Homeostasis and Excretion PDF

Okay, here is the conversion of the provided text and images into a structured markdown format. I have focused on extracting the key information, structuring it well, and using LaTeX for mathematical formulas where appropriate. ## Homeostasis and Excretion ### Homeostasis Inside our bodies, condi...

Okay, here is the conversion of the provided text and images into a structured markdown format. I have focused on extracting the key information, structuring it well, and using LaTeX for mathematical formulas where appropriate. ## Homeostasis and Excretion ### Homeostasis Inside our bodies, conditions are kept relatively constant. This is called **homeostasis**. The kidneys are organs which have a major role to play in both homeostasis and in the removal of waste products, or **excretion**. They filter the blood, removing substances and controlling the concentration of water and solutes (dissolved substances) in the blood and other body fluids. If you were to drink a litre of water and wait for half an hour, your body would soon respond to this change by producing about the same volume of urine. In other words, it would automatically balance your water input and water loss. Drinking is the main way that our bodies gain water, but there are other sources. Some water is present in the food that we eat, and a small amount is formed by cell respiration. The body also loses water, mostly in urine, but also smaller volumes in sweat, faeces, and exhaled air. Every day, we gain and lose about the same volume of water, so that the total content of our bodies stays more or less the same. This is an example of homeostasis. The word 'homeostasis' means 'steady state', and refers to keeping conditions inside the body relatively constant. The inside of the body is known as the **internal environment.** You have probably heard of the 'environment', which means the 'surroundings' of an organism. The internal environment is the surroundings of the cells inside the body. It particularly means the blood, together with another liquid called **tissue fluid**. ### Daily Water Balance **Image Description:** A diagram illustrating water gain and water loss in a human. * **Water Gain:** * Food: 800 cm³ * Drink: 1400 cm³ * Cell Respiration: 400 cm³ * **Total:** 2600 cm³ * **Water Loss:** * Exhaled Air: 400 cm³ * Sweat: 600 cm³ * Urine: 1500 cm³ * Faeces: 100 cm³ * **Total:** 2600 cm³ Tissue fluid is a watery solution of salts, glucose, and other solutes. It surrounds all the cells of the body, forming a pathway for the transfer of nutrients between the blood and the cells. Tissue fluid is formed by leakage from blood capillaries. It is similar in composition to blood plasma but lacks the plasma proteins. It is not just water and salts that are kept constant in the body. Many other components of the internal environment are maintained. For example, the level of carbon dioxide in the blood is regulated, along with the blood pH, the concentration of dissolved glucose, and the body temperature. Homeostasis is important because cells will only function properly if they are bathed in a tissue fluid that provides them with their optimum conditions. For instance, if the tissue fluid contains too many solutes, the cells will lose water by osmosis and become dehydrated. If the tissue fluid is too dilute, the cells will swell up with water. Both conditions will prevent them from working efficiently and might cause permanent damage. If the pH of the tissue fluid is not correct, it will affect the activity of the cell's enzymes, as will a body temperature much different from 37 °C. It is also important that excretory products are removed. Substances such as urea must be prevented from building up in the blood and tissue fluid, where they would be toxic to cells. ### Urine An adult human produces about 1.5 dm³ of urine every day, although this volume depends very much on the amount of water drunk and the volume lost in other forms, such as sweat. Every litre of urine contains about 40 g of waste products and salts. ### Solutes in urine **Table 8.1**: Some of the main solutes in urine. | Substance | Amount / g per dm³ | | :---------------------- | :----------------- | | Urea | 23.3 | | Ammonia | 0.4 | | Other nitrogenous waste | 1.6 | | Sodium chloride (salt) | 10.0 | | Potassium | 1.3 | | Phosphate | 2.3 | Notice the words *nitrogenous waste*. Urea and ammonia are two examples of nitrogenous waste. It means that they contain the element nitrogen. All animals have to excrete a nitrogenous waste product. The reason behind this is quite complicated. Carbohydrates and fats only contain the elements carbon, hydrogen, and oxygen. However, proteins also contain nitrogen. If the body has too much carbohydrate or fat, these substances can be stored, for example, as glycogen in the liver, or as fat under the skin and around other organs. Excess proteins, or their building blocks (called amino acids) cannot be stored. The amino acids are first broken down in the liver. They are converted into carbohydrate (which is stored as glycogen) and the main nitrogen-containing waste product, urea. The urea passes into the blood, to be filtered out by the kidneys during the formation of urine. Notice that the urea is made by chemical reactions in the cells of the body (the body's metabolism). 