9 2 Vertebrate Circulatory Systems All vertebrates have closed circulatory systems that serve the same basic functions but their structures are highly adapted to the behavior and environment of each taxa Vertebrate circulatory systems include a heart made of cardiac muscle which is divided into multiple chambers This muscle squeezes the blood driving it through the system via pressure gradients Blood flows away from the heart through large blood vessels called arteries which branch to form many smaller arterioles which branch further into more numerous smaller capillaries The capillary networks or capillary beds are the site of diffusion where molecules are exchanged between the blood and interstitial fluid based on their concentration gradients The capillaries coalesce to form fewer wider venules which coalesce to form veins which carry the blood back to the heart The structure of a blood vessel varies depending on where it is relative to the heart Its position relative to the heart determines how much pressure it experiences and the walls of the blood vessels have structural adaptations that reflect these differences For example when blood initially leaves the heart it is being pumped with great force The arteries that are closest to the heart must have thick walls to withstand this force without breaking They also have thick layers of muscle to maintain this pressure and propel the blood into the capillary beds Each vessel has a hollow core a lumen surrounded by several layers of cells From outside to inside they are the tunica externa tunica media tunica intima and endothelium which may be considered as part of the tunica intima Each layer has a different structure and function and its thickness varies depending on what kind of blood vessel it is Smaller blood vessels arterioles venules lack some layers and capillaries lack all layers beside the endothelium Tunica externa composed of collagen which anchors the blood vessels to the surrounding tissue Collagen is a structural protein found in the connective tissue of animals that exists in cells as long fibrils composed of triple helices of amino acids It makes up over 25 of all protein in mammals and is found in many tissues including bones skin and cartilage It is highly elastic and provides the blood vessel wall with great tensile strength it can be pulled and stretched without being damaged In arteries an elastic membrane underlies the collagen allowing the collagen layer to expand and contract with resilience Tunica media composed of layers of smooth muscle cells sometimes mixed with elastic tissue There are more smooth muscle cells in arteries than veins where the tunica media is mostly layers of elastic connective tissue However as arteries get closer to the heart they get larger and must withstand great pressure These arteries have more elastic tissue than smaller ones like the arterioles These differences allow arteries to exert pressure on the blood to help drive it into the capillaries and allows veins to expand and contract to accommodate changes in blood volume Tunica intima composed of a single layer of endothelial cells anchored to an elastic layer of connective tissue and an internal membrane an elastic lamina In larger arteries there is a substantial sub endothelial layer of connective tissue but this is completely absent in capillaries The flat smooth endothelial cells line the lumen of the vessel so they are in direct contact with the blood The structural characteristics of the tunica intima determine what particles can diffuse between the blood and the interstitial fluid Capillaries lack all of the layers besides the endothelial cells Recall that capillaries are very narrow numerous vessels that vascularize all of the animal s tissues Each tissue will have a capillary bed that dips into the interstitial fluid bringing the blood close to the cells These structural adaptations increase the rate of diffusion of particles between the blood and interstitial fluid either by going between the endothelial cells or through them In all other vessels the other layers make diffusion either inefficient or impossible The endothelial cells that comprise capillary walls vary between tissues which determines what particles can diffuse In continuous capillaries the cells are held together by tight junctions complexes of proteins that connect the cells into a sheet preventing diffusion of any particles larger than H2O or ions between the cells These types of capillaries are found in skin and muscle Fenestrated capillaries are made of endothelial cells that are porous The size of the pore will allow smaller particles to diffuse across the capillary wall based on their concentration gradients but larger particles will be occluded and unable to diffuse These are found in areas where exchange between the blood and tissues is crucial such as the kidneys intestines and endocrine organs These pores may expand and contract in response to hormonal input Sinusoidal capillaries have few tight junctions and are extremely porous This allows even large proteins and entire cells to diffuse across the capillary wall based on their concentration gradients These are found in areas where proteins and new cells are synthesized to be released into the circulation so they can be transported to other areas where their receptors are expressed This includes tissues such as the liver which synthesizes plasma proteins and bone marrow which synthesizes red and white blood cells All vertebrates have closed circulatory systems that use pressure gradients to serve the same basic functions of supplying oxygen nutrients and signaling molecules like hormones to cells and removing CO2 and waste However vertebrates live in a diversity of environments that affect these needs Between habitats there are significant differences in variables such as atmospheric pressure oxygen availability and flow of environmental medium Further species differ in behavior diet size shape and activity level so their metabolic needs can be very different These selective pressures have shaped the circulatory systems of vertebrates creating many different arrangements of hearts and vessels All closed circulatory systems have two sets of capillary beds One set of capillary beds vascularizes the respiratory surface or the tissue where O2 is brought into the body and exchanged with the blood This could be the gills the skin or the lungs depending on the taxa In these capillary beds the
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