Homeostasis PDF

# Homeostasis ## 14.1 Homeostasis in Mammals ### On these pages you will learn to: - Discuss the importance of homeostasis in mammals and explain the principles of homeostasis in terms of internal and external stimuli, receptors, central control, coordination systems, effectors (muscles and glands...

# Homeostasis ## 14.1 Homeostasis in Mammals ### On these pages you will learn to: - Discuss the importance of homeostasis in mammals and explain the principles of homeostasis in terms of internal and external stimuli, receptors, central control, coordination systems, effectors (muscles and glands) - Define the term 'negative feedback' and explain how it is involved in homeostatic mechanisms - Outline the roles of the endocrine system in coordinating homeostatic mechanisms ### As species of organisms evolved from simple, single-celled organisms into complex, multicellular ones, these organisms evolved to perform a specialist function. With specialisation in one function came the loss of the ability to perform other functions. Different groups of cells each carried out their own function, and this made the cells dependent on each other. Cells specialising in reproduction, for example, depend on different groups of cells for their survival: one group to obtain oxygen for their respiration, another to provide glucose and another to remove their waste products. This means that the more complex, multicellular organisms have a division of labour that is seen in tissues, organs and organ systems. These different functional systems must be coordinated if they are to perform efficiently. ### There are two forms of coordination in most multicellular animals: nervous and endocrine. - The nervous system allows rapid communication between specific parts of an organism. - The endocrine system usually provides a slower, less specific form of communication. Both systems need to work together. The increased complexity of multicellular organisms meant the development of an internal environment at the same time. This internal environment is made up of extracellular fluids that bathe each cell, supplying nutrients and removing wastes. By maintaining this fluid at levels which suit the cells, the cells are protected from the changes that affect the external environment and so give the organism a degree of independence. ### What is homeostasis? Homeostasis is the maintenance of a constant state. More specifically, it refers to the internal environment of organisms and involves maintaining the chemical make-up, volume and other features of blood and tissue fluid within narrow limits, sometimes called normal ranges. ### Homeostasis ensures that the cells of the body are in an environment that meets their needs and allows them to function normally despite external changes. - This does not mean that there are no changes - on the contrary, there are continuous fluctuations brought about by variations in internal and external conditions. - These changes, however, occur around a set point. - Homeostasis is the ability to return to that set point and so maintain organisms in a balanced equilibrium (Figure 2). ## The importance of homeostasis - The enzymes that control the biochemical reactions within cells, and other proteins such as membrane channel proteins, are sensitive to changes in pH and temperature (Topic 3.2). - Any change to these factors reduces the efficiency of enzymes or may even prevent them working altogether, e.g. may denature them. - Changes to membrane proteins may mean that substances cannot be transported into or out of cells. - Changes to the water potential of the blood and tissue fluids may cause cells to shrink and expand (even to bursting point) owing to water leaving or entering by osmosis. In both situations the cells cannot operate normally. ## 14.2 Excretion and kidney structure ### On these pages you will learn to: - Describe the deamination of amino acids and outline the formation of urea in the urea cycle - Describe the gross structure of the kidney ### Excretion is the removal of the waste products of metabolism from the body. This is distinct from elimination (egestion), which is the removal of substances such as dietary fibre that have never been involved in the metabolic activities of cells. ### Excretory substances - An adult human produces about 500 dm³ of carbon dioxide and 400 cm³ of water each day as a result of respiration. - Other excretory products include bile pigments and mineral salts as well as the nitrogenous excretory product urea CO(NH2)2. ### Urea is used as the nitrogenous excretory product of organisms that have some access to water, but not in large volumes, e.g. animals living on land, such as mammals. Urea is produced in the liver from excess amino acids in three stages: - Amino groups (NH2) are removed from the amino acids in a process called deamination and made into ammonia. - The remainder of the amino acid can be respired to give ATP. - The ammonia is converted to urea by the addition of carbon dioxide in a pathway called the ornithine cycle. ATP is required for this process. ### Figure 1 summarises how this waste product is formed from the main elements in organic compounds. ### Extension #### Other excretory substances - Ammonia (NH3) is the easiest product to form from the amino groups (NH2) produced when amino acids are oxidised. - Its production requires no ATP and it is very soluble in water and so is easily dissolved and washed out of the body. It is, however, extremely poisonous - 800 times more so than carbon dioxide. - Only organisms such as freshwater fish with access to large volumes of water are therefore able to use ammonia as their nitrogenous excretory product. - Uric acid is almost insoluble in water and cannot diffuse into cells, making it hardly poisonous at all. - However, it takes seven ATP molecules to produce it. - As almost no water is needed for its removal, it is used by organisms living in very dry conditions. - As it is low in mass when stored it is also used by flying organisms. - Animals such as birds and reptiles that lay eggs have an additional reason for using it - to remove their nitrogenous waste. As the young develop within the egg, their wastes cannot be removed and so anything more toxic than uric acid would kill them. ## 14.3 The structure of the nephron ### On these pages you will learn to: - Describe the detailed structure of the nephron with its associated blood vessels using photomicrographs and electron micrographs - Describe the gross structure of the kidney ### The nephron is the functional unit of the kidney. It is a narrow tube, closed at one end, with two twisted regions separated by a long hairpin loop. Each nephron is made up of a: - **Renal (Bowman's)** capsule- the closed end at the start of the nephron. - It is cup-shaped and contains within it a mass of blood capillaries known as the glomerulus. Its inner layer is made up of specialised cells called podocytes (Figure 1). - **Proximal (first)** convoluted tubule - a series of loops surrounded by blood capillaries. Its walls are made of cuboidal epithelial cells with microvilli (Figure 1). - **Loop of Henlé** - a long, hairpin loop that extends from the cortex into the medulla of the kidney and back again. It is surrounded by blood capillaries (Figure 2). - **Distal (second)** convoluted tubule - a series of loops surrounded by blood capillaries. Its walls are made of cuboidal epithelial cells, but it is surrounded by fewer capillaries than the proximal tubule. - **Collecting duct** - a tube into which a number of distal convoluted tubules empty. It is lined by cuboidal epithelial cells and becomes increasingly wide as it empties into the pelvis of the kidney. ### Associated with each nephron are a number of blood vessels (Figure 3): - **Afferent arteriole** - a tiny vessel that is a branch of the renal artery and supplies the nephron with blood. The afferent arteriole enters the renal capsule of the nephron where it forms the - **Glomerulus** - a many-branched knot of capillaries from which fluid is forced out of the blood (Figure 4). The glomerular capillaries recombine to form the - **Efferent arteriole** - a tiny vessel that leaves the renal capsule. It has a smaller diameter than the afferent arteriole, which causes an increase in blood pressure within the glomerulus. The efferent arteriole carries blood away from the renal capsule and later branches to form the - **Peritubular capillaries** - a concentrated network of capillaries that surrounds the proximal convoluted tubule, the loop of Henlé and the distal convoluted tubule and from where they reabsorb mineral salts, glucose and water. The peritubular capillaries merge together into venules (tiny veins) that in turn merge together to form the renal vein. ### Summary test 14.2 Excretion is the removal of metabolic waste products from the body, whereas the removal of non-metabolic material such as roughage is known as (1). Respiration in humans produces around 400 cm³ of (2) and 500 dm³ of (3) that need to be removed from the body. Other excretory products include mineral salts, bile pigments and nitrogenous wastes. Urea is made by removing amino groups from amino acids and converting them to ammonia - a process called (4). The ammonia is then converted to urea by the addition of (5) in a pathway called the (6) cycle. The mammalian kidney is surrounded by a protective (7) and in cross section is seen to be made up of a lighter coloured outer region called the (8) and a darker inner region called the (9). These regions are made up of around a million tubular structures called (10). Blood is brought to the kidney by the vessel called the (11), and urine leaves it via a tube called the (12). ### Summary test 14.3 The nephron is the structural unit of the kidney. It comprises a cup-shaped structure called the (1) that contains a knot of blood vessels called the (2) which receives its blood from a vessel called the (3) arteriole. The inner wall of this cup-shaped structure is lined with specialised cells called (4) and from it extends the first, or (5), convoluted tubule whose walls are lined with (6) epithelial cells that have (7) to increase their surface area. The next region of the nephron is a hairpin loop called the (8) which then leads onto the second, or (9), convoluted tubule. This in turn leads onto the (10) which empties into the renal pelvis. Around much of the nephron is a dense network of blood vessels called the (11) capillaries.