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Biology 30 Module 2 Body Actions and Systems: Maintaining Life Lesson 6 Body (Cell) Wastes Copyright: Ministry of Education, Saskatchewan May be reproduced for educational purposes Biology 30 83 Lesson 6 Biology 30 84 Lesson 6 Lesson 6 Body (Cell) Wastes Directions for completing the lesson: Text References for suggested reading: Read BSCS Biology 8th edition Section 14.10 - Pages 359-360 Section 16.10 – Pages 418-420 OR Nelson Biology Pages 204-214 Study the instructional portion of the lesson Review the vocabulary list Do Assignment 6 Biology 30 85 Lesson 6 Vocabulary ADH Bowman’s capsule closed digestive system deamination dialysis egesthan excretion flamecell glomerolus green glands guttation Biology 30 Henle;s loop kidney malpighian tubules nephridial system nephron open digestive system secretion transpiration urea uric acid 86 Lesson 6 Body (Cell) Wastes Introduction Two important processes going on in the bodies of living organisms have to do with the release of energy and with the building and repair of cell and tissue parts. In comparing these to similar types of actions carried out by humans on a larger scale, certain common characteristics appear. When humans "produce" electrical, heat and other forms of energy, the process is usually accompanied by the formation of water or chemical vapors (steam or smoke) or ash, tar and other liquid or solid byproducts of burning fuels. In following through all phases in the construction and finishing of a house, apartment complex or office building, one would see many left-overs. Such items as pieces of boards, insulation, empty or partially empty glue or caulking containers, bits of masonry, electrical wires and pipes are just some of the things which may no longer be viewed as useful items and are discarded. Similarly, release of energy and the synthesis of matter inside bodies result in some substances being regarded as wastes. In the actions occurring on a broader scale outside of bodies, wastes are either removed by some physical forces in the natural environment (wind or water) or are periodically gathered and hauled away to some dump site. Leaving or not attending to such wastes in some way could reduce the efficiency of some operation or clog or clutter it so badly as to stop it completely. Conditions within the bodies of living organisms follow the same pattern. Wastes must be continually removed to certain "dump sites" which may be permanent or, more frequently, which are temporary as the wastes are eventually released from the bodies. A serious effect of waste accumulation can be seen during kidney failure, due to injury or disease, which can lead to death. Methods of disposing of wastes range from simple to complex, according to the nature and complexity of organisms themselves. In general, the removal of wastes becomes more critical and the methods more active and complex as body sizes and activity levels increase. Biology 30 87 Lesson 6 After completing this lesson you should be able to: • define what "wastes" are. • discuss the importance of removing wastes from living organisms. • indicate the types or kinds of substances which can be wastes to particular organisms at certain times. • distinguish between the processes of secretion, excretion and egestion. • describe the ways in which plants deal with wastes. • describe some of the kinds of excretory systems in some representative animal groups: - • Biology 30 unicellular (protist) organisms. colonial-type, simple multicellular organisms. worms arthropods vertebrates identify some other functions or roles of excretory organs or systems in bodies. 88 Lesson 6 The Meaning of "Waste" Commonly in everyday life, anything which no longer has any use and is simply "cluttering" up an area, is regarded as waste or garbage. A similar meaning can be applied to some substances in organisms Waste: anything which has no useful value to a body at a particular time and/or of which there is an excess Some forms of matter can be useful to a body at one time and of no value or even harmful at another time. Further mention of this will be made shortly, but oxygen and carbon-dioxide gases fit this category in relation to green plants. Depending on whether their dominant activities are photosynthesis or respiration at certain times, green plants could be alternating in their manner of using each of these gases as a nutrient or as a waste. Too much of a particular substance at a particular moment, even though it is usually an essential nutrient, can result in attempts at its removal. Excess amounts of vitamins or minerals taken by some people as supplements are generally removed with the urine. Excess water in plant or animal bodies is also treated as waste. Plant and animal body wastes originate or accumulate from two general types of actions: internal metabolic processes, where energy is converted from one form to another or where body parts are either built up or broken down, result in waste products. In addition, the different metabolic processes require different kinds of nutrients. Ingestion of nutrients can bring in excess amounts of the nutrients themselves or of other substances combined with those nutrients but not needed or utilized – such as the extra amounts