|
II - Circulatory System of the Rat
The vertebrate circulatory system consists of the blood vascular and lymphatic systems. We shall be concerned only with the former in the laboratory dissections. It is important to realize that in the vertebrates, the blood vascular system is a closed system within which blood circulates, much as a given volume of water might circulate continuously through a complicated system of pipes of various sizes, shapes and diameters. These pipes are so arranged, such that at one point in the circuit all of the circulating liquid is channeled through a central pumping station (the heart) which provides the pressure needed to maintain a more or less constant flow throughout the entire system.
In general, blood leaves the heart and enters one or more large arteries which branch repeatedly into more and more arteries of gradually decreasing size which lead out towards all of the body tissues. Collectively, these many small arteries are able to carry the same volume of blood per unit time as do the smaller number of large arteries (sometimes just one) leaving the heart. Within the tissues the small arteries (or arterioles) lead on to smaller blood passages, the capillaries, where metabolites and gases are exchanged between the blood and the tissue. The portion of the blood vascular system between the heart and the capillaries is the arterial system. Upon leaving the capillaries, the blood again enters small blood vessels (the venules) which are part of the venous system. The venules soon join together with others, thus forming larger veins that lead back towards the heart. The venous system thus consists of that portion of the blood vascular system leading from the capillaries of the body tissues and lungs to the heart.
Several modifications in the above simple plan occur in the vertebrates but the same basic concept of flow through a closed system applies to all. For example, in fish, a special capillary bed in the gills is interposed in the arterial system between the heart and the terminal tissues. In the rat, all blood flows through the heart twice during the course of one complete circuit of the system, once entering the heart after going through the lungs, and once after being distributed to the tissues as described earlier; hence the term double circulatory system is used in describing the blood vascular system of the rat.
A portal system may also occur, where venous blood is carried directly from one capillary bed to another before it enters the heart. A portal system is thus typically interposed within the venous system.
The circulatory system has three major roles in the body:
| Excretion: | transporting waste products to the kidneys; this may involve a renal portal system in some vertebrates. |
| Respiration: | carrying oxygen from the respiratory surfaces to tissues and carbon dioxide from the tissues to the respiratory organs. Since the breathing mechanisms vary among vertebrates, extensive changes occur in the anterior circulatory system. |
| Nutrition: | conveying nutrients from the alimentary canal to the tissues. |
In common with other mammals, as well as birds, the rat is endothermic (homeothermic), and is henceforth able to maintain a relatively high and more or less constant body temperature via physiological means. This results in relatively high rates of enzymatic reactions involved in metabolism. It follows that these organisms must also have a highly efficient circulatory system, capable of supplying the relatively arge quantities of metabolites required to maintain tthis high metabolic rate. It is the double circulatory system which provides the required degree of efficiency.
Functionally, the rat heart may be considered to consist of two separate pumps, made up of the right and left halves, which as a result of their evolutionary history are located together in the same organ. The left half receives oxygenated blood from the lungs and pumps it out to the body tissues. The deoxygenated blood returning from the tissues passes through the right half of the heart, which pumps it out to the lungs. Each half of the heart consists of an atrium which receives the blood and a thick-walled, muscular ventricle that pumps it out, hence a four chambered heart. A hepatic portal system is present, but a renal portal system is lacking in the rat.
