The role of the circulatory system in animals is the transportation of nutrients and oxygen to every cell that is in an animal organism, and to also remove waste products. The heart, blood vessels and blood are three vital components the body needs to survive. There are also other major roles the circulatory system has which are later discussed.
Mammals have a double circulatory system meaning two circuits that blood journeys through; pulmonary and systemic.
Pulmonary: Pulmonary circulation is the transportation of blood from the heart, to the lungs, and back to the heart again. The pulmonary circuit transports blood to the lungs for it to be oxygenated and then transported back to the heart. In the lungs, carbon dioxide is taken away from the blood, and oxygen taken up by the haemoglobin in the red blood cells.
Systemic: The systemic circuit transports blood around the body to deliver the oxygen and returns de-oxygenated blood to the heart. Systemic circulation provides nutrition to all of the tissue located in the organism, with the exclusion of the heart and lungs as they have their own systems.  See below for a diagram of the circulatory system.
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This diagram is showing the circulatory system; the pulmonary circuit where it is picking up oxygen from the lungs, and the systemic circuit which is transporting oxygen to the body.
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Heart: The heart is a muscular pump, when the heart is beating it is pumping blood to the lungs and around your body. The amount of blood pumped can be calculated.
Heart rate x stroke volume = cardiac output.
Below is a diagram of the heart.
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This diagram of the heart is shown from in the front. So the right side is shown on the left. The left side is on the right side of the diagram. The heart has four chambers. The two atria gather the blood. The two ventricles pump blood out of the heart. Valves stop the blood from flowing backwards. The septum splits the two sides of the heart. The right side of the heart pumps de-oxygenated blood to the lungs to pick up oxygen. The left side of the heart pumps the oxygenated blood from the lungs around the rest of the body.
Blood Vessels: There are three different types of blood vessel:
Arteries
Arteries transport oxygenated blood from the heart, except from the pulmonary artery which goes to the lungs where the blood would be deoxygenated.
Arteries have thick muscular walls and have small lumen and they contain blood which is under high pressure.
Veins
Veins transport blood to the heart which is always de-oxygenated except the pulmonary vein which goes from the lungs to the heart where the blood would be oxygenated. Veins have thin walls and larger lumen and they contain blood which is under low pressure. Veins also have valves to stop blood from flowing backwards.
Capillaries
Capillaries are located in the lungs and muscles, when capillaries are looked at under a microscope they are one cell thick, blood is of very low pressure. The capillaries are where oxygen passes through the capillary wall into the tissues and where carbon dioxide passes from tissues in to the blood.
Blood: Animal organisms can’t survive without blood. Without blood, organs wouldn’t get oxygen and nutrients that they need to live; animals wouldn’t be able to keep warm or cool down, fight infections, or get rid of waste products. Without enough blood, animals would weaken and die.
The circulatory system works carefully with other systems in animal organisms. It delivers oxygen and nutrients to organisms by working with the respiratory system. The circulatory system assists carrying waste and carbon dioxide out of the organism.
The circulatory system also has its part in fighting disease in carrying specialised cells which are made in the organs of the immune system.
The circulatory system is responsible for the transportation of hormones. Hormones control vast amount of things such as growth, the reproductive cycle and glucose metabolism. Hormones are produced in one part of the body, such as the brain or the liver, and then must be moved to another part of the body by the circulatory system for them to transport their message.
The circulatory system’s other main role is to regulate body temperature, if body temperature rises then blood vessels close to the skin increase in size so that more heat is directed in to the air and vice versa if body temperature drops the blood vessels decrease in size so the heat will retain in the body.
Factors which can influence transportation and circulation in animals
High blood pressure (Essential hypertension) where there is no specific cause.
High blood pressure of a known cause (secondary hypertension).
Low fluid volume which will also include low blood pressure.
Low cardiac output. (Β-Adrenoreceptor antagonists).
Obstruct membrane changes and cardiac output, will cause widening of the blood vessels.
Aneurysms, where there is weakening in the artery walls, mainly the aorta.
Arteriosclerosis is where the artery walls are hardening and thickening: loss of elasticity which is part of aging.
Atherosclerosis which is the process in the progression of plaques in the lumen which is located in blood vessels.
 All of these may also be influenced by physiological factors for e.g. diet, exercise, disease, drugs or alcohol, obesity and excess weight.
Control mechanisms in animals
Self-regulating mechanisms, where biological systems try to uphold stable internal conditions e.g., blood pressure and body temperature, when there are changes in the external environment. Internal environment of any living organism was upheld constant within certain restrictions. Homeostasis is usually achieved through two types of regulating systems: on-off control and feedback control. Hormones often play a main role in keeping homeostatic constancy. Homeostasis is carried out around the whole body; reaches every cell up to organs and systems.
Enzymes could not work properly, which means nothing could operate correctly if there is not a constant internal environment this would mean the living organism would die.
Every single cell is bathed in a watery solution, which is made by some blood plasma which is allowed to escape out of blood. This will carry away any waste back into the blood.
The balance in tissue fluid is vital for the cells and the organism. There are six things that must be controlled in an organism for good health:
Carbon dioxide
Additional carbon dioxide must be taken away or else the body becomes too acidic. Carbon dioxide is mainly lost in the air we breathe out, but a minor amount is lost in the urine.
Urea
Urea is poisonous and must be removed from the organism; this chemical is made when amino acids are digested in the liver. This is a waste chemical and is mainly removed through urine and sweat.
Ions
Cells can end up swollen, shrivelled or sometimes burst, if the right balance is not kept in ions. Sodium, potassium, hydrogen and phosphate are important ions. These are controlled through how much water is drunk by the organism; some are lost like sodium ions, through faeces and sweat.
Sugar
There has to be enough glucose for respiration and satisfactory stores of glycogen. When blood glucose levels fall too low the organism will die.
Water
70% of body mass is water. Not keeping the right amount of water the organism would die.
Temperature
Enzymes that control all the chemical reactions in an organism work best at the temperature of approx. 37 degrees centigrade, if the organism was to get too hot or too cold the enzymes would die concluding in the organism to die.