The Heart and Associated Vessels
Be extremely careful during this portion of the dissection as the arteries and veins are extremely fragile. The arteries of these rats are not injected. You will therefore examine the heart and locate only the major blood vessels coming out of and going into the heart. Locate the thymus gland, which overlies the anterior portion of the heart. Carefully remove the thymus gland.
The heart of the rat consists of four chambers (Figures 11 and 12). There is a right ventricle, and a left ventricle, which are not easy to distinguish externally. Also locate the easily distinguishable right atrium and left atrium which are dark ear-shaped structures on each side of the anterior portion of the heart. Entering the right atrium are three main blood vessels which bring the deoxygenated blood back to the heart from all regions of the body. These blood vessels are the right superior vena cava, the left superior vena cava and the inferior vena cava. The inferior vena cava may be located by lifting the heart and carefully separating the lobes of the lung. It is a large vein running from the diaphragm to the right atrium. The left superior vena cava may be seen running across the dorsal surface of the thoracic cavity to enter the right atrium close to the point of entry of the inferior vena cava and the right superior vena cava. The right and left superior venae cavae return deoxygenated blood to the heart from the right and left side of the head and neck. The thoracic cavity is drained by the azygous vein which empties into the left superior vena cava near its entry into the atrium.
The deoxygenated blood in the right atrium is pumped to the right ventricle through an opening guarded by the tricuspid valve. The blood is pumped from the right ventricle through the pulmonary semilunar valve into the pulmonary trunk (usually colourless) which divides into right and left pulmonary arteries going to the lungs.
The oxygenated blood returns from the lungs to the left atrium via the right and left pulmonary veins. These can be found on the concave surface of the lungs but are difficult to trace to the atrium (do not attempt).
From the left atrium the blood enters the large muscular left ventricle through the bicuspid valve.
FIGURE 11 -- HEART OF THE RAT - DORSAL VIEW
FIGURE 12 ARTERIES AND VEINS OF THE HEAD REGION OF THE RAT
The Arteries and Veins Of The Head - Figure 12.
The oxygenated blood leaves the left ventricle through the aortic semilunar valve and enters the aorta. Carefully tear away any connective tissue and expose this artery. The aorta immediately divides into the innominate artery, and the aortic arch which sends off two branches; the left common carotid artery and the left subclavian artery. The aortic arch continues as the dorsal aorta.
Locate the innominate artery and carefully trace its path to the point at which it branches. The right common carotid artery is the branch which carries the blood to the right anterior portion of the head and the right subclavian artery carries blood to the right front leg. The right and left subclavian arteries give off a number of branches which carry oxygenated blood to the thoracic cavity, the vertebral column, the brain, the muscles of the neck and shoulder and continue on as the right and left axillary arteries.
The venous system in the head region carries deoxygenated blood to the heart. The right and left superior venae cavae, which you have already located are parts of this system.
Thus endeth the course!