Homeostasis in Mammals PDF - Exam Guide

Okay, here is the converted text from the images into a structured markdown format: # Homeostasis ## 14. 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 in...

Okay, here is the converted text from the images into a structured markdown format: # Homeostasis ## 14. 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. ** Image Description:** * Figure 1. A cheetah catching it's gazelle prey. * Figure 2. Penguins in the Antarctic above and Camels in the desert below. 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 Homeostasis is essential for the proper functioning of organisms because: * 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. **Figure 3 Components of a typical control system** | Input Change to the system | Receptor Measures level of a factor | Control Unit Operational information is stored here and used to coordinate effectors | Effector Brings about changes to the system in order to return it to the set point | Output System returned to set point | | :------------------------------------------------ | :------------------------------------------------------------ | :--------------------------------------------------------------------------------------------------------- | :------------------------------------------------------------------------------------------------ | :---------------------------------------------------------------- | | Room temperature drops from 20°C to 18°C | Room thermostat signals that the temperature is bellow the set point | Programmer checks that heating should be on at this time. If so, it starts boiler and circulation pump | Boiler fires up, pump circulates water, radiators become hot | Room temperature is raised to 20 °C | | **FEEDBACK LOOP** in this case negative feedback as it turns system off | | | | | * Biochemical reactions in organisms are in a state of dynamic equilibrium between the forward and reverse reactions. Changes to the environment of cells can upset this equilibrium to the harm of the organism. * Organisms with the ability to maintain a constant internal environment are more independent of the external environment. They have a wider geographical range and therefore have a greater chance of finding food, shelter, etc. Mammals, for example, with their ability to maintain a constant temperature, are found in most habitats from hot arid deserts to cold, frozen poles. * Maintaining a constant concentration of glucose in the blood means that an organism can release energy needed for various activities at a constant rate. The regulation of blood glucose is covered in Topic 14.8. ### Control mechanisms and feedback The control of any self-regulating system involves a series of stages: * **set point** the desired level at which the system operates. This is monitored by a * **receptor** that detects internal and external stimuli which indicate any deviation from the set point and informs the * **central control** that coordinates information from various sources and sends instructions to a suitable * **effector**, often a muscle or gland, that brings about the necessary change needed to return the system to the set point. This return to the desired level creates a * **feedback loop** that informs the receptor of the changes to the system brought about by the effector. Figure 3 shows the relationship between these stages using the everyday example of controlling a central heating system. Most systems, including biological ones, use **negative feedback**, i.e. the information fed back turns the system off. We shall see examples of negative feedback in the following topics. **Positive feedback** occurs when a deviation from the set point causes changes that result in an even greater deviation from the normal. Examples are rare, but include: * In neurones, a stimulus causes a small influx (movement into the cell) of sodium ions (Topic 15.4). This influx increases the permeability of the neurone to sodium, more ions enter, causing a further increase in permeability and even more rapid entry of ions. In this way, a small stimulus can bring about a large and rapid response. * Oxytocin causes contractions of the uterus at childbirth. The contractions stimulate the release of more oxytocin, causing even more contractions. The increasing frequency of contractions leads to the birth of the baby. ### Coordination of control mechanisms Systems normally have many receptors and effectors. It is important to ensure that the information provided by receptors is analysed by the central control before action is taken. Receiving information from a number of sources allows a better degree of control. For example, temperature receptors in the skin may signal that the skin is cold and that body temperature should be raised. However, information from the hypothalamus in the brain may indicate that blood temperature is already above normal. This situation could occur during strenuous exercise when blood temperature rises but sweating cools the skin. By analysing the information from all the detectors, the brain can decide the best course of action in this case not to raise the body temperature further. In the same way, the central control must coordinate the action of the effectors so that they operate together. For example, sweating would be less effective in cooling the body if vasodilation did not occur at the same time. ## 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. ** Image Description:** * Shows: $CO_2$ carbon dioxide, 3 ATP, Urea fairly toxic, fairly soluble, excreted by mammals, Water $H_2O$ * Shows the position of the kidneys in humans 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 $500 dm^3$ of carbon dioxide and $400 cm^3$ 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 - $CO(NH_2)_2$. 