The heart is considered as the body's circulatory pump, located in the thoracic cavity. It is made up of four chambers, two upper chambers known as Atria and two lower, thicker walled chambers, known as Ventricles. It has a strong muscular wall and is made up of a special type of muscle known as myocardium, which is only found in the heart. The septum divides the two sides of the heart and four one way valves control the blood flow. The blood flow carries oxygen and vital nutrients to all parts of the body. The blood then returns to the heart via the veins. The blood returns to the heart at the same rate as it is pumped out, it takes just one minute for it to complete a full circuit of the entire body. (Refer to diagram) (Taylor, 1999-2013). 
The flow of blood through the heart:
De-oxygenated blood returning from the body enters the superior and inferior vena cava. It enters the right atrium, is pumped through the tricuspid valve into the right ventricle, from the right ventricle it is pumped through the pulmonary semilunar valve to the pulmonary trunk. The pulmonary trunk carries the blood to the lungs, releases carbon dioxide and absorbs oxygen. The blood returns to the heart through pulmonary veins. From the veins, it enters the heart again through the left atrium. The left atrium contracts and pumps blood through the bicuspid valve (mitral) into the left ventricle. The left ventricle pumps blood through the aortic semilunar valve into the aorta. From the aorta blood enters the systemic circulation throughout body tissues until it returns to the heart. Then the cycle begins again. The heart beat sound is known as the 'lubb' and 'dupp', this is where the blood is pushing on the valves of the heart. The 'lubb comes first and is longer. The 'dupp' is shorter and comes second. This is caused by the opening and closing of the semilunar valves. (Cancer Research, 2013). 
The heart wall is made up of three layers:
The muscular middle layer, containing striated cardiac muscle tissue and is responsible for pumping blood and the heart's pumping action. (Taylor, 1999-2013)
This lines the inside of the heart and is very smooth and made up of loose connective tissue and simple squamous epithelial cells. It prevents blood from sticking to the inside of the heart and also prevents blood from clotting. (Taylor, 1999-2013)
This is the outermost layer of the heart, made up of connective tissue. It protects the
Inner structure of the heart and is also known as the visceral pericardium. It helps hold the other muscles close together and also acts as an anchor for the heart. (Taylor, 1999-2013). 
Blood Tissue, Its Function and Circulation
Blood is the body's transport system and works to perform several functions. Although it is a fluid it is made up of connective tissue and is vital for the body as it could not survive or remove bodily wastes, without it cells and body tissues would die.(Anglin, 2014) The circulatory system is pumped by the heart and always circulates through the body in the same direction. Arteries carry blood full of oxygen to all parts of the body, the further away from the heart the smaller they become, eventually turning into capillaries. Blood helps to keep the body at the right temperature and maintains blood pressure and volume. It transports oxygen and carbon dioxide for delivery and disposal. It transports oxygen and nutrients to cells, removes waste and transports hormones. (Cancer Research, 2013) 
Plasma is the fluid part of blood and makes up the most volume. It is a pale straw coloured liquid of which 90% is water. It contains proteins, glucose and other dissolved nutrients. Its function is to make sure cells flow throughout the body and also helps to maintain the body's blood pressure and volume. (Ivy Rose, 2003-2013) 
Red Blood Cells:
Erythrocytes are flexible disc shaped cells with no nucleus, their main function is to transport oxygen to the body's cells and deliver carbon dioxide to the lungs. It contains enormous amounts of protein called haemoglobin, which is responsible for its dark or bright red colour. They are also important in determining your blood type. (Ivy Rose, 2003-2013). 
White Blood Cells:
Leucocytes are the body's defence army within the blood tissue and make up less than 1% of the blood. They are the only true cells in the blood and are not limited to the blood stream. There are many types of white blood cells, each one has a different structure and function and are part of the immune system. (Ivy Rose, 2003-2013). 
Thrombocytes these are cell fragments that help in blood clotting. At the site of an injury, platelets will stick to the exposed collagen and trap other blood cells in the form of a clot and stop blood from flowing and preventing excessive blood loss. The platelets contract and pull the injury back together, the blood will clot and aid in healing. (Ivy Rose, 2003-2013).
Blood Tissue Function
Blood is mainly for transportation of dissolved gases and of metabolic waste. It also helps to maintain the body's temperature. It removes toxins from the body, via the kidneys in the form of urine or sweat. (Pearson, 2014). 
