The heart is a muscular double pump which is roughly conical in shape. It contracts to supply blood to the low pressure pulmonary circulation and the high pressure systemic circulation simultaneously. To do this it has four chambers: the left and right atria and the left and right ventricles. It has a dorsal base and a ventral apex.
Position of the Heart
The heart is positioned mostly within the pericardium (which we'll discuss later) which lies within the mediastinum. The mediastinum is a space which is formed by the division of the thorax into left and right pleural cavities. The heart usually lies between the third and sixth intercostal spaces (the spaces between ribs), but this varies between species. The heart is also tilted so that the base faces craniodorsally and the apex caudoventrally, this gives the heart a longitudinal axis which lies at 45 degrees from verticle in a dog. This angle does vary between species, however. In addition, the heart is slightly biased towards the left side of the chest and approximatley 60% of the heart lies to the left of the midline. The most dorsal point of the heart, which is the left atrium, is approximately level with an imaginary line bisecting the first rib. Meanwhile, the most ventral part of the heart, the apex, lies close to the sternum, left of the second-last sternebra. In dogs and cats the heart is attached to the sternal portion of the diaphragm by the phrenicopericardiac ligament. In larger domestic species the heart is attached to the sternum by the sternopericardiac ligament.
Various organs and structures lie around the heart. Above the canine heart exists the oesophagus, trachea as well as the tracheobronchial lymph nodes. The aorta is present on the left hand side above the heart while the azygous vein, and the cranial and caudal vena cavae are on the right above the heart. The vagus nerve is also present above the heart. Below the heart lies the sternum as well as the internal thoracic artery and vein. On the right of the heart lies a lung as well as the right phrenic nerve. To the left of the heart lies a lung as well as the left phrenic nerve. Cranial to the heart lies the cranial lobes of the lungs, the cranial vena cava, brachiocephalic trunk and the left subclavian artery. A thymus is also present here, but only in young animals. Caudal to the heart is the diaphragm, the liver, the rumen and reticulum in ruminants as well as the accessory lobe of the right lung.
This is also known as the auricular aspect because both of the auricles are visible.
The left and right auricles look like hands which are grabbing the pulmonary artery which arises from the periphery of the heart base. This is the only vessel to do so. The aorta, on the other hand, arises more from the centre of the heart. The funnel shaped portion of the right ventricle which leads to the pulmonary trunk is the conus arteriosus. The groove next to it is the paraconal groove. This groove is static during the contraction of the heart and it marks the interventricular septum. Coronary arteries are present here. The word "coronary" means "crown" and this helps us to remember where the coronary groove is as it sits above the left ventricle. Multiple pulmonary veins lead to the left atrium. The pulmonary artery and the aorta appear close together because they develop from a single tube which later splits in two. These two vessels are held together by the ligamentum arteriosis.
The right side of the heart is also known as the atrial aspect of the heart because it is dominated by the atria. This aspect is also characterised by the merging of the cranial and caudal vena cavae (these are called the superior and inferior vena cavae respectively in humans), which are known as the 'great veins'. The azygous vein has two conformations depending on the species. Dogs and horses have a right azygous vein which is unpaired and enters the heart near the ending of the cranial vena cava. Pigas and ruminants have a left azygous vein which is also unpaired and winds under the caudal vena cava to enter the coronary sinus along with the great coronary vein. The subsinuosal groove gets its name from the fact that it is below the coronary sinus.
The pericardium is a membrane which encloses the pericardial cavity which contains the heart. The pericardium helps to hold the heart in position while still allowing it to contract and relax. It consists of two main layers:
- The Fibrous Pericardium: this is tough, thin and inelastic. Its inelasticity prevents restriction of the lung during contraction and relaxation. This pericardium blends with the adventitia of the blood vessels situated dorsally as well as the sternopericardiac or phrenicopericardiac ligaments at the apex. This causes the pericardium to create a vacuum effect. When the heart reduces size during contraction, a vacuum is created which sucks blood into the atria. The fibrous pericardium also helps to protect the heart from damage.