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 ($NH_2$) 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 ($NH_3$)** is the easiest product to form from the amino groups ($NH_2$) 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. **Image Description:** * Figure 3. Detailed structure of mammalian kidney * Figure 3. Structure of mammalian kidney ### Structure of the mammalian kidney In mammals there are two kidneys found at the back of the abdominal cavity, one on each side of the spinal cord (Figure 2). In humans each kidney is usually surrounded by fat that gives it some physical protection. Weighing only 150 g each, they filter your blood plasma every 22 minutes of your life. A section through the kidney (Figure 3) shows it is made up of the: * **fibrous capsule** - an outer membrane that protects the kidney * **cortex** - a lighter coloured outer region made up of renal (Bowman's) capsules, convoluted tubules and blood vessels * **medulla** - a darker coloured inner region made up of loops of Henlé, collecting ducts and blood vessels * **renal pelvis** - a funnel-shaped cavity that collects urine into the ureter * **ureter** - a tube that carries urine to the bladder * **renal artery** - supplies the kidney with blood from the heart via the aorta * **renal vein** - returns blood to the heart via the vena cava. A microscopic examination of the cortex and medulla reveals around one million tiny tubular structures in each kidney. These are the basic structural and functional units of the kidney - the **nephrons**. **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)**. ## 14. The structure of the nephron ### 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 **Image Description:** * Photomicrograph of cortex of human kidney. Three Glomeruli can be seen. * Photomicrograph of medulla of human kidney showing loops of Henlé. Around them are blood capillaries containing red blood cells. 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**. * **Proximal(first) convoluted tubule** - a series of loops surrounded by blood capillaries. Its walls are made of cuboidal epithelial cells with microvilli. * **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. * **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: * **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 * **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 * **peritibular 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. **Image Description:** * Figure 3 Regions of the nephron and associated blood vessels. Showing the Afferent arteriole, Efferent arteriole, Distal convoluted tubule, Branch of renal artery, Branch of renal vein, Proximal convoluted tubule, Collecting duct, Peritubular capillaries Loop of Henlé, descending limb and ascending limb. * Figure 4. Colourised scanning electron micrograph of a glomerulus surrounded by the renal capsule seen as a white brown membrane. Part of the proximal convoluted tubule is seen. #### 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**) 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. ## 14.4 Kidney function-ultrafiltration and selective reabsorption **On these pages you will learn to:** * Describe how the processes of ultrafiltration and selective reabsorption are involved with the formation of urine in the nephron The functions of the kidney are listed in **Table 1**. The main one, that of regulating the composition of blood, is carried out by the nephrons in a series of stages - ultrafiltration, selective reabsorption and the reabsorption of water and minerals. **Table 1 The functions of the kidneys** | **Function** | | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Regulating the composition of the blood and maintaining a constant water potential. | | Maintaining a constant volume of water | | Removing wastes such as urea | | Maintaining the concentration of mineral ions and other substances constant | | Regulating blood pressure | | Maintaining the body's calcium level | | Stimulating the production of red blood cells | ### Ultrafiltration Blood enters the kidney through the renal artery, which branches frequently to give around one million tiny arterioles, each of which enters a renal (Bowman's) capsule of a nephron. This is called the **afferent arteriole** and it divides to give a complex of capillaries known as the **glomerulus**. The glomerular capillaries later merge to form the **efferent arteriole**, which then sub-divides again into capillaries (the peritubular capillaries), which wind their way around the various tubules of the nephron before combining to form the renal vein. The walls of the glomerular capillaries are made up of endothelial cells with pores between them. As the diameter of the afferent arteriole is greater than that of the efferent arteriole, there is a build up of hydrostatic pressure within the glomerulus. As a result, water, glucose, mineral ions and other substances up to a relative molecular mass of up to 68000 are squeezed out of the capillary to form the **glomerular filtrate**. The movement of this filtrate out of the glomerulus is resisted by the: * capillary endothelium * basement membrane of the epithelial layer of the renal (Bowman's) capsule * epithelial cells of the renal (Bowman's) capsule * the hydrostatic pressure of the fluid in the renal capsule space the intracapsular pressure * the low water potential of the blood in the glomerulus. This total resistance would be sufficient to prevent filtrate leaving the glomerular capillaries, but there are some modifications to reduce this barrier to the flow of filtrate: * The inner layer of the renal (Bowman's) capsule is made up of highly