'Excretion' means getting rid of waste of this kind. When the body gets rid of solid waste from the digestive system (faeces), this is not excretion, since it contains few products of metabolism, just the 'remains' of undigested food, along with bacteria and dead cells. So, the kidney is, in fact, carrying out two functions. It is a homeostatic organ, controlling the water and salt (ion) concentration in the body as well as an excretory organ, concentrating nitrogenous waste in a form that can be eliminated. ### The Urinary System The human urinary system as shown in Figure 8.2. Each kidney is supplied with blood through a short **renal artery**. This leads straight from the body's main artery, the aorta, so the blood entering the kidney is at a high pressure. Inside each kidney the blood is filtered, and the 'cleaned' blood passes out through each **renal vein** to the main vein, vena cava. The urine leaves the kidneys through two tubes, the **ureters**, and is stored in a muscular bag called the **bladder**. The bladder has a tube leading to the outside, called the **urethra**. The wall of the urethra contains two ring-shaped muscles, called **sphincter muscles**. They can contract to close the urethra and hold back the urine. The lower sphincter muscle is under conscious control, or 'voluntary', while the upper one is involuntary - it automatically relaxes when the bladder is full. ### Kidney structures **Image Description:** Diagram of the human urinary system (Figure 8.2) showing the kidneys, ureters, bladder, urethra, renal artery, and renal vein. Other diagram showing the internal structure of a kidney (Figure 8.3), highlighting the cortex, medulla, pyramids, renal artery, renal vein, ureter, and pelvis. If you were to cut a kidney lengthwise you would be able to see the structures shown in Figure 8.3. There is not much that you can make out without the help of a microscope. The darker outer region is called the **cortex**. This contains many tiny blood vessels that branch from the renal artery. It also contains microscopic tubes that are not blood vessels. They are the filtering units, called **kidney tubules** or **nephrons** (from the Greek word nephros, meaning kidney). The tubules then run down through the middle layer of the kidney, called the **medulla**. The medulla has bulges called pyramids pointing inwards towards the concave side of the kidney. The tubules in the medulla eventually join up and lead to the tips of these pyramids, where they empty urine into a funnel-like structure called the **pelvis**. The pelvis connects with the ureter, carrying the urine to the bladder. ### Structure of the Nephron **Image Description**: image with the main parts of nephron: glomerulus, Bowman's capsule, Loop of henle, collecting duct etc. By careful dissection, biologists have been able to work out the structure of a single tubule and its blood supply (Figure 8.4). There are about a million of these in each kidney. At the start of the nephron is a hollow cup of cells called the **Bowman's capsule.** It surrounds a ball of blood capillaries called a **glomerulus** (plural glomeruli). It is here that the blood is filtered. Blood enters the kidney through the renal artery, which divides into smaller and smaller arteries. The smallest arteries (arterioles) supply the capillaries of the glomerulus (Figure 8.5). ### Ultrafiltration in the Bowman's Capsule **Image Description**: microscopic view of a Bowman's capsule and glomerulus, including cells. A blood vessel with a smaller diameter carries blood away from the glomerulus, leading to capillary networks which surround the other parts of the nephron. Because of the resistance to flow caused by the glomerulus, the pressure of the blood in the arteriole leading to the glomerulus is very high. This pressure forces fluid from the blood through the walls of the capillaries and the Bowman's capsule, into the space in the middle of the capsule. Blood in the glomerulus and the space in the capsule are separated by two layers of cells: the capillary wall and the wall of the capsule. Between the two cell layers is a third layer called the **basement membrane**, which is not made of cells. These layers act like a filter, allowing water, ions, and small molecules like glucose and urea to pass through, but holding back blood cells and large molecules such as proteins. The fluid that enters the capsule space is called the **glomerular filtrate**. This process, where the filter separates different-sized molecules under pressure, is called **ultrafiltration**. The kidneys produce about 125 cm³ (0.125 dm³) of glomerular filtrate per minute. This works out at 180 dm³ per day. Remember though, only 1.5 dm³ of urine is lost from the body every day, which is less than 1% of the volume filtered through the capsules. The other 99% of the glomerular filtrate is reabsorbed back into the blood. We know this because scientists have actually analysed samples of fluid from the space in the middle of the nephron. Despite the diameter of the space being only 20µm (0.02 mm), it is