries. The aortic valve divides the LV from the aorta. Each valve leaflet has a small nodule in the middle of its leading edge (the corpora Aranti). Slit like fenestrations are often found along the leading edge of the leaflets, but these are a normal finding. The leaflets are termed the right coronary, left coronary and non-coronary cusps, to reflect their position relative to the coronary arteries. The pulmonary valve divides the RV from the pulmonary artery and is similar in structure to the aortic valve. Similar nodules (corpora Morgoni) and fenestrations may be seen. The pulmonary valve leaflets are much thinner structures than the aortic valve leaflets.