specialised cells called **podocytes**. These cells, which are illustrated in Figure 1, are lifted off the surface membrane on little 'feet' ('podo' = feet). This allows filtrate to pass beneath them and through gaps between their branches. Filtrate passes between these cells rather than through them. * The endothelium of the glomerular capillaries has spaces up to 100 nm wide between its cells (Figure 1). Again, fluid can therefore pass between, rather than through, these cells. As a result, the hydrostatic pressure of the blood in the glomerulus is high enough to overcome the resistance and so filtrate passes from the blood into the renal capsule. This filtration under pressure is known as **ultrafiltration**. The filtrate has much the same composition as blood plasma, with the exception of the plasma proteins which are too large to pass across the basement membrane. Many of the substances in the 125 cm3 of filtrate passing out of blood each minute are extremely useful to the body and need to be reabsorbed. **EXTENSION - The glomerulus - a unique capillary bed** In mammals, the glomerulus is the only capillary bed in which an arteriole (the afferent arteriole) supplies it with blood and an arteriole (the efferent arteriole) also drains blood away. In all other mammalian capillary beds it is a venule that drains away the blood. Why then do we not make life simpler and call the efferent arteriole a venule? The reason is that the efferent arteriole later divides up into a second capillary bed - the peritubular capillaries - and these are drained by a venule. In any case, the structure of the wall is that of an arteriole and not a venule. The glomerular capillaries need to merge into an efferent arteriole because this increases the hydrostatic pressure within the glomerulus and allows ultrafiltration to occur. ### Selective reabsorption In the proximal convoluted tubule nearly 85% of the filtrate is reabsorbed back into the blood. Why then, you may ask, allow it to leave the blood in the first place? Ultrafiltration operates on the basis of size of molecule: all below 68000 relative molecular mass are removed. Some are wastes, but most are useful. About $180 dm^3$ of water enters the nephrons each day. Of this volume, only about $1 dm^3$ leaves the body as urine. Eighty-five per cent of the reabsorption of water occurs in the proximal convoluted tubule. **Image Description:** #### The cells of the proximal convoluted tubules are adapted to reabsorb substances into the blood by having microvilli which give them a large surface area and many mitochondria to provide ATP for active transport of sodium ions. The process is as follows: A picture shows a podocyte and ultrafiltration. A second picture shows the cells of the proximal convoluted tubule with a list of the process as follows: * Sodium ions are actively transported out of the cells lining the proximal convoluted tubule into blood capillaries which carry them away. This is by the action of a membrane carrier protein, the sodium-potassium pump. The sodium ion concentration of these cells is therefore lowered. * Sodium ions now diffuse along a concentration gradient from the lumen of the proximal convoluted tubule into the lining cells but only through special carrier proteins. * These carrier proteins are of different types, each of which carries another molecule (glucose or amino acids or chloride ions, etc.) along with the sodium ions. This is known as co-transport (Topic 4.5). Water follows osmotically down the water potential gradient that is created. * The molecules which have been co-transported into the cells of the proximal convoluted tubule then diffuse into the blood. As a result, all the glucose, amino acids, chloride ions and most other valuable molecules are reabsorbed as well as water. * **Image Description:** A colourised scanning electron micrograph of podocyte cells around a glomerulus in a human kidney. Details of cells from the wall of the proximal convoluted tubule. ## 14. The kidney function-loop of Henlé and reabsorption of water ### 14.5 Kidney function-loop of Henlé and reabsorption of water **The loop of Henlé** The loop of Henlé is a hairpin-shaped tubule that extends into the medulla of the kidney. It is responsible for creating the conditions in the surrounding interstitial fluid that lead to the reabsorption of water from the distal convoluted tubule and the collecting duct. This results in concentrating the urine so that it has a lower water potential than the blood. The concentration of the urine produced is directly related to the length of the loop of Henlé. It is short in mammals whose habitats are in or by water (e.g. beavers) and long in those whose habitats are dry regions (e.g. kangaroo rat). The loop of Henlé has two regions: * The descending limb, which is narrow, with thin walls that are highly permeable to water. * The ascending limb, which after a short distance is wider, with thick walls that are impermeable to water. The loop of Henlé acts as a counter-current multiplier. To understand how this works it is necessary to consider the following sequence of events using Figure 1, to which the numbers refer. **Image Description:** Counter-current multiplier of the loop of Henlé 1. Sodium and chloride ions are actively pumped out of the ascending limb of the loop of Henlé using ATP provided by the many mitochondria in the cells of its wall. 2. This creates a low water potential (high ion concentration) in the region of the medulla between the two limbs (called the interstitial region). In normal circumstances water would pass out of the ascending limb by osmosis. However, the thick walls are almost impermeable to water and so very little, if any, escapes. 