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THE EXCRETION INTRODUCTION Waste substances are produced as a result of the nutrition of the cells in our body. The removal of waste from the body is called excretion. It is really a cleaning process in which the blood is filtered to get rid of all the dangerous substances it may contain. The excretory system also eliminates toxic substances that come from the exterior of the body such as the chemicals in food additives, medicines, air pollution, smoking, cosmetics… EXCRETORY ORGANS We eliminate waste through different organs of the body: kidneys (urine), sweat glands (sweat), lungs (CO2) and liver (bile). - Sweat glands: Our skin has millions of sweat glands which produce sweat, a liquid similar to urine. It is produced by filtering the blood that reaches the skin. Another function of the sweat glands is to cool the skin: when the temperature is high, sweat increases. As it evaporates from the skin, the skin is cooled. - Liver: Some waste products and other toxic substances are added to bile, which drains into the intestine. From here, it is expelled from the body. - Lungs: They remove carbon dioxide (CO2) from the blood and expel it from the body. - Urinary system: This is the most important part of the excretory system. It eliminates waste products (mainly urea, ammonia and uric acid) and other toxic substances in the form of urine. In adittion to its excretion function, it regulates the amount of water and the concentration 1 and type of minerals in the blood (it expels non waste products that are in excess such as water and mineral salts). Curiosity: these 4 excretory organs are the most known but there are many other forms to eliminate toxic substances: the cells that cover the skin and the walls of the intestine are renewed very fast, being able to accumulate toxic substances and when the cells are shed, the toxic substances are eliminated outside the body (some eczema in the skin and some inflammatory bowel diseases are caused by the accumulation and elimination of significant amounts of toxic substances by these pathways). Tears can also eliminate toxic substances. Besides toxic substances can be accumulated in nails and hair (nail testing and hair testing can be used to detect drug consumption in the last few months or weeks). The urinary system is made up of two kidneys, two ureters, a urinary bladder and a urethra. Kidneys filter the blood to produce urine. The ureters are two narrow tubes (around 28 cm long) connecting the kidneys to the bladder. The urinary bladder is an elastic and muscular sac where urine collects before it is expelled from the body thanks to the contraction of the bladder that pushes the urine out via the urethra, a tube that connects the urinary bladder to the outside of the body. In men the urethra is connected to the reproductive system and in women the urethra is separated from the reproductive system. This means that in men urine and semen pass through it (at different times) and the men’s urethra is longer than women’s urethra because it goes through the penis. Exercise: What are the differences between men’s and women’s urethra? Exercises: What substances are concentrated in urine than in blood? Why? more Why do proteins and glucose not appear in urine? 2 THE CIRCULATORY SYSTEM INTRODUCTION The circulatory system is often seen to include two separate systems: the cardiovascular system, which circulates blood, and the lymphatic system, which circulates lymph. The essential components of the human cardiovascular system are the blood, blood vessels and the heart, and the main components of the lymphatic system are lymph, lymphatic vessels and lymphatic nodes. THE CARDIOVASCULAR SYSTEM It is made up of the blood, blood vessels and the heart. The heart pumps blood through the blood vessels mainly to carry to all the tissues the nutrients (included O2) our cells need and to take away from the tissues the waste produced by our cells (included CO2). BLOOD The average human adult has more than 5 litres of blood in his or her body. Blood is the transport medium of nearly everything within the body. It transports hormones, nutrients, oxygen, antibodies, and other important things needed to keep the body healthy. Blood is a thick, red liquid composed of plasma and blood cells. Blood plasma is a yellowish liquid made up of water containing dissolved molecules such as nutrients (mineral salts, glucose, amino acids, vitamins…), waste products (CO2, urea, uric acid…), important proteins such as antibodies, hormones… - - There are three types of blood cells: Red blood cells or erythrocytes: These are the most common type of blood cell (around 5 million per mm3 of blood). They are disc-shaped and have no nucleus. They contain haemoglobin, a red molecule that contains an atom of iron. This molecule is responsible for the blood´s red colour and transports oxygen from the alveoli of the lungs to the body´s cells, and carries CO 2 from these cells back to the alveoli. White blood cells or leukocytes: They protect the body from infections. There are around 8,000 per microlitre (mm3) of blood. 