possible to pierce the tubule with microscopic glass pipettes and extract the fluid for analysis. Figure 8.6 shows the structure of the nephron and the surrounding blood vessels in more detail. There are two coiled regions of the tubule in the cortex, separated by a U-shaped loop that runs down into the medulla of the kidney, called the **loop of Henle**. After the second coiled tubule, several nephrons join up to form a **collecting duct**, where the final urine passes out into the pelvis. **Image Description**: Image of a nephron and blood supply with samples 1-4 showing what is happening to the fluid as it travels along the nephron. ### Summary of the fluids composition in a nephron * Sample 1 (blood plasma) * protein = a * glucose = b * urea = c * $Na^+$= d * Sample 2 (glomerular filtrate) * Flow rate = 100% * protein = nil * glucose = b * urea = c * $Na^+$ = d * Sample 3 (end of first coiled tubule) * Flow rate = 20% * protein = nil * glucose = nil * urea = 3c * $Na^+$= d * Sample 4 (collecting duct) * Flow rate = 1% * protein = nil * glucose = nil * urea = 60c * $Na^+$= 2d Samples 1-4 show the results of analysing the blood before it enters the glomerulus, and the fluid at three points inside the tubule. The flow rate is a measure of how much water is in the tubule. If the flow rate falls from 100% to 50%, this is because 50% of the water in the tubule has passed back into the blood. To make the explanation easier, the concentrations of dissolved protein, glucose, urea, and sodium are shown by different letters (a to d). You can tell the relative concentration of one substance at different points along the tubule from this. For example, urea at a concentration '3c' is three times more concentrated than when it is 'c'. In the blood (sample 1) the plasma contains many dissolved solutes, including protein, glucose, urea, and salts (just sodium ions, $Na^+$, are shown here). As mentioned before, protein molecules are too big to pass through into the tubule, so the protein concentration in sample 2 is zero. The other substances are at the same concentration as in the blood. Now look at sample 3, taken at the end of the first coiled part of the tubule. The flow rate that was 100% is now 20%. This must mean that 80% of the water in the tubule has been reabsorbed back into the blood. If no solutes were reabsorbed along with the water, their concentrations should be five times what they were in sample 2. Since the concentration of sodium hasn't changed, 80% of this substance must have been reabsorbed (and some of the urea too). However, the glucose concentration is now zero - all of the glucose is taken back into the blood in the first coiled tubule. This is necessary because glucose is a useful substance that is needed by the body. Finally, look at sample 4. By the time the fluid passes through the collecting duct, its flow rate is only 1%. This is because 99% of the water has been reabsorbed. Protein and glucose are still zero, but most of the urea is still in the fluid. The level of sodium is only 2d, so not all of it has been reabsorbed, but it is still twice as concentrated as in the blood. This description has only looked at a few of the more important substances. Other solutes are concentrated in the urine by different amounts. Some, like ammonium ions, are secreted into the fluid as it passes along the tubule. The concentration of ammonium ions in the urine is about 150 times what it is in the blood. ### The loop of Henlé You might be wondering what the role of the loop of Henlé is. The full answer to this is rather complicated. You may meet it again if you study biology beyond International GCSE, but for now a brief explanation will be enough. It is involved with concentrating the fluid in the tubule by causing more water to be reabsorbed into the blood. Mammals with long loops of Henlé can make a more concentrated urine than ones with short loops. Desert animals have many long loops of Henlé, so they can produce very concentrated urine, conserving water in their bodies. Animals which have easy access to water, such as otters or beavers, have short loops of Henle. Humans have a mixture of long and short loops. Here is a summary of what happens in the kidney nephron: Part of the plasma leaves the blood in the Bowman's capsule and enters the nephron. The filtrate consists of water and small molecules. As the fluid passes along the nephron, all the glucose is absorbed back into the blood in the first coiled part of the tubule, along with most of the sodium and chloride ions. In the rest of the tubule, more water and ions are reabsorbed, and some solutes like ammonium ions are secreted into the tubule. The final urine contains urea at a much higher concentration than in the blood. It also contains controlled quantities of water and ions. ### Control of the Body's Water Content Not only can the kidney produce urine that is more concentrated than the blood, but it can also control the concentration of the urine, and regulate the water content of the blood. The kidneys respond to this 'upset' to the body's water balance by making a larger volume of more dilute urine. Conversely, if the blood becomes too concentrated, the kidneys produce a smaller volume of urine. These changes are controlled by a hormone produced by the pituitary gland, at the base of the brain. The hormone is called **antidiuretic hormone, or ADH**. 