Circulatory System.docx

Humans The Circulatory System and Blood A circulatory is needed to ensure all cells can obtain enough oxygen and nutrients, and that their metabolic waste is remove before it can cause damage. There are two types of circulatory systems, Open systems, and Closed systems. Open Systems In open systems,...

Humans The Circulatory System and Blood A circulatory is needed to ensure all cells can obtain enough oxygen and nutrients, and that their metabolic waste is remove before it can cause damage. There are two types of circulatory systems, Open systems, and Closed systems. Open Systems In open systems, the heart pumps a fluid called haemolymph into an open central cavity called the haemocoel. As the haemolymph moves across the haemocoel, it exchanges materials with the surrounding cells. It then reaches collecting vessels at the other side, through which it is returned to the heart and pumped back into the other side of the haemocoel. This is found in animals like snails and insects. Closed Systems Blood is always enclosed in blood vessels. Tissue fluid bathes the cells and acts as a medium through which substances are exchanged between the blood and the cells e.g., earthworms and vertebrates. The closed circulatory systems are more efficient than open ones because: - The blood can be pumped around the body faster and therefore the exchange of food and oxygen is faster. This allows the animal to be more active. - It allows the flow of blood to different organs to be increased or decreased Two-circuit Circulatory system In advanced animals, like mammals, a two-circuit system is used. A pulmonary system driven by the right side of the heart pumps deoxygenated blood from the heart to the lungs, where it gets rid of CO2 and collects oxygen before returning to the other side of the heart. A systemic circuit driven by the left side of the heart pumps oxygen-rich blood to all the cells of the body and delivers CO2 rich blood back to the opposite side of the heart. Both sides are interconnected but separated by the septum of the heart. Blood Vessels There are three main types of blood vessel: Arteries, Capillaries and Veins (3) Arteries Veins Capillaries Lumen Small Large Tiny Wall Thick Wall – Outer layer of collagen and inner layer of smooth muscle and elastic fibres Thin Wall 1 cell thick – Endothelium allows the exchange of materials between blood and cells Valves None (blood under pressure) Present (prevents backflow) None Direction of blood flow Transports blood away from the heart Transport blood to heart Link arteries to veins Blood Type Carries Oxygenated Blood (except pulmonary artery) Carries deoxygenated blood (except pulmonary vein) Oxygenated and deoxygenated Blood Flow Rapid flow under high pressure Slow flow under low pressure - assisted by squeezing action of nearby arteries and muscles Slow, pressure falling (allows time for exchange of substances to occur) Pulse Blood flows in pulses Blood flows at a steady rate No pulse Arterioles Arterioles are branches of arteries or small arteries. They have the same structure as arteries but are smaller. The muscle layer surrounding the arterioles can contract, causing the lumen of the arteriole to constrict. This can reduce blood flow. Venules Venules are small veins that are formed when capillaries start to join again. The blood in venules is under low pressure and does not pulse. It is also rich in CO2 and low in O2. The blood is moving back towards the heart. Blood Flow through the body (4) - Deoxygenated blood (dark red) leaves the right ventricle of the heart under high pressure through the semilunar valves into the pulmonary artery. This is the only artery that carries deoxygenated blood. It passes through the capillaries of the lungs, losing CO2 and collecting oxygen - Oxygen rich blood (bright red) leaves the lungs through the pulmonary vein and enters the left atrium. This is the only vein that carries oxygenated blood - The blood passes through the bicuspid valve into the left ventricle. It is pumped through the semilunar valves into the aorta. Some goes to the head through the carotid artery, the rest passes downwards to the body through the aorta - The hepatic artery goes to the liver and the mesenteric artery goes to the intestines The renal artery supplies blood to the kidneys where it is purified. The rest goes to the gonads and the legs - In each of these organs, the blood passes through capillaries and loses all its pressure and most of its oxygen before it starts its journey back to the heart through the interior vena cava (5) - Deoxygenated blood from the head and arms enters the heart through the superior vena