3 - Platelets or thrombocytes: These are not true cells, but cell fragments. They allow the blood to coagulate which prevents haemorrhages. There are around 300,000 per mm3 of blood. Blood cells are produced in the bone marrow, which is found inside certain bones. Some leukocytes (lymphocytes) mature in the lymph nodes. - The functions of blood are: Blood carries oxygen and nutrients to all cells and takes away their waste products. It defends our body from infections. It contains platelets that can form a plug in a damaged blood vessel to prevent blood loss. It regulates the body temperature, moving body heat from the warmer areas to the cooler ones. It transports hormones around the body. BLOOD VESSELS There are three main types of vessel: arteries, capillaries and veins. Arteries carry blood from the heart to the body’s tissues, while veins carry blood back from the tissues to the heart. Small arteries are called arterioles and small veins are referred to as venules. The third and smallest type of vessel, capillaries, form a network connecting the smallest arterioles with the smallest venules. So the route is: heart → arteries → arterioles → capillaries → venules → veins → back to the heart and the route starts again. Arteries have relatively thick elastic walls that enable them to withstand the high pressure of blood pumped from the heart. By the time the blood has been forced through capillaries and arrived in veins, its pressure has dropped, so veins have thinner walls. Veins have one-way valves to stop blood from running back the wrong way. Capillaries are so small that blood cells can only move through them one at a time and their walls are very narrow (formed by a single layer of flat cells), allowing substances (nutrients and waste) to be exchanged quickly and easily between the blood and the tissues. Oxygen and food nutrients pass from these capillaries to the interstitial fluid (liquid between the cells) of the tissues and waste pass from the interstitial fluid to the capillaries. Cells exchange these substances with the interstitial fluid (the interstitial fluid is the intermediary between blood and cells). Exercise: Fill in the gaps (using the words arteries, capillaries or veins): ……………. carry blood from arterioles to venules, ………….. carry blood from the heart to the body’s tissues and …………….. carry blood from the body’s tissues to the heart. ……………. have thick elastic walls, ………….. have thinner walls and ………………walls are very narrow to allow the exchange of substances between blood and tissues. The blood pressure is high in ………. and low in ……….……. Valves in the …..……….. prevent backflow of blood. 4 THE HEART The heart is a hollow organ made up of muscle tissue (cardiac muscle). The cardiac muscle contracts and then relaxes, on average, 75 times per minute. Each contraction is called a heartbeat. Parts of the heart It is divided into two unconnected parts, the right and the left, by an internal wall. Each part is divided into two chambers: the upper chamber is the atrium (auricle) and the lower chamber is the ventricle. The heart therefore has 4 chambers: two atria and two ventricles. Each atrium and ventricle are connected by a valve (atrioventricular valve) to stop blood from running back the wrong way. The valve between the right atrium and the right ventricle is called the tricuspid valve, and the one between the left atrium and the left ventricle is called the mitral valve. Vessels of the heart Several veins lead to the atria: a superior and a inferior vena cava on the right and four pulmonary veins on the left. Two arteries lead away from the ventricles: the pulmonary artery on the right and the aorta on the left. At the beginning of each of these arteries are the pulmonary valve and the aortic valve, respectively; these are called the semilunar valves. They prevent blood from flowing back to the heart after it has left. Blood Circulation: A double circuit The human blood circulation is double because the blood has to pass through the heart twice in order to complete its circuit. The heart pumps oxygenated blood to the body (systemic circulation) and deoxygenated blood to the lungs (pulmonary circulation). In the human heart there is one atrium and one ventricle for each circulation: • Pulmonary circulation: The pulmonary circuit carries de-oxygenated blood to the lungs and returns oxygenated blood to the heart. This circulation stars in the right ventricle, where the pulmonary artery leads away from the heart and carries oxygen-depleted blood to the lungs for reoxygenation and removal of carbon dioxide in the dense network of capillaries that surrounds the alveoli. The capillaries turn into venules, which join together to form four pulmonary veins which 5 lead to the left atrium. From here the blood flows through the mitral valve to the left