of fibre or cellulose which we may consume with our plant foods. Biology 30 89 Lesson 6 Waste Substances Keeping in mind that some of the following forms of matter could also be nutrients at certain times, wastes may include: • gases released by the processes of photosynthesis and respiration: oxygen and carbon-dioxide. • water, as a byproduct of respiration or as an excess of ingestion. • mineral salts, such as those of sodium, chlorine and potassium. • ammonia or nitrogen products, primarily from cellular respiration involving proteins. • excess organic substances ingested into bodies, such as cellulose or vitamins. • parts of cells or tissues which are no longer functioning, such as plant bark, animal skin cells or red blood cells. The Importance of Removing Wastes Allowing wastes to accumulate either inside cells or in the fluids around the edges could be detrimental in a number of ways. At the cellular levels, such accumulations could slow or even stop the processes of diffusion, osmosis or active transport of nutrients or wastes into or out of the cells. Shortages of nutrients or excesses of toxic wastes could lead to cellular deaths. A comparable example may be that of a person who has frostbite on skin tissue, with unprotected ears being common spots. Such a person actually suffers the first damage when intercellular lymph solidifies. This prevents the movements of nutrients and wastes into and out of cells and causes their deaths. Certain substances can become toxic in higher concentrations. Accumulations of nitrogen compounds, such as ammonia, can be particularly harmful in animal organisms where protein respiration is common. Toxic wastes can cause cell and tissue deaths in smaller, localized areas. On a larger scale, wastes can produce muscle paralysis or convulsions, improper organ functioning and possibly coma and death. Biology 30 90 Lesson 6 A term which has been mentioned in previous lessons is homeostasis. Maintaining specific kinds of internal body conditions and maintaining them at uniform levels, is important to the general functioning and health of organisms. Cells, tissues and organs generally operate best within a narrow range of conditions. A rise or fall in body temperature of only a few degrees Celsius, a rise or fall in the concentration of certain mineral (salt) ions in lymph or blood, or a change in the pH or acidity levels of fluids, can have significant effects on actions of body parts and the overall body. Waste removal affects these conditions, as well as others, and must be fairly precise in what is removed and in what quantities. Removal of too much of some substance can be as harmful as an over-accumulation. Excretion, Secretion and Egestion Some terms used with the movements of substances from cells or from bodies should perhaps be clarified before going on. References could vary but most follow the definitions or explanations stated here. Secretion: a material that is useful to the body in some way Certain cells produce, accumulate and then release material to the outside of their boundaries or membranes. If the material is considered as useful to the body in some way, it is regarded as a secretion. Digestive enzymes released into the mouth, stomach or small intestine are secretions. Excretion: Matter which leaves cells and is considered to be of no value and possibly even harmful is waste Ammonia, from protein respiration, is an excretion. Carbon-dioxide gas from the same process is also an excretion. Egestion/Elimination: the final removal of a substance out of a body Biology 30 91 Lesson 6 The movement of a material to the outside of a cell does not necessarily mean that it has left a body. Frequently, both secretions and excretions enter body fluids or certain body cavities which are still inside the body. These may include lymph, blood, parts of the digestive tract, or a number of glands or the bladder. The final removal of a substance out of a body is egestion or elimination. Removal of urine or feces from a body is egestion. Many references use the term excretion for this process as well. On occasion, it may be difficult to choose the best term to describe the removal of a particular substance. Tears in many marine organisms could be excretions which contain extra salt which bodies are trying to get rid of. Some organisms adapted to land and fresh water may shed tears for the same reason. For others, tears may keep the surfaces of the eyes moist for better functioning of eyelids and also in acting as a mild antiseptic. In these situations, tears could be considered as secretions. Bile, produced in the liver, contains dead red blood cells as well as bile salts used in digestion. Since bile eventually leaves the body in the feces it could have all three terms applied to it. Plants Dealing with wastes is not as critical an action with plants as it is with animals. This does not mean that there are necessarily fewer processes taking place within plants or that some of their metabolic rates are lower; in fact, some of these may equal those of animals. There are a number of characteristics unique or more common to plants make excretion or egestion less of a problem for them. 1. Non-living areas do not form wastes Large portions of plant parts and organs are non-living in perennial