Animal Form & Function II - Respiratory & Circulatory Systems

II - Circulatory System of the Rat

Dissection of the Rat Circulatory System

Heart and Arterial System

In the centre of the thorax, locate the heart, within its coelomic cavity, the pericardial sac. Study the heart in position to see its major features. The apex of the heart is formed by the large, muscular, left ventricle. The right ventricle is not clearly distinguished externally from the left, but is smaller and has thinner walls. The pulmonary trunk (artery) carrying blood to the lungs, leads anteriorly from the right ventricle. The trunk crosses under and passes behind the aorta, which arises from the large left ventricle. The aorta passes forward then loops to the left and back.

The left and right atria lie anterior to the ventricles. They can be seen on either side of the pulmonary trunk and aorta. Find the two superior venae cavae which drain the anterior body regions, and the inferior vena cava which returns blood to the heart from the abdominal region, all of these enter the right atrium. Pulmonary veins from the lungs enter the left atrium. Make a sketch of the heart and label the chambers and major vessels.

Examine the major arteries surrounding the rat heart

Locate the dorsal aorta which may be readily seen where it leaves the heart and curves to the left and dorsally into the thoracic cavity. Trace the dorsal aorta as it leaves the heart and locate the following branches: the innominate artery is the first major branch of the aorta. The innominate is short and if followed subdivides into the right common carotid artery to the head, and the right subclavian artery to the right forearm. The second branch off the aorta is the left common carotid artery. Both the right and left common carotids divide into an internal carotid artery and an external carotid artery which supply the brain and outer surface of the head, respectively. The third artery leaving the aorta is the left subclavian artery to the left forearm. As the aorta passes back through the thorax, it gives off numerous pairs of small, intercostal arteries to the dorsal body wall.

View the rat abdominal cavity

Continue to follow the dorsal aorta as it passes through the diaphragm from the thoracic cavity into the abdominal cavity. The aorta runs alongside the inferior (posterior) vena cava. In the abdominal region, you can find several major unpaired branches of the aorta that supply various parts of the digestive tract. The most anterior (first branch in the abdominal cavity after the diaphragm) of these is the coeliac artery which supplies the spleen, pancreas and stomach. A short distance posterior to the coeliac artery is the superior (often called anterior) mesenteric artery which supplies the small intestine. The inferior (posterior) mesenteric artery is the most posterior unpaired branch of the aorta, arising from it just before it passes into the hind legs and tail regions. The inferior mesenteric supplies the colon and rectum.

Posterior to the superior mesenteric artery, locate the pair of arteries to the kidneys, the renal arteries. Locate the spermatic or ovarian arteries , which usually arise from the abdominal aorta just posterior to the kidneys. The spermatic arteries supply the testes, while the ovarian arteries supply the ovaries. Next, locate a pair of small iliolumbar arteries that arise from the aorta posterior to the genital arteries and send branches to the dorsal musculature of the body wall. The right and left common iliac arteries arise at the posterior end of the abdominal cavity, where they form the two major terminal branches off the aorta supplying the legs. A third terminal branch, which is a continuation of the aorta into the tail, is the caudal artery. Make a drawing to show all of these major branches of the aorta.

Venous System

The venous system consists of those vessels returning blood from the capillaries to the heart, including the portal systems, of which the hepatic portal system, carrying blood from the alimentary tract to the liver (image). The hepatic portal system consists of the hepatic portal vein and its tributaries from the abdominal viscera. Two main veins the superior mesenteric vein and the inferior meseteric vein drain the viscera ultimately joining to form the hepatic portal vein which enters the liver. The superior mesenteric vein arises as many small branches from the small intestine and runs towards the liver. It is joined by the inferior mesenteric vein from the colon. Three smaller veins the pancreatic, duodenal and gastric veins also enter the hepatic portal vein before it enters the liver. Trace the hepatic portal vein forward to where it enters the liver. Note that the mesenteric veins are paralleled by the mesenteric arteries, seen previously.

The remainder of the venous system, with minor exceptions noted below, is bilaterally symmetrical. Locate again the left and right superior vena cava, which enter the right atrium of the heart. Each receives blood from the neck and head region of one side of the body. In the neck region, locate the corresponding external jugular veins, the largest vein coming from the neck and head region. If you did your dissection of the thorax carefully you should be able to follow the external jugular into the thoracic cavity. The external jugular is joined by the subclavian vein from the forelimb, and a small internal jugular vein from deep in the neck. Once joined, these veins form the superior vena cava, which continues to the heart.

The inferior vena cava travels posteriorly from the heart. Note its passage through the diaphragm and on into the abdomen. The inferior vena cava branches into the tributaries of the renal veins (to the kidneys) and genital veins (spermatic or ovarian) from the gonads. Likewise, a pair of iliolumbar veins originate from the dorsal abdominal muscles and enter the inferior vena cava just posterior to the genital veins.

Posteriorly, the inferior vena cava is formed by the union of two common iliac veins, one from each hind limb. They enter the hind limbs as the femoral veins, while the caudal vein comes from the tail and is continuous with the inferior vena cava.

Examine the rat thoracic cavity
View the rat abdominal cavity


There is fundamental plan to all but the smallest vessels (capillaries) of the circulatory system. The walls of each is usually composed of three concentric tunics or layers as outlined.

i) Arteries

Examine a vein and artery

Tunica intima
(tunica internal)
This is the internal layer of the vessels and is quite elastic. It is composed of elements usually arranged longitudinally.
Tunica media This is the intermediate layer in which elements are mostly circularily arranged. This is the thickest coat of an artery and consists of smooth muscle interspersed with elastic tissue.
Tunica adventitia This is the external layer in which elements are usually longitudinally arranged. This coat is thinner than the tunica media. It contains the outer elastic membrane and there are seldom any muscle fibres in it.

ii) Veins

Veins show much greater variation in structure than do arteries but the three basic tunics are still present. Identify the three tunics; tunica intima, tunica media and tunica adventitia. The boundaries of the layers are often indistinct. Compare arteries and veins for the following features and relate differences to the functions of the vessel.

  1. Ratio of thickness of wall to size of lumen.
  2. Relative thickness of the tunics of the walls.

iii) Cardiac Muscle

Examine sections of heart muscle and note the branched fibres forming a fairly open, interconnecting network. Cardiac muscles although not under the control of the will, are striated. Characteristic of cardiac muscle are the intercalated discs, which are the junction between the fibres, a single nucleus and the branched nature of the cells.