Circulatory System

  • The circulatory circulates around the animals body, transporting:
    • Oxygen to all cells
    • Nutrients from the digestive system to all cells
    • Waste products from cells and returns them to the respiratory, digestive, and urinary systems for disposal
    • Hormones from where they are secreted to their target organs
    • Is involved in immune response and thermoregulation
  • Sponges and most cnidarians use water as their circulatory fluid
  • Invertebrates with a pseudocoelom, the coelomic fluid acts as the circulatory fluid
    • Nematodes use this method and movement of muscles around the body cavity moves the fluid.
  • In an open system the extracellular fluid is circulated around the body
    • In molluscs there is a vessel from the heart that supplies the gills and one that supplies the rest of the body. The vessels divided into finer vessels and deliver blood to large sinuses where it bathes tissues
    • In insects haemolymph leaves the heart through short branched arteries that open into large sinuses, the haemolymph moves around the organs directly bathing cells, it then returns to the heart directly or through short veins
  • In a closed system blood is enclosed in vessels and circulated around the body by the heart
    • Cephalopods have three hearts, two branchial hearts send blood to the gills and a systemic heart that sends blood to the body
    • In annelids blood flows away from the heart through ventral vessels and returns through dorsal vessels, most have five hearts
  • Closed circulatory systems have advantages:
    • Higher pressure
    • More rapid blood flow
    • Blood flow can be targeted to individual organs
    • Blood flow to organs can be adjusted depending on the need of the body
  • The most ancestral vertebrate circulation system is the fish which has a single circuit. From the heart, blood circulates through the gills and gill capillaries, then through the system capillaries before returning to the heart, which lowers the blood pressure. Their heart has two chambers composed of the sinus venosus and atrium, and ventricle and conus arteriosus
    • The sinus venosus evolved into the sinoatrial node in birds and mammals
    • The conus arteriosus becomes incorporated into the ascending aorta and pulmonary trunk in crocodilians, birds, and mammals
  • The development of lungs necessitated a change in the pattern of circulation by adding a second pumping circuit. One circuit goes to the lungs, where the blood is oxygenated. This oxygenated blood returns to the heart to be circulated to the body.
  • This is called double circuit circulation. Ideally, the system should keep the mixing of oxygenated and deoxygenated blood to a minimum, according to the metabolic needs of the individual creature
  • Amphibians, reptiles, birds and mammals have double circuit circulation patterns. First blood is circulated in the pulmonary circuit to the lungs (where blood is oxygenated), then returned to the heart before being circulated to the rest of the body in the systemic circuit.
    The trend in evolution of the vertebrate heart is towards increasing separation of oxygenated and deoxygenated blood
  • Amphibians have a three-chambered heart, with two atria and one ventricle. Oxygenated and deoxygenated blood don’t mix in the atria. The ventricle is partly divided to reduce the mixing.
  • Amphibians also have cutaneous respiration, and so have a second set of capillaries in the skin where blood is oxygenated and carried back to the heart (pulmocutaneous circuit)
  • Lizards and snakes have a septum that partly divides the oxygenated and deoxygenated blood in the ventricle. This is important because reptiles have watertight skin and rely completely on their lungs for oxygen. They also tend to be more active than amphibians. They have a shunt (the right aorta), the purpose of which is not well understood
  • Crocodilians possess two aortas, but they have 2 ventricles
  • The increased efficiency of the heart of birds and mammals is thought to be related to the evolution of endothermy.
  • Birds and mammals are also very active and have a high metabolism, and require good circulation of oxygenated blood to their tissues to allow this
  • Blood vessels include arteries, veins, arterioles, venules and capillaries. The first four all have the same basic structure:
    • Endothelium
    • Elastic fibers (two layers in arteries)
    • Smooth muscle (thicker in arteries)
    • Connective tissue
  • Arteries are under much higher pressure then veins. The elastic fibers in larger arteries allow them to recoil and not burst each time a pulse of blood comes through. The smaller arteries and arterioles are less elastic but are protected by their muscle layer.
  • This muscle layer allows for vasoconstriction, which lowers blood flow through a particular artery, and vasodilation to to open it up again. This can be used in thermoregulation and to regulate the amount of blood going to a particular organ
  • Blood pressure in veins is significantly lower than in arteries.
  • In bipeds, blood has a long vertical way to go from the feet back up to the heart. Venous pressure alone is not strong enough to push the blood back up. Your leg muscles help by squeezing the veins; this is called the venous pump. In addition to the valves in the veins, this moves the blood back up
  • Blood composition
    • Plasma: carries endocrine molecules, proteins, solutes, that includes nutrients, gasses, and wastes
    • Formed elements: Erythrocytes, thrombocytes, leukocytes
  • Capillaries are where gas, nutrient and waste exchange happens between the blood and the cells.
    • They consist of a layer of endothelium and a basement membrane; their diameter is barely larger than that of an erythrocyte.
    • Blood slows right down when passing through capillaries because of the small diameter of the vessels, allowing more time for the exchange of materials.
  • There are three types of capillary, and they vary in their permeability and location
    • Continuous: complete basement membrane and continuous endothelium with intercellular clefts. Least amount of exchange. Found in muscle, skin, lungs and CNS
    • Fenestrated: complete basement membrane and continuous, fenestrated endothelium. Greater exchange. Found in endocrine glands, small intestine and kidneys.
    • Sinusoid: discontinuous basement membrane and discontinuous, fenestrated endothelium. Maximum exchange. Found in liver, spleen and bone marrow
  • The circulatory part of the lymphatic system consists of lymphatic capillaries, vessels and ducts
  • Fluid enters the blind lymph capillaries that have very permeable walls. The vessels get progressively larger and have valves similar to those in veins. The ducts empty back into the bloodstream at the junctions of the jugular and subclavian veins on each side of your body
  • Movement of the body muscles moves the lymph along. Some fish, all amphibians and reptiles and some birds have lymph “hearts.
  • Vessels in the lymphatic system function as part of the circulatory system.
  • A significant amount of water and solutes leave the capillaries to form interstitial fluid. Some of it goes back into the capillaries, but more comes out than goes back in.
  • This fluid is returned to the bloodstream by the lymphatic system. This helps to maintain normal blood pressure and volume and prevents fluid from building up in tissues (edema)
  • As it travels through the lymphatic vessels, the lymph passes through lymph nodes, which filter out all the waste and bad things. The filtered lymph then enters the bloodstream
  • Lymph can contain many substances:
    • Cancer cells
    • Damaged cells
    • Fats
    • Bacteria and viruses
    • White blood cells (lymphocytes)