plants such as woody shrubs and trees and to a lesser extent in annual and biannual plants. Inner pith and the xylem cells of heartwood and sapwood, or of fibrovascular bundles, are no longer living at maturity. Even the outer cellulose walls of living cells have this characteristic. Such non-living areas do not form wastes themselves. In some plants, pith or xylem cells even become areas where some solid wastes such as mineral crystals, gums and resins are deposited permanently. Biology 30 92 Lesson 6 2. Cycles of photosynthesis and cellular respiration The occurrence of both photosynthesis and cellular respiration, even though they are likely taking place at different rates at different times, also sets up cycles not possible in animals. Waste materials released by one process could become nutrients for the other. For instance, oxygen released by photosynthesis can be used in cellular respiration. Cellular respiration, or the breakdown of organic compounds, forms carbon-dioxide and water which can be used to synthesize new organic compounds later on. The system is not perfect in that not all wastes can be reused. However, it greatly reduces the total amount of waste present that the plant eventually has to cope with. 3. End products of cellular respiration in plants and aniimals Catabolism in plants centers largely on carbohydrates. The endproducts of starch and sugar digestion can be used again in photosynthesis. Even if not used, these endproducts or wastes are not usually harmful or toxic to plant cells. In animals, a considerable amount of cellular respiration uses protein as a nutrient. When a protein is broken down – either to provide energy or to supply building parts to form an organism's own proteins, the amino or nitrogen containing group has to first be broken off from the rest of the molecule. This process is called deamination. Many of the amino groups then combine with carbon to form ammonia. This is a very toxic waste to cells and tissues. Animal bodies therefore have a more serious type of waste to deal with. Some plants or plant parts do break down a limited amount of protein. New seedlings, before the ability to photosynthesize becomes developed, and plants experiencing shortages of sunlight, will break down some proteins. However, some plants have the ability to reuse the ammonia or nitrate waste products later on. Other plants can temporarily or permanently convert the toxic nitrogen compounds into nontoxic amides or alkaloids, which can be safely stored in leaves or fruits. Nicotine, morphine, caffeine and digitalis are all such stored wastes. 4. Cell Physiology A dominating feature of many plant cells is a large, central vacuole, which may take up 80 to 90% of a cell's volume. This fluid-filled space contains stored nutrients and foods; but it can also store wastes for the life of the cell and possibly the life of the plant. Plant Cell Vacuole Biology 30 93 Lesson 6 Plant Excretion and Egestion Since plants do not usually have large concentrations of wastes, there is very little in the way of development or adaptation of cells specifically for handling wastes. Some excretions and egestions do occur using the same pathways or movement processes that nutrients use. Solid or fluid wastes: These wastes, which may be stored in pith or xylem cells, can reach their final destinations along the xylem vessels, pith or vascular rays. Water is the most common waste fluid egested or released to the outside. This loss of water to the atmosphere is called transpiration. Most of the water departs from the vascular bundles (veins) in leaves by passing through cell walls. Vaporizing, the water droplets diffuse through the same stomata which allow movements of gases. Some plant varieties have stomata-like openings, which are at the ends of veins, at the leaf edges or surfaces. Remaining open, these modified "stomata" permit actual droplets of water to ooze out of the ends of veins in a process called guttation. Wet soil conditions, along with a high atmospheric humidity, produce the guttations commonly observed some mornings on the leaves of potatoes, tomatoes, cabbages, strawberries and lawn grasses. (Guttation may be confused with dew but, whereas dew is a film of moisture over the whole plant, guttation is apparent as a drop or droplets along a leaf edge.) The actual driving force behind guttation appears to be root pressure. Gaseous wastes: Gas wastes, which include carbon-dioxide or oxygen at different times, takes place by diffusion through leaf stomata and stem lenticels. Lenticels enable gases within the stem to have fairly easy access to the outside by diffusion alone. (They also permit gases to enter when needed.) In woody trees this is further assisted by the characteristic of living cells existing as a layer towards the outside area of the stem while non-living cells are located towards the interior. Biology 30 94 Lesson 6 Why do Trees Lose their Leaves? Leaf fall is actually a form of excretion. Deciduous plants which commonly lose or drop their leaves periodically, are really removing or getting rid of plant matter which is no longer needed. The actual mechanism of leaf fall is centered on a special row of parenchyma cells found near the base of a leaf petiole. This separation layer, or abscission