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Transport in animals (Features of the transport system (A system of…
Transport in animals
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The heart
The cardiac cycle
Ventricular systole
The ventricle walls contract and increase blood pressure in the ventricles. This forces blood up through the semilunar valves, out of the heart, into the pulmonary artery and the aorta. The blood cannot flow back from the ventricles into the atria because the tricuspid and bicuspid valves are closed by the rise in ventricular pressure. The pulmonary artery carries deoxygenated blood to the lungs and the aorta carries oxygenated blood to the rest of the body.
Diastole
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The left atrium contracts, pushing the remaining blood into the left ventricle, through the valve.
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When full, the pressure forces open the bicuspid valve between the atrium and the ventricle.
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With the left atrium relaxed and the bicuspid valve closed, the left ventricle contracts. Its strong muscular wall exerts high blood pressure.
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This pressure pushes blood up out of the heart, through the semi lunar valves into the aorta and closes the bicuspid valve, preventing backflow of blood into the left atrium.
Atrial systole
The atrium walls contract and the blood pressure in the atria increases. This pushes the blood through the tricuspid and bicuspid valves down into the ventricles, which are relaxed.
The sides ofth heart work together. The atria contract at the same time, followed by the ventricles contracting together. A complete contraction and relaxation of the whole hart is called a heartbeat.
Valves
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The atria-ventricular valves, semilunar valves (at base of the aorta) and pulmonary artery and semilunar valves in veins all operate by closing under high blood pressure, preventing blood flowing backwards.
Control of heartbeat
Contraction of cardiac muscle myogenic. The wall of the right atrium has cluster of cardiac cells, the Sino-atrial node, acts as pacemaker.
The impulses cause the cardiac muscle in each ventricle to contract simultaneously, from the apex upwards.
The AVN passes the excitation down the nerves of the bundle of His and the left and right bundle branches and to the apex of the heart. The excitation is transmitted to purknje fibres in the ventricle walls, which carry it upwards through the muscles of the ventricle walls.
The ventricles are insulated from the atria by a thin layer of connective tissue, at the atria-ventricular node. The AVN introduces a delay in the transmission of the electrical impulse. The muscles of the ventricles do not start to contract until the muscles of the atria have finished contracting.
A wave of electrical stimulation arises at the SAN and spreads over both atria, so they contract together.
Blood
Red blood cells
They are red because they contain the pigment haemoglobin, the main function in which is to transport oxygen from the lungs to the respiring tissues. Red blood cells are unusual in 2 ways.
Biconcave discs (larger surface area and thin centre for diffusion) and have no nucleus (more room for haemoglobin).
Plasma
A pale yellow liquid, 90% water, containing solutes such as food molecules (glucose, amino acids, vitamins and minerals), waste products, hormones and plasma proteins (distributes heat).
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Transport of oxygen
Haemoglobin binds to oxygen in the lungs, and releases it in the respiring tissues.
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Circulatory systems
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The blood doesn't move around the body in blood vessels but it bathes the tissues directly while held in a cavity called the haemocoel.
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