Blood Vessels Structure and Function
Arteries: these carry blood from the heart under high pressure. They have thick outer walls and thick layers of muscle with elastic fibres. (Pearson, 2014). 
Veins: these work under low pressure and have thin walls and thin layers of muscle with elastic fibres. They have one way valves so that blood continues to move in the right direction. (Pearson, 2014). 
Capillaries: their function is to allow food and oxygen to diffuse cells and waste is diffused from the cells. They have thin walls, only one cell thick which allows them to perform their job more effectively. (Pearson, 2014). 
The Respiratory System
The primary function of the respiratory system is to supply the body with oxygen through breathing, this action is involuntary. By inhaling oxygen and exhaling carbon dioxide, through gas exchange. This process is achieved through the alveoli, which are adapted to make gas exchange happen easily and efficiently. The alveoli has an enormous surface area due to the blood vessels and capillaries, the walls are moist and thin, this is because of a substance secreted inside the alveoli known as surfactant and is produced by specialized cells. Because the surface area of the alveoli is so big this enables gas exchange to take place more efficiently. Gases move by diffusion, from high concentration to low concentration, oxygen diffuses from the air in the alveoli into the blood and carbon dioxide diffuses from the blood into the air in the alveoli. Your respiratory system is made up of airways, your lungs and blood vessels and muscles attached to them. Air is sucked in through the mouth and nose, through the voice box, down the windpipe, and into the two lungs. Tiny mucous covered hairs known as cilia, act as filters to remove any foreign particles or germs. Once in the lungs the air goes to blood vessels that are connected to veins and arteries, which then carry it throughout your whole body. When you exhale carbon dioxide leaves the body the same way oxygen was taken in. Coughing or sneezing is our body's way of removing these particles and germs. Abdominal muscles help the lungs to expand and contract in order for us to inhale and exhale. (Taylor, 1999-2013). 
Our respiratory systems are made up of many components: (Pearson, 2014). 
Cellular respiration is made up of three main stages, glycolysis, which means 'splitting sugars'. This can occur with or without oxygen, without oxygen glycolysis makes only small amounts of ATP, this is known as fermentation. (Bailey, 2014). 
The second stage is known as Krebs cycle or the citric acid cycle. Two molecules of the three carbon sugars produced in glycolysis is converted into acetyls COA, known as NAD and FAD. These reduced forms carry 'high energy' electrons to the next stage. Citric acid only occurs when oxygen is present, but does not use oxygen directly. (Bailey, 2014). 
The third stage is electron transport and requires oxygen directly. It's a series of reactions where 'high energy' electrons are passed into oxygen. This process produces ATP. (Bailey, 2014).
Equation for Cellular Respiration: (Sharata, 2011).
Aerobic and Anaerobic Respiration
Aerobic respiration happens when glucose reacts with oxygen; it takes place in the mitochondria. For one molecule of glucose aerobic respiration produces 38 ATP, anaerobic only produces 2 ATP molecules. The end product of aerobic respiration is carbon dioxide and water and the end product of anaerobic respiration is ethyl alcohol or lactic acid and carbon dioxide. Therefore, aerobic respiration produces significantly more ATP than anaerobic respiration. (Scoville, 2014).
Effects of Exercise on Cardiovascular and Respiratory System
When you begin exercising your energy requirements increase instantly, the increased demand places stress on the cardiovascular and respiratory system to deliver more oxygen to working tissue cells and to remove carbon dioxide to remove carbon dioxide and other waste from tissue cells. (Ohara 2009). Blood flow must increase and the heart must pump more blood. Regular exercise results in increased cardiac output, this results in an improved delivery of oxygen to active muscles. The more you exercise the more efficient cardio-respiratory system becomes at supplying oxygenated blood throughout your body. Exercise can also form an addiction, which could also be detrimental to your health. (Kochen, 2014).
Excessive exercise can have an adverse effect on the heart, scarring, enlargement and stiffening of the heart can occur with excessive training. There could be an increased risk of stroke and possibly developing heart rhythms. (Web MD, 2005-2014). Also you could suffer from exercise induced asthma or a condition known as bronchial hyper-responsiveness, which is obstruction of the airways after exercising. (Troosters, et al, 2014).
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