This is an extract of our Plasticityin Cardiac Design document, which we sell as part of our Environmental Physiology of Animals Notes collection written by the top tier of University Of Manchester students.
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Plasticity in Cardiac Design Any animal larger than 1mm will not be able to rely on diffusion alone and must need some sort of circulatory system. This must normally include: o A propulsive organ (usually a heart) that forces blood around the body. o An arterial system that distributes blood and acts as a pressure reservoir o Capillaries to facilitate transfer of materials between blood and tissues o A venous system; a volume reservoir which returns blood to the heart. The heart is the thickening of a vessel; musclarised, degree of synctiation and uni-directional Open circulatory system Hemolymph pumped by a heart enters the arteries and then empties into a fluid space (the hemocoel). Hemolymph in the hemocoel is not distributed through capillaries but rather baths the tissues directly. They are chacterised by large blood volumes (~30% body weight) and low blood pressures (<1KPa). Extracellular spaces are confluent with the hemolymph, which moves between loose sinuses within the tissues and larger sinuses between the tissues. Occurs in small animals, arthropods and molluscs. No specific pump needed. Limitations of the open circulatory system o Low blood pressure so cannot alter velocity or distribution of blood flow o Slow changes in O2 uptake o Low maximal uptake in O2 This is all equates to a low aerobic scope (the difference between BMR and maximum MR) compared with animals with closed circulation, and is usually coupled with low activity. However, this is not the case in insects, as the oxygen transport has been de-coupled from circulation via the tracheal system, meaning that insects can exhibit higher levels of activity. Ciculatory design affects cardiac design In invertebrates, namely arthropods, the absence of piped venous return to the heart is solved with valved holes called ostia within the heart itself. These ostia are closed during contraction and thus negative pressure builds up in the rigid pericardium. When the heart relaxes the negative pressure, the blood is drawn in. In the earthworm, there are many flexibly placed dorsal hearts along the main dorsal vessel that show rhythmic contractility. This can be augmented by accessory hearts. The movement is peristaltic and is aided by contractions within the body wall. The case is slightly different in molluscs (excluding cephalopods-closed). Again, the heart is slow pressure and there is a rigid pericardium (which is a key feature in open circulatory systems. However, the mollusc heart consists of two chambers, separated by the rigid pericardium. Blood is sucked into the atrium via negative pressure and the rigid pericardium, and is expelled with a powerful muscular ventricle.
Closed circulatory systems o Occurs in all vertebrates and some invertebrates (cephalopods). o Continuous circuit from arteries to veins to capillaries o Smaller blood volumes (around 8%) of total body weight o Higher pressures (10KPa) o Huge amount of peripheral resistance o Circulating fluids (blood) is highly specialised and is separate from other extracellular fluid o Blood travels in vessels lined with endothelial cells and only comes into contact with tissues in capillary beds o Requires a powerful pump to propel blood o Usually unidirectional apart from in tunicates (sea squirts) where the direction flips o Greatest diversity of cardiac structure of any group Implications of closed circulation o Allows ultrafiltration of blood at the glomerulus in the kidney o Forces blood into a dense capillary bed o Capillaries work in parallel giving fine control of tissue distribution and thus the control of tissue oxygenation. The vertebrate heart The fish heart Consists of four chambers in series within a rigid pericardium. A divided ventricle allows pressure separation (a key feature of the vertebrate heart). It is the only truly venous heart in vertebrates and as a consequence it is a low pressure system.
the fish heart is a single circuit. The sinus venosus receives blood from the body and this is then directed into a contractile atrium then though a powerful ventricle and finally through the bulbus aerteriosus, which controls pressure by acting as a dampener, smoothing out pulsatile flow to protect the delicate microcirculation at the gills. The blood must go through the gills at high resistance to be oxygenated - this limits pressure generated by the rigid pericardium and affects the aerobic scope of the fish. Sharks, however, can alter their pericardial pressure.
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