BIOLOGY Volume 2 - A Guide to General Biology - 2004

14. TRANSPORT IN ANIMALS

14.4. Blood Circulation

A generalized diagram of human Blood Circulation is shown in Fig. 14.2, A and is characterized by the following features.

1. Humans have a double Circulatory system. This means that blood passes through The Heart twice during a complete circuit of the body. The advantage of such a system is that blood can first be oxygenated in the Lungs (the Pulmonary Circulation), then returned to The Heart and pumped out again to the rest of the body (the systemic circulation). The reason for this is that blood pressure drops significantly in the pulmonary capillaries, and without a secondary boost, Blood supply to the major part of the body would be highly inefficient. This pattern is not characteristic of all vertebrates. In fish, for example, blood flows from the heart to the gills where it is oxygenated, then distributes throughout the body, and only afterwards returns to the heart; in other words, fish have only a single circulation (Fig. 14.2, B). A double circulatory system first appears in evolutionary history in amphibians, but becomes completely separate only in birds and mammals. It is no coincidence that these latter two vertebrate groups are warm-blooded. Endothermy requires a High Metabolic Rate, which is only possible with an efficient supply of oxygen to Tissues—oxygen being essential for aerobic Respiration (which is energetically far more superior to Anaerobic respiration). In turn, a high metabolic rate sustains a high level of overall organismal activity across A wide variety of environmental conditions. Having two completely separated circuits requires the heart to be divided into two functional halves. One pumps deoxygenated blood to the lungs, while the other pumps oxygenated blood to the rest of the body. In effect, we possess two hearts (right and left) that have fused together and contract simultaneously. In amphibians, the heart is undivided, whereas in reptiles it is incompletely partitioned (with crocodiles being the exception).

2. Organs are supplied with blood in parallel rather than in series (Fig. 14.2, A). Otherwise, blood flowing from organ A to B, then to C, and so on, would progressively lose pressure, oxygen, and nutrients at each stage, meaning that certain PARTS OF THE body would eventually be deprived. Furthermore, damage to a blood vessel at any single point would disrupt the blood supply to all downstream tissues.

3. A portal vein leads from the gut to the Liver (Fig. 14.2, A). Portal Veins are defined as veins connecting two organs neither of which is the heart (a similar system links the Hypothalamus to the Pituitary Gland). Thus, the gut and the liver are connected in series rather than in parallel, which entails the drawbacks mentioned above. However, these are offset by a major advantage. Specifically, the composition of blood draining from the gut varies greatly depending on what an individual has eaten or drunk. One of the liver's Functions is to filter the blood to maintain its composition within physiologically acceptable limits. For instance, excess glucose is removed from the blood here and stored as Glycogen.

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Fig. 14.2. A. Diagram of the human circulatory system, comprising two circuits. Major Blood Vessels are shown. LV — left ventricle; LA — left atrium; RV — right ventricle; RA — right atrium. B. Diagram of the fish circulatory system, possessing a single circuit.