Elimination (1948)

because living processes are taking place within it the body produces many substances some are waste materials and the body has to get rid of them they are passed out through four different routes through the skin the kidneys the lungs and the cola let us consider the first route the skin this is a drawing of a much enlarged surface view of the skin as you can see by the size of the hairs this is a drawing of a section of the skin also much enlarged the top part of the skin is called the epidermis and the lower part is called the dermis the sweat glands lie in the dermis each sweat gland is a little tube one end is coiled into a ball the other end opens onto the surface of the epidermis as a pore a network of blood vessels surrounds the coiled portion of the glass water is taken from the blood in these vessels by the sweat glands which pass it out onto the surface of the skin as sweat sweat is mainly water but it has very small quantities of other materials dissolved in it although water is dispersed off through the skin the chief purpose for this elimination is to help control the temperature of the body there are about two million sweat glands in the human skin they are found abundantly in the armpits in the palms of the hands and in the soles of the feet although the skin excretes water most of the water passed out of the body leaves through the second root of excretion the kidneys embedded in protective fat the two kidneys lie one on each side of the spine in the small of the back each kidney is covered by a very thin membrane called the capsule let us look at the kidney in section the outer part is called the cortex and the inner part is called the medulla the kidney has a hollow interior basin which is called the pelvis this is formed by the expanded upper part of a muscular tube the ureter which leads away from the kidney the kidney substance is made up of a number of tubules each kidney contains about a million of them let us look at a tubule greatly simplified each tubule starts in the cortex where this end forms a small cup the tubule follows a very complicated course through both parts of the kidney the kidneys receive a copious supply of blood branches from a large artery lead into little ball-like clusters of capillaries which lie inside the cups of the tubules blood flowing through the capillaries in the cup passes into a second network of capillaries wrapped around other parts of the tubule the blood then leaves the kidney through small veins which unite until they form a large vein carrying the blood back toward the heart nine tenths of the fluid part of blood is water containing many dissolved substances such as simple mineral salts and urea water and some of the dissolved substances pass out of the blood through the walls of the capillary clusters and into the cup the fluid then passes along the tubule which is lined with special cells in some parts of the tubule these cells select and take back from the fluid some of the water and any other substances needed by the body these pass out of the tubule through the walls of the second capillary network and back into the blood the remainder of the fluid flows on through the tubule this fluid water with waste substances such as urea dissolved in it is called urine the tubule joins up with other tubules and a collecting duct is formed the urine flows down the duct into the pelvis of the kidney the urine then passes down the ureters into the bladder the bladder acts as a reservoir in which the urine is contained until it's excreted from the body the third route of excretion is provided by the lungs air drawn in through the nose or mouth is carried to the lungs by the trachea which divides into two tubes called the bronchi the bronchi divide and subdivide forming a large number of smaller tubes the very smallest tubes lead into clusters of air sacs the lungs contain millions of these aspects a network of small capillary blood vessels is pressed closely around the sex surplus carbon dioxide in the blood passes through the walls of the blood vessels into the sex carbon dioxide is produced in the oxidation of food as we breathe out the air sacs contract and some of the carbon dioxide is carried in the expired air up the bronchial tubes through the trachea and then out through the nose or mouth a certain amount of water vapor is also excreted by this root the fourth route of excretion is the colon indigestible foodstuffs with water and salts pass from the small into the large intestine the colon in a semi-liquid state the contraction and relaxation of the muscular wall of the colon moves this material slowly along towards the rectum through the walls of the ascending colon much of the water and some of the salts are absorbed into blood vessels the remainder is waste material chiefly made up of indigestible ingredients from food bacteria worn out cells from the lining of the intestine and substances from the digestive juices this material finally leaves the body as a semi-solid as we have seen water is lost to the body through the lungs and through the skin when we breathe out the body loses water in the form of vapor water is also lost in the regulation of temperature by the production of sweat the tissues of the body are largely made up of water it is therefore very important there should always be enough water in the blood to supply the tissues the amount of water passed into or out of the blood is regulated by the kidneys in hot weather much water leaves the body of sweat to compensate for this more water is passed out of the kidney tubule back into the blood the amount of urine leaving the kidneys is therefore less in cold weather less water leaves the body as swift a smaller quantity of water is taken back into the blood and the flow of urine therefore increases this adjustment in the amount of fluid excreted by the kidneys in proportion to the amount of water leaving the body by other roots is a good example of the way in which different systems of the body can work together you