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The Mammalian Heart and Circulatory System

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Introduction

The Mammalian Heart and Circulatory System The heart is a muscular structure that contracts in a rhythmic pattern to pump blood. Hearts have a variety of forms: I am going to look at the mammalian heart which is comprised of four chambers. The heart is the centre of the cardiovascular system. On average the heart beats over 100 000 times a day and pumps 3780 litres of blood through 100 000 kilometres of blood vessels. The circulatory system functions in the delivery of oxygen, nutrient molecules, and hormones and the removal of carbon dioxide, ammonia and other metabolic wastes. It also helps to maintain fluid balance, regulate body temperature, and it assists in the defence of the body against invading microorganisms. Mammals have a double circulatory system: a pulmonary system, which carries blood between the heart and lungs, and a systemic system, which carries blood between the heart and the rest of the body. Capillaries are the points of exchange between the blood and surrounding tissues. Material crosses in and out of the capillaries by passing through or between the cells that line the capillary. The structure of the heart is shown in the diagram below: Mammals have a double circulation, which means that the right hand side of the heart pumps deoxygenated blood to the lungs in the pulmonary artery to pick up oxygen and release carbon dioxide. The oxygenated blood then returns to the left-hand side of the heart in the pulmonary vein. ...read more.

Middle

E.g. during heavy exercise, the heart rate and the breathing rate will increase significantly. When the atria and ventricles are in diastole, blood at a low pressure in the veins flows into the atria which fill with blood and increase this pressure, meanwhile the atrioventricular valves are shut. With rising atrial pressure these valves are then pushed open and the ventricles start to fill with blood. During the 0.3 seconds of contraction, there is both atrial systole followed by ventricular systole. The sinoatrial node initiates an electrical impulse that travels across the atrial walls. This contracts the myocardium of each atrium and atrial systole then forces all remaining blood past the atrioventricular valves into the ventricles. The semi-lunar valves are closed during this. The electrical impulse passes from the atrioventricular node to the bundle of His then to the purkinje fibres. There is a slight delay at the AV node to ensure that both contract at the same time. The myocardium of the ventricles contract, the bicuspid and tricuspid valves remain closed, and the pressure increases in the ventricles and pushes open the semi-lunar valves. This whole contraction results in blood flow out of the heart and into the pulmonary and systemic circulatory systems. Cardiac contractions are initiated by an electrical impulse that originates from the sinoatrial node (pacemaker). The waves spread out over the two atrial walls so that they contract. There is a band of fibres between the atria and ventricles, which have a high electrical resistance so the waves cannot spread from the atria to the ventricles. ...read more.

Conclusion

Nutrients, wastes, and hormones are exchanged across the thin walls of capillaries. Capillaries are microscopic in size, although blushing is one manifestation of blood flow into capillaries. Control of blood flow into capillary beds is done by nerve-controlled sphincters. This is the capillary structure: The extensive network of capillaries in the human body is estimated at between 50,000 and 60,000 miles long. Blood leaving the capillary beds flows into a progressively larger series of venules that in turn join to form veins. Veins carry blood from capillaries to the heart. With the exception of the pulmonary veins, blood in veins is deoxygenated. The pulmonary veins carry oxygenated blood from lungs back to the heart. Venules are smaller veins that gather blood from capillary beds into veins. Pressure in veins is low, so veins depend on nearby muscular contractions to move blood along. The veins have valves that prevent back-flow of blood. The structure of veins: As blood gets farther from the heart, the pressure likewise decreases. Each contraction of the ventricles sends pressure through the arteries. Elasticity of lungs helps to keep pulmonary pressures low. Systemic pressure is sensed by receptors in the arteries and atria. Nerve messages from these sensors communicate conditions to the medulla in the brain. Signals from the medulla regulate blood pressure. In these blood vessels vital substances are transported in the blood to the tissues around the body. This is for processes such as respiration to take place. Substances include glucose, oxygen taken to the tissues, and carbon dioxide and metabolic waste, which are taken back from the tissues as waste products. ...read more.

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