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Early Circulatory Systems
- In small, early organisms, diffusion again provided an easy method for the spread of nutrients and wastes throughout the body.
- The physics of diffusion make it a poor solution for larger organisms. As the rate of diffusion is proportional to the square of distance traveled, something else was needed to get nutrients where they were needed in a short time.
Circulatory Systems in Fish and Sharks
- A simple, single-circuit circulatory system evolved in fish to minimize the distance nutrients and wastes would have to diffuse to and from each cell.
- In this type of system, a two chambered heart cycles the blood through the gills, body cells, and heart in one direction. This comparatively-draggish circulation rate is aided by the movement of the muscles of the organism.
Circulatory Systems in Mammals
- Mammals evolved a double-circulation circuit that utilizes a two-chambered heart to pump both oxygenated blood to the body capillaries and deoxygenated blood through the pulmonary circuit.
- This setup allows for a much greater volume of blood and material to be moved about the system, satifying the resource-intensive nature of endotherms.
Circulatory Systems of Amphibians
- Like mammals, amphibians utilize a double circulatory circuit, pump oxygenated blood out to body capillaries and pulling oxygen-poor blood back through. Unique to many amphibians compared to mammals is the different type of pulmonary circuit used, called a pulmocutaneous circuit.
Unlike the mammalian pulmonary circuit, the pulmocutaneous circuit sends oxygen-poor blood through both the lungs and the skin as both can serve as gas exchange mediums when it is advantageous to do so. For instance, many reptiles shift gas exchange to primarily cycle by the skin when submerged.
Respiratory Systems of Early aquatic life
- Early aquatic life was generally small enough for the outer layer of an organism to be of a sufficiently large surface area to carry out efficient gas exchange.
- Another theory regarding the reason for the evolution of gills is that they evolved as a solution to a slightly-acidic ocean environment. To balance a lower-than-homeastatic pH, gills filter Co2 from the blood to get it back up.
- The fact that lampreys, which have gills, only get about 10% of their oxygen from them (the rest is taken in through the skin,) and that they are well-apt to survive in highly-acidic waters seems to give truth to this idea.
- Sometime before the evolution of the hagfish and the common ancestor of lampreys, gills evolved as a solution to the size problem.
- Gills allowed larger organisms to subsist, providing a much greater surface area and an efficient countercurrent exchange system for gas exchange.
- Oxygen could diffuse out of the body, and carbon dioxide could simply diffuse out. This proved to be a successful system, requiring no extra developmental effort until larger, fish-like organisms evolved.
Evolution of Terrestrial Respiratory Systems
- Common to many reptiles and mammals is the interior alveolar structures at the termini of the bronchi. This suggests a lineage that correlates with the known path of descent to mammals from reptiles.
- Early lungs were thought to have evolved as a supply pouch for air to supplement gas exchange in gills and to provide an easy means to control depth and buoyancy.
- These may have also provided a solution to gas exchange issues in oxygen-depleted waters.
- As aquatic, lung-bearing organisms lived in shallower water, respiratory adaptation to terrestrial life began to unfold.
- Tetrapods began to make more use of their lungs, selecting them over gills ultimately.
- Modern amphibians show a transitional phase of respiratory development, using gills, lungs, and their skin as a respiratory medium.