'Diuresis' means the flow of urine from the body, so 'antidiuresis' means producing less urine. ADH starts to work when your body loses too much water, for example, if you are sweating heavily and not replacing lost water by drinking. The loss of water means that the concentration of the blood starts to increase. This is detected by receptor cells in a region of the brain called the hypothalamus, situated above the pituitary gland. These cells are sensitive to the solute concentration of the blood and cause the pituitary gland to release more ADH. The ADH travels in the bloodstream to the kidney. At the kidney tubules, it causes the collecting ducts to become more permeable to water, so that more water is reabsorbed back into the blood. This makes the urine more concentrated, so that the body loses less water and the blood becomes more dilute. The action of ADH illustrates the principle of **negative feedback**. A change in conditions in the body is detected and starts a process that works to return conditions to normal. When the conditions are returned to normal, the corrective process is switched off. When the water content of the blood returns to normal, this acts as a signal to 'switch off' the release of ADH. The kidney tubules then reabsorb less water. Similarly, if someone drinks a large volume of water, the blood will become too dilute. This leads to lower levels of ADH secretion, the kidney tubules become less permeable to water, and more water passes out of the body in the urine. In this way, through the action of ADH, the level of water in the internal environment is kept constant. ### Control of Body Temperature You may have heard mammals and birds described as 'warm-blooded'. A better word for this is homeothermic. It means that they keep their body temperature constant, despite changes in the temperature of their surroundings. For example, the body temperature of humans is kept steady at about 37 °C, give or take a few tenths of a degree. This is another example of homeostasis. All other animals are 'cold-blooded'. For example, if a lizard is kept in an aquarium at 20 °C, its body temperature will be 20 °C too. If the temperature of the aquarium is raised to 25 °C, the lizard's body temperature will rise to 25 °C as well. We can show this difference between homeotherms and other animals as a graph (Figure 8.8). In the wild, lizards keep their temperature more constant than in Figure 8.8, by adapting their behaviours. For example, in the morning, they may stay in the sun to warm their bodies, or at midday, if the sun is too hot, retreat to holes in the ground to cool down. The real difference between homeotherms and all other animals is that homeotherms can keep their temperatures constant by using physiological changes for generating or losing heat. For this reason, mammals and birds are also called **endotherms**, meaning 'heat from inside'. An endotherm uses heat from the chemical reactions in its cells to warm its body. It then controls its heat loss by regulating processes like sweating and blood flow through the skin. Endotherms use behavioural ways to control their temperature too. For example, penguins 'huddle' together in groups to keep warm, and humans put on extra clothes in winter. What is the advantage of a human maintaining a body temperature of 37 °C? It means that all the chemical reactions taking place in the cells of the body can go on at a steady, predictable rate. The metabolism doesn't slow down in cold environments. If you watch goldfish in a garden pond, you will notice that in summer, when the pond water is warm, they are very active, swimming about quickly. In winter, when the temperature drops, the fish slow down and become very sluggish in their actions. This would happen to a mammal too if its body temperature varied. It is also important that the body does not become too hot. The cells' enzymes work best at 37 °C. At higher temperatures enzymes, like all proteins, are destroyed by denaturing. Endotherms have all evolved a body temperature around 40 °C (Table 8.2) and enzymes work best at this temperature. **Table 8.2** Body temperatures of a range of mammals and birds | Species | Average and normal range of body temperature / °C | Species | Average and normal range of body temperature / °C | | :---------- | :-------------------------------------------------- | :--------- | :-------------------------------------------------- | | Brown bear | 38.0 ± 1.0 | Shrew | 35.7 ± 1.2 | | Camel | 37.5 ± 0.5 | Whale | 35.7 ± 0.1 | | Elephant | 36.2 ± 0.5 | Duck | 43.1 ± 0.3 | | Fox | 38.8 ± 1.3 | Ostrich | 39.2 ± 0.7 | | Human | 36.9 ± 0.7 | Penguin | 39.0 ± 0.2 | | Mouse | 39.3 ± 1.3 | Thrush | 40.0 ± 1.7 | | Polar bear | 37.5 ± 0.4 | Wren | 41.0 ± 1.0 | ### Monitoring Body Temperature In humans and other mammals the core body temperature is monitored by a part of the brain called the thermoregulatory centre. This is located in the hypothalamus of the brain. It acts as the body's 'thermostat'. If a person goes into