Physiology: Diversity of Circulatory Systems

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Circulatory systems move fluid by increased pressure on fluid in one part of the body.

Flud flows through body down a pressure gradient. 

Methods of propelling blood: constriction and valves. 

Closed circulatory systems evolved independently from ancestral open systems in several lineages of animals. Convergent evolution. 

Molluscs have diverse circulatory systems: all have hearts or contractile organs, most have at least some blood vessels, almost all have open circulatory systems, cephalopods have closed circulatory systems.

Closed CS of a cephalopod:

Three muscular chambered hearts, one systemic heart: pumps oxygenated blood to the body, two branchial hearts: pump blood to gills and back to systemic heart. 

Cephalopods have evolved closed CS systems: 

Swimming cephalopods abundant in oceans before appearance of marine fish, swimming power: jet propulsion, inherently efficient, gills in mantle cavity obtain plenty of O2, limitations of circulatory system: haemocyanin, 1/2 O2 carrying capacity of haemoglobin.

At rest almost no venous O2 reserve, start jet propulsion: sole option to increase O2 is to increase blood flow by increased cardiac work. Squid increase cardiac output by 4-fold during exercise would increase rate of circulating O2 delivery by 4-fold. Compared to fish: increase 10-fold by using venous O2 reserve. Low resting venous O2 reserve by increasing the amount of O2 delivery only increasing rate of blood flow. Squid heart bears responsibility for increased O2 delivery. 

Crustaceans have open circulatory systems: 

Very simple in smaller less active species to extremely complex in larger, more active species. Small/immature crustaceans have no heart or blood vessels. Branchiopod crustaceans. Decapod crustaceans e.g. lobster, crab and crayfish. Lacunae: small spaces among tissue cells. Sinuses: larger blood spaces, representing thoroughfare channels for the blood. 

Decapod crustaceans: 

Adult has a single chambered heart. Several branching arteries, ultimately emptying out into lacunae and sinuses. Heart encased in a sac, pericardial sinus. Branchiopoericardial vein brings blood from gills to pericardial sinus. Ostia. 

Open versus closed circulatory systems: 

Open CS are low to medium pressure systems. Often thought that blood is sluggish. But rate of flow…

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