3. The walls of the descending limb are, however, very permeable to water and as the cells have many membrane protein channels known as aquaporins it passes out of the filtrate, by osmosis, into the interstitial space. This water enters the blood capillaries in this region by osmosis and is carried away. 4. The filtrate progressively loses water in this way as it moves down the descending limb lowering its water potential. It reaches its minimum water potential at the tip of the hairpin. 5. At the base of the ascending limb, sodium and chloride ions diffuse out of the filtrate and as it moves up the ascending limb these ions are also actively pumped out (see point 1) and therefore the filtrate develops a progressively higher water potential. 6. In the interstitial space between the ascending limb and the collecting duct there is a gradient of water potential with the highest water potential in the cortex and an increasingly lower water potential the further into the medulla one goes (see Topic 14.6). 7. The collecting duct is permeable to water and so, as the filtrate moves down it, water passes out of it by osmosis. This water passes by osmosis into the blood vessels that occupy this space, and is carried away (see Topic 14.6). 8. As water passes out of the filtrate its water potential is lowered. However, the water potential is also lowered in the interstitial space and so water continues to move out by osmosis down the whole length of the collecting duct. The counter-current multiplier ensures that there is always a water potential gradient drawing water out of the tubule. The water that passes out of the collecting duct by osmosis does so through aquaporins (water channels). The hormone ADH (Topic 14.6) can alter the number of these channels and so control water loss. By the time the filtrate, now called urine, leaves the collecting duct on its way to the bladder, it has lost most of its water and so it has a lower water potential than the blood. ### The distal (second) convoluted tubule The cells that make up the walls of the distal (second) convoluted tubule have microvilli and many mitochondria that allow them to reabsorb material rapidly from the filtrate, by either diffusion or active transport. The main role of the distal tubule is to make final adjustments to the water and salts that are reabsorbed and to control the pH of the blood by selecting which ions to reabsorb. To achieve this, the permeability of its walls becomes altered under the influence of various hormones (Topic 14.6). A summary of the processes taking place in the nephron is given in Figure 2. ### Counter-current multiplier When two liquids flow in opposite directions past one another, the exchange of substances (or heat) between them is greater than if they flowed in the same direction next to each other. In the case of the loop of Henlé, the counter-current flow means that the filtrate in the collecting duct with a lower water potential meets interstitial fluid that has an even lower water potential. This means that, although the water potential gradient between the collecting duct and interstitial fluid is small, it exists for the whole length of the collecting duct. There is therefore a steady flow of water into the interstitial fluid, so that around 80% of the water enters the interstitial fluid and hence the blood. If the two flows were in the same direction (parallel) less of the water would enter the blood. **Image Description:** Relative concentrations of three substances in the filtrate as it passes along a nephron NB Scale is not linear ## 14.6 Control of water and solute concentration of the blood **On these pages you will learn to:** * Describe the roles of the hypothalamus, posterior pituitary, ADH and collecting ducts in osmoregulation The quantity of water and salts we take in varies from day to day, as does the quantity we lose. **Table 1** shows the daily balance between loss and gain of salts and water for a typical human. The blood, however, needs to have a constant volume of water and concentration of salts to avoid osmotic disruption to cells. The homeostatic control of water and solute concentrations in the blood is achieved by hormones that act on the distal (second) convoluted tubule and the collecting duct. **Table 1** Daily water and salt balance in a typical human | WATER | | SALT | | | :-----------------------: | :-------------------------------: | :----------------------------: | :-----------------------------: | | Volume of water / cm³ day-1 | | Mass of salt / g day-1 | | | Water gain | Water loss | Salt gain | Salt loss | | Diet 2300 | Urine 1500 | Diet 10.50 | Urine 10.00 | | Metabolism 200 | Expired air 400 | - | Faeces 0.25 | | respiration | Evaporation from skin 350 | | Sweat 0.25 | | Faeces | - | - | - | | Sweat | | - | - | | TOTAL 2500 | TOTAL 2500 | TOTAL 10.50 | TOTAL 10.50 | As a result of facilitated diffusion and active transport by cells in the ascending limb of the loop of Henle, sodium and chloride ions are concentrated in the interstitial fluid surrounding the distal convoluted tubule and the collecting duct. There is a gradient of water potential within this interstial region, with the highest water potential in the cortex and the lowest in the medulla region closest to the renal pelvis (see Topic 14.5, Extension, for further details). Some urea passes out of the filtrate in the collecting duct into the interstitial region, so further increasing the concentration of solutes and decreasing the water potential. At all points, the interstitial region has a lower water potential than the filtrate passing down the collecting duct. ### The naming of antidiuretic hormone The name antidiuretic hormone (ADH) may, at first, seem