cava. Deoxygenated blood from the rest of the body enters the heart through the inferior vena cava. - Both vessels empty into the right atrium. The contraction of the atrium causes this blood to flow through the tricuspid valve into the right ventricle. When the ventricle contracts, it forces the blood out through the semilunar valve into the pulmonary artery, which goes into the lungs, where the blood loses CO2 and collects oxygen - Oxygenated blood leaves the lungs through the pulmonary veins and enters the heart at the left atrium. The contraction of the atrium causes this blood to flow through the bicuspid valve into the left ventricle. When the left ventricle contracts, it forces the oxygenated blood into the aorta, through the semilunar valve. Blood from the aorta travels around the whole body Hepatic Portal System Veins that do not return to the heart but go to a second capillary bed are called portal veins. The vein leading from the intestines to the liver is called the hepatic portal vein. The hepatic portal vein brings blood rich in digested food, but lacking in oxygen, from the intestine directly to the liver. The blood is modified in the liver and then transported back to the heart. (6) Heartbeat The heart muscle is a specialised tissue. It is strong, branched, and never tires. The normal rate of the human heart when resting is about 75 beats per minute. CO2 levels in the blood are monitored by the Medulla Oblongata in the brain and this information is used to control the heartbeat. Each beat of the heart sends a pulse of pressure along the artery. This is most easily detected at the wrist or neck and is normally taken at the wrist using the first two fingers of the left hand. The heartbeat is the sound made by the valves of the heart closing. It is often described as a ‘lub-dub’ sound. The ‘lub’ is made when the bicuspid and tricuspid valves shut under the backpressure from the blood in the contracting ventricles. The ‘dub’ sound is made when the semilunar valves of the aorta and pulmonary artery slam shut at the backpressure from the arteries. Heart-muscle tissue is auto-rhythmic and will beat on its own. However, there is a pacemaker in place to coordinate the contraction of cells in sequence. It consists of a group of cells in the wall of the right atrium, called the sino-atrial (SA) node which initiates the actual contraction, and a relay station in the wall between the left and right atria near the ventricles, called the atrioventricular (AV) node. The Cardiac Cycle The Cardiac cycle is one complete sequence of blood filling and emptying the heart. - Diastole is the period of relaxation during which the heart fills - Systole is the period of contraction during which the heart empties - Both sides of the heart beat in unison During diastole, the muscles of the atria and ventricles relax, allowing blood to flood from the vena cava and pulmonary veins into the atria and ventricles. - Once the chambers are full, the SA node initiates contraction, which forces the last of the blood from the atria into the ventricles. Once the atria is completely empty, the AV node initiates systole. - This contraction slams the cuspid valves shut, preventing blood being forced back into the atria and forcing blood out of the heart through the semilunar valves into the aorta and pulmonary arteries - Both atria and ventricles relax, and diastole starts again. The semilunar valves prevent backflow of blood under pressure from the arteries Blood pressure - Blood pressure is the force exerted by the blood on the walls of the arteries due to the contracting of the heart. It depends on the volume of blood within the system and the space available within the blood vessels - Blood pressure is measured in an artery of the upper arm using a sphygmomanometer. - An inflatable cuff is used to measure the pressure required to stop the blood flow at this point. Two pressures are measured: systolic and diastolic pressures of the ventricles (120/80 mm Hg – for a healthy adult). - These values normally rise with age. If the lower of the two is above 95 the person is suffering from high blood pressure (hypertension). High blood pressure is often caused by blockages in arterioles or small arteries. A healthy circulatory system - Smoking: Smoking can cause cancer, emphysema, atherosclerosis, and high blood pressure which can lead to heart attacks and strokes - Diet: A diet rich in saturated fats can lead to obesity which can cause high blood pressure and a blockage of coronary arteries, leading to heart disease - Exercise: Regular exercise improves the efficiency of the circulatory system and reduces the risk of heart disease