- The Serous Pericardium: this pericardium is enclosed by the fibrous pericardium and consists of two highly elastic layers which are created from the same membrane. These are the parietal serous layer (which is firmly stuck to the fibrous pericardium) and visceral serous layers (which coats the heart and is also called the epicardium).
The left and right atria are separated by the thin interatrial septum. Each atrium has two chambers: the smooth-walled main reservoir called the sinus venarum, and the blind ending auricle. The muscular walls of the atria are thin because they only resist low venous pressures. Pectinate muscles strengthen mainly the auricles.
The right atrium receives blood from the large systemic veins and the smaller coronary vein. The caudal vena cavae enters dorsocaudally while the cranial vena cava enters dorsocranially. The sinus venarum is the remnant of the foetal sinus venosus. The crista terminalis is the thickened ventral edge of the cranial vena cava. It defines the entrance to the right auricle. The tuberculum intervenosum is a muscular crest that points ventrally and divides the convergence of the vena cavae to divide the blood streams to the atrioventricular orifice. Caudal to the tuberculum intervenosum is the fossa ovalis which is an oval, membranous depression in the interatrial septum. It is the site of the foramen ovale in the foetus.
The left atrium is similar to the right atrium. The main difference is that the left atrium has a number of pulmonary veins which drain into it. Thus there are a larger number of venous openings known as ostia.
The ventricles are much more muscular than the atria and make up most of the mass of the heart. The left ventricle forms a circular muscular core while the thinner-walled right ventricle partially wraps around it.
The myocardium of the ventricles may have small projections into the lumen which are called papillary muscles and these act as anchors for the chordae tendineae.
The right ventricle has a crescent-shaped cross section with walls that are significantly thinner than that of the left ventricle. This is because it operates at a much lower pressure as it pumps blood to the lungs as opposed to the systemic circulation.
The right atrioventricular valve (aka tricuspid) has two cusps in the dog. Three pairs of chordae tendinae attach to three or four papillary muscles so that each muscle attaches to two cusps and each cusp attaches to two muscles. The trabecula septomarginalis traverses the ventricle from the interventricular septum to the wall. It carries Purkinje fibres and provides a shortcut for electrical impulses.
At the apex of the heart, many ridges of myocardium form the trabeculae carneae which give the wall a corrugated appearance. These may reduce turbulence during blood flow.
This ventricle has a circular cross section and forms the apex of the heart. The left atrioventricular valve has two flaps and is also called the bicuspid or mitral valve. Two large papillary muscles are present on the outer wall.
The fibrous skeleton is formed by a series of fibrous rings around each valve and their interconnecting tendons. The connection between the aorta and the atrioventricular valves forms two triangular tendons called the left and right fibrous trigones. These may contain cartilage which may ossify in older animals to form the ossa cordis.
The fibrous skeleton is the origin an insertion points of the cardiac muscle fibres. They also act as a fibrous barrier between the atria and ventricles and prevent the muscle fibres from communicating. Thus the atria and ventricles are separated electrically and this allows contractions at different times. The skeleton also shapes and stabilises the orifices of the valves. It provides a fixed origin for the valve cusps and anchors the great vessels to the base of the heart.
Structure of the Valve Cusps
The atrioventricular valves are irregular, serrated fibrous sheets that flap closed to prevent the influx of blood into the atria during systole. Left AV valve = tricuspid, Right AV valve = bicuspid. The left valve has a heavier construction due to the greater forces which are exerted on it.
The arterial valves include the aortic and pulmonary valves and each have three semilunar cusps. The luminal border of the cusps contains a central thickened nodule. Above each cusp, the arterial wall is slightly dilated by a sinus which prevents the cusps from sticking to the wall. The sinuses create an eddy effect in the blood flow which aids valve closure. The left and right coronary arteries arise from the left and right aortic sinuses.
That's all for this post, see you next time :)