unusual. However, it describes its function precisely. Diuresis is the production of large volumes of dilute urine. It is a symptom of a disease called diabetes insipidus (so called because the urine from sufferers did not taste sweet!). The disease was successfully treated with pituitary extract. Therefore it was suggested that a hormone existed that was given the name 'antidiuretic hormone'. As the effect of ADH is to increase the permeability of collecting ducts so that more water is reabsorbed into the blood, it causes the production of small volumes of concentrated urine. This is the opposite of diuresis - hence the name antidiuretic hormone. ### Regulation of the water potential of the blood The water potential of the blood is determined by the balance of water and salts within it. A rise in solute concentration lowers its water potential. This may be caused by: * too little water being consumed * much sweating occurring * large amounts of salt being taken in (ingested). The body responds to this decrease in water potential as follows: * Sensory cells called **osmoreceptors** in the hypothalamus of the brain detect the decrease in water potential. * It is thought that, when the water potential of the blood is low, water is lost from these osmoreceptor cells by osmosis. Owing to this water loss the osmoreceptor cells shrink, a change that stimulates the neurosecretory cells in the hypothalamus to produce a hormone called **antidiuretic hormone (ADH)**. * ADH passes along the neurones (nerve cells) to the posterior pituitary gland, from where it is secreted into the capillaries. * ADH passes in the blood to the kidney, where it increases the permeability to water of the cell surface membrane of the cells that make up the walls of the distal (second) convoluted tubule and the collecting duct. * Receptors on the cell surface membrane of these cells bind to ADH molecules, activating a second messenger system within the cell (cyclic AMP - Topic 14.7). This results in the activation of a protein kinase, an enzyme that adds phosphate groups to other proteins to activate them. * The action of protein kinase causes vesicles within the cell to move to, and fuse with, its cell surface membrane. (ADH binding also leads to an increase in transcription of the gene coding for the aquaporin protein, increasing the number of available aquaporins.) ### A decrease in the solute concentration of the blood increases its water potential. This may be caused: * by large volumes of water being consumed * by salts used in metabolism or excreted not being replaced in the diet. The body responds to this increase in water potential as follows: * The osmoreceptors in the hypothalamus detect the increase in water potential and stimulate the pituitary gland to reduce its release of ADH. * ADH, via the blood, decreases the permeability of the collecting ducts to water and urea. * Less water is reabsorbed back into the blood from the collecting duct. * More dilute urine is produced and the water potential of the blood decreases. * When the water potential of the blood has returned to normal, the osmoreceptors in the hypothalamus cause the pituitary to raise its release of ADH back to normal (= negative feedback). These events are summarised in Figure 1. **Image Description:** Summary: * Diagram 1 regulation of water potential of the blood by antidiuretic harmone ADH. * Diagram 2 show walls of distal(second) and collecting duct become more permeable or less permeable to water **SUMMARY TEST 14.6** A human gains around **(1)** cm³ of water each day, of which **(2)** cm³ comes from the diet, with the remainder being produced in metabolic processes such as **(3)**. More than half this water is lost from the body as **(4)**. The same typical human needs around 10.5 g of salt in the diet, of which 10g is lost in the urine and 0.25 g in faeces. The remainder is lost in **(5)**. Despite daily fluctuations in water and salt intake, the water potential of the blood remains relatively constant as a result of **(6)** control achieved by hormones that act on the **(7)** and collecting duct. If too little water or too much salt is consumed, or if **(8) is** excessive, the water potential of the blood will **(9)**. In response to this, osmoreceptors in the **(10)** of the brain detect the change and produce antidiuretic hormone (ADH) that passes to the **(11)** gland from where it is secreted. ADH passes via the blood to the kidney where it increases the **(12)** of the distal convoluted tubule and collecting duct to water and **(13)**. As a result more water is reabsorbed and enters the blood. The osmoreceptors also stimulate a thirst response and so more water is drunk and the water potential of the blood therefore **(14)**. When the water potential returns to normal, the osmoreceptors detect this and ADH production is reduced to normal an example of the principle of **(15)**. ## 14.9 Biosensors ### 14.9 Biosensors On these pages you will learn to: * Explain the principles of operation of dip sticks containing glucose oxidase and peroxidase enzymes, and biosensors that can be used for quantitative measurements of glucose in blood and urine * Explain how urine analysis is used in diagnosis with reference to glucose, protein and ketones Biosensors are devices that use immobilised biological molecules, such as enzymes or antibodies, or biological systems, such as whole cells, to detect a specific chemical and in most cases, measure the concentration of the chemical. In its simplest form, a biosensor can be a simple dipstick, but commonly biosensors are now taken to mean those devices that are coupled with microelectronics so that results are rapid, the measurements are extremely accurate,