a warm or cold environment, the first thing that happens is that temperature receptors in the skin send electrical impulses to the hypothalamus, which stimulates the brain to alter our behaviour. We start to feel hot or cold, and usually do something about it, such as finding shade, having a cold drink, or putting on more clothes. If changes to our behaviour are not enough to keep our body temperature constant, the thermoregulatory centre in the hypothalamus detects a change in the temperature of the blood flowing through it. It then sends signals via nerves to other organs of the body, which regulate the temperature by physiological means. ### Did you know? A thermostat is a switch that is turned on or off by a change in temperature. It is used in electrical appliances to keep their temperature steady. For example, a thermostat in an iron can be set to 'hot' or 'cool' to keep the temperature of the iron set for ironing different materials. ### The Skin and Temperature Control The human skin has a number of functions related to the fact that it forms the outer surface of the body. These include: * forming a tough outer layer able to resist mechanical damage * acting as a barrier to the entry of pathogens * forming an impermeable surface, preventing loss of water * acting as a sense organ for touch and temperature changes * controlling the loss of heat through the body surface. **Image Description**: Diagram of a section through human skin (Figure 8.9) showing different its different layers and components. The outer epidermis consists of dead cells that stop water loss and protect the body against invasion by microorganisms such as bacteria. The hypodermis contains fatty tissue, which insulates the body against heat loss and is a store of energy. The middle layer, the dermis, contains many sensory receptors. It is also the location of sweat glands and many small blood vessels, as well as hair follicles. These last three structures are involved in temperature control. Imagine that the hypothalamus detects a rise in the central (core) body temperature. Immediately it sends nerve impulses to the skin. These bring about changes to correct the rise in temperature. First of all, the sweat glands produce greater amounts of sweat. This liquid is secreted onto the surface of the skin. When a liquid evaporates, it turns into a gas. This change needs energy, called the latent heat of vaporisation. When sweat evaporates, the energy is supplied by the body's heat, cooling the body down. It is not that the sweat is cool - it is secreted at body temperature. It only has a cooling action when it evaporates. In very humid atmospheres (e.g., a tropical rainforest) the sweat stays on the skin and doesn't evaporate. It then has very little cooling effect. Secondly, hairs on the surface of the skin lie flat against the skin's surface. This happens because of the relaxation of tiny muscles called hair erector muscles attached to the base of each hair. In cold conditions, these contract and the hairs are pulled upright. The hairs trap a layer of air next to the skin, and since air is a poor conductor of heat, this acts as insulation. In warm conditions, the thinner layer of trapped air means that more heat will be lost. This is not very effective in humans because the hairs over most of our body do not grow very large. It is very effective in hairy mammals like cats or dogs. The same principle is used by birds, which 'fluff out' their feathers in cold weather. Lastly, there are tiny blood vessels called capillary loops in the dermis. Blood flows through these loops, radiating heat to the outside, and cooling the body down. If the body is too hot, arterioles (small arteries) leading to the capillary loops dilate (widen). This increases the blood flow to the skin's surface which is called vasodilation. **Image Description**: Diagram comparing vasodilation and vasoconstriction and how blood flows through vessels near the skin in the process. In cold conditions, the opposite happens. The arterioles leading to the surface capillary loops constrict (become narrower) and blood flow to the surface of the skin is reduced, so that less heat is lost. This is called vasoconstriction. Vasoconstriction and vasodilation are brought about by tiny rings of muscles in the walls of the arterioles, called sphincter muscles, like the sphincters you read about earlier in this chapter, at the outlet of the bladder. There are other ways that the body can control heat loss and heat gain. In cold conditions, the body's metabolism speeds up, generating more heat. The liver, a large organ, can produce a lot of metabolic heat in this way. The hormone adrenaline stimulates the increase in metabolism. Shivering also takes place, where the muscles contract and relax rapidly. This also generates a large amount of heat. Sweating, vasodilation and vasoconstriction, hair erection, shivering and changes to the metabolism, along with behavioural actions, work together to keep the body temperature to within a few tenths of a degree of the 'normal' 37 °C. If the difference is any bigger than this, it shows that something is wrong. For instance, a temperature of 39 °C might be due to an illness.