ventricle, where systemic circulation begins. • Systemic circulation: The systemic circuit carries oxygenated blood around the body to deliver the oxygen and returns de-oxygenated (poor in oxygen) blood to the heart. This circulation stars in the left ventricle and flows along the aorta, which splits into arteries, arterioles and then into capillaries. These vessels lead to all body tissues and organs except the lungs. The blood in the capillaries provides cells with the oxygen and nutrients they need, and takes away waste products (mainly carbon dioxide). It then returns via the venules, veins and venae cavae, which lead to the right atrium. From here the blood flows through the tricuspid valve to the right ventricle, where pulmonary circulation begins. Blood circulation in humans is double, closed and complete. Double because we have the pulmonary and the systemic circulation, closed because the blood is always inside the vessels and complete because oxygenated and deoxygenated blood are never mixed. The right and the left part of the heart are separated by an internal wall, so the oxygenated blood in the left part does not mix with the deoxygenated blood of the right part of the heart. The left ventricle is thicker (more muscle) than the right ventricle because it has to pump the blood stronger (with more pressure) so that the blood gets all the parts of the body except the lungs. Exercise: What are the main differences between the pulmonary and the systemic circulation? Exercise: Use arrows to indicate all the places that a red blood cell goes through since it is into the right atrium until it gets there again. Besides, indicate the circuit that contains oxygenated blood and the circuit that contains de-oxygenated blood and indicate the pulmonary and systemic circuits. 6 The Cardiac Cycle The sequence of events in a single heartbeat is called the cardiac cycle. Since the average heart beats about 75 beats per minute, the length of each cardiac cycle is less than one second. During the cycle the chambers of the heart may be relaxed (diastole) or contracted (systole) and two sounds are produced: “lub” and “dub”. Each cardiac cycle (heartbeat) has 3 phases: atrial systole, ventricular systole and diastole. • Atrial Systole: The atria (left atrium and right atrium at the same time) contract, the atrioventricular valves (tricuspid and mitral) open and the blood moves into the ventricles. • Ventricular Systole: The ventricles (left and right at the same time) contract, the atrioventricular valves (tricuspid and mitral) close causing the first sound: “lub” and preventing the blood from going back to the atria. The semilunar valves (aortic valve and pulmonary valve) open, letting the blood out through the aortic and pulmonary arteries. • Diastole: The ventricles and the atria are relaxed. The semilunar valves (aortic valve and pulmonary valve) close causing the second sound: “dub” and preventing the blood from going back to the ventricles. The blood goes into the atria through the pulmonary veins and cavas. THE LYMPHATIC SYSTEM Water and substances from blood plasma filter out of the capillaries into the spaces between the cells in the body tissues. These spaces are filled with a fluid known as the interstitial fluid. Excess interstitial fluid is collected, and then returned to the bloodstream (veins), by the lymphatic system. Lymph is a clear liquid transported by the lymphatic system and it is formed from the interstitial fluid. Lymph also contains many lymphocytes, so the lymphatic system has a protective function; in addition, it regulates the interstitial fluid levels in the tissues (the lymphatic system drains off any extra fluid to stop the tissues from puffing up). Exercise: What are the two main functions of the lymphatic system? The lymphatic system is made up mainly of lymphatic vessels and lymphatic nodes. Lymph travels along the lymphatic vessels until it is emptied in a vein close to the neck (subclavian vein). Along this route there are lymph nodes, especially in the armpits, chest and abdomen. Lymph 7 nodes contain many lymphocytes. Lymph is filtered through the lymph nodes before being emptied into the blood. Bacteria, viruses and cancer cells picked up from the tissues by the lymph are trapped in the lymph node. Lymphocytes can then attack and kill the bacteria. This is why your lymph nodes tend to swell if you have an infection. Lymphatic vessels have valves (similar to the valves in veins), which stop the lymph from running back the wrong way. Lymphatic vessels have valves (similar to the valves in veins), which stop the lymph from running back the wrong way. The lymphatic system does not have a pumping organ like the heart. So, the lymph moves very slowly. It is propelled by the movement of the body (the contraction of the muscles that surround the lymph vessels pushes it, this is similar to what happens in veins). Exercise: What could happen in the tissues of sedentary people in the long term? 8