Rat Circulatory System

The Heart and associated vessels

Be extremely careful during this portion of the dissection as the arteries and veins are extremely fragile. You will 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 out of the way.
The heart of the rat consists of four chambers. 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 (Figure 20).
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Figure 20: Ventral view of the heart of the rat.
Use forceps and carefully dissect the muscle and fatty tissue away from the major arteries and veins in the neck region. This is a tedious process and will take some time. Your rat has been double-injected with latex. Blue latex was injected into the vains and red latex was injected into the arteries. 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 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 inferior vena cava may be located by lifting the heart and carefully separating the lobes of the lung. It returns deoxygenated blood to the heart from the lower part of the body. The inferior vena cava is a large vein running from the diaphragm to the right atrium. The thoracic cavity is drained by the azygous vein which empties into the left superior vena cava near its entry into the atrium. It is found only on the left side of the thoracic cavity (Figure 21).
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.
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Figure 21: Dorsal view of the heart of the rat.

The Arteries and Veins Of The Head

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 (Figure 22). 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.
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Figure 22: Arteries and veins in the head region of the rat.
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The Visceral Arteries and Veins

Locate the dorsal aorta again. Notice the numerous intercostal arteries which arise from the dorsal aorta and go to the ribs. Just posterior to the diaphragm locate the aorta. The first large unpaired abdominal artery which branches from the aorta is the coeliac artery (Figure 23). After a short distance the coeliac artery gives rise to three smaller arteries - the splenic, hepatic and the gastric arteries. The splenic artery supplies the pancreas and spleen. The hepatic artery supplies the liver with oxygenated blood and the gastric artery serves the stomach.
The second unpaired branch of the dorsal aorta is the superior mesenteric artery. Trace the superior mesenteric artery. It gives rise to a number of blood vessels, which lead to the pancreas, duodenum, small intestine and large intestine.
Locate the paired renal arteries which come off the aorta after the superior mesenteric artery and carry blood to the right and left kidneys.
Just posterior to the renal arteries are the small paired ovarian arteries (female) or the testicular arteries (male) which supply blood to the ovaries or testes. Locate the two iliolumbar arteries which carry blood to the dorsal body wall. In the region of the iliolumbar arteries, the dorsal aorta gives off the inferior mesenteric artery which carries blood to the colon and rectum.
The dorsal aorta splits into two branches; the left and right common iliac arteries which divide into a number of branches supplying blood to the reproductive organs and hind limbs.
Follow one of the common iliac arteries as it goes out towards the leg. It will first give off a small artery ventrally which runs to the uterus or to the accessory glands of the male. Look deep under this vessel - here you will find the internal iliac artery. The main artery continues out to the leg as the external iliac where it will further subdivide to serve the pelvic region and the leg itself.
From the branching of the common iliac arteries, the dorsal aorta is know as the caudal artery which supplies blood to the tail (Figure 23).
In many instances the venous system has comparable blood vessels to that of the arterial system (Figure 24). Locate the left and right common iliac veins which flow into the inferior vena cava. Trace the inferior vena cava anteriorly and locate the paired iliolumbar veins, testicular veins (male), ovarian veins (female), and the renal veins. The inferior vena cava enters the liver where the hepatic vein enters it. (This is usually buried deep within the liver tissue so do not attempt to locate the hepatic vein). The inferior vena cava continues on to enter the right atrium.
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