layer, has the pectins in its cells' walls dissolved by enzymes at the end of the growing season. The most likely triggering actions appear to be length of photoperiod (length of daylight and darkness in a certain time period) and temperature. The cells in the layer separate from each other once the pectins dissolve and the leaf remains attached only by its vascular bundles. Shrinking or expansion due to moisture changes, wind or snow and rain eventually cause final separation. Comparison of Excretory Systems Following is a discussion of excretory systems progressing from the systems of simple unicellular organisms to the more complex systems of vertebrates. It is important that you are able to understand: The progression in development of the excretory systems The key factors for each organisms’ system How organisms in the arthropods and vertebrates manage soluble nitrogenous wastes Biology 30 95 Lesson 6 Unicellular (Protist) Organisms Due their small sizes, diffusion and osmosis are generally sufficient for the excretion-egestion of gases and fluids. This process is made easier by the presence of a water, or fluid, environment. vacuoles Some varieties have radiating canals leading to a central vacuole, as in the case of Paramecium. The canals collect excess water and transfer it to the vacuole. Upon reaching a certain size, the vacuole contracts, forcing the water out. Depending on the salt or mineral concentration of the surrounding water, the frequency of the contractions of the vacuoles could increase or decrease (refer to lesson 3 application). The Paramecium has two contractile vacuoles, one at each end. What about solid particles that remain after the digestive enzymes have completed their work? In many unicellular organisms, these could be passed out in their vacuoles through any point along the surface membrane. Some varieties, such as the Paramecium, have localized spots, or anal pores through which solid waste particles can pass. Colonial-Type or Simple, Multicellular Organisms Excretion can be accomplished through diffusion or osmosis since most of the individual cells border an external fluid environment. Since the wastes are deposited directly into the external environment, both terms excretion and egestion can be used. Organisms such as Hydra, or many of the jellyfish, have central or gastrovascular cavities. However, since external fluids circulate regularly through these cavities, there are no increased difficulties in removing matter. Gastrovascular cavity of a hydra. Biology 30 96 Lesson 6 Worms The excretory systems of worms range from those with no, or few developments to those having the beginnings of excretory systems similar to vertebrates. This range in complexity is due to the variations in habitats and manners of living and the resulting adapations for removing wastes. Flatworms, roundworms and the common earthworm (Phylum Annelida) will be discussed a) Flatworms can be divided into two main catagories: Free living worms such as Planaria Parasitic worms such as flukes and tapeworms Planaria and flukes have either a pharynx or mouth which leads to a branched gastrovascular cavity. Theses organisms actively ingest organic matter into their cavities for digestion. Since they have a closed or incomplete digestive tube, solid waste particles are egested through the same opening through which nutrients enter. Segmented parasitic worms, such as tapeworms, have an even greater reduction in their systems. In addition to lacking respiratory and circulatory systems, they lack mouths and digestive tubes. Nutrients from their hosts’ intestines simply diffuse in, although diffusion does have to occur through a much thicker cuticle or outer membrane. (This thickened cuticle is necessary to protect the parasite from being digested by its own host). The wastes of both the host and worm diffuse in and out just as the nutrients do. Flame cell system A flame cell system is a specially developed excretory system existing in some tapeworms and in the planarians and flukes. Flame cell in a Planaria. Biology 30 97 Lesson 6 A flame cell system consists of a series of collecting tubules running along both sides of a body. Collecting tubules have many "pockets" along their length. Each is a flame cell. Movements of cilia (or flagella) inside the cell, which to some observers resemble a flickering "flame", tend to draw water and possible nitrogen waste into the cell and collecting tubule. These excess wastes are then passed out through openings or excretory pores, where the collecting tubules come to the surface of the organism's outer membrane. b) roundworms or nematodes can be free living, scavengers, or parasites This group shows a digestive system which is complete or open, having both mouth and anus. Solid waste particles are therefore able to move completely through the tube for eventual egestion. Two excretory tubes, one along each side, seem to function much like the collecting tubules in the previous group. However, there do not appear to be any flame cells. Instead, the two tubes join together at the anterior end and empty through a common excretory pore. c) Phylum Annelida shows another specialized excretory system, the nephridial system, an advance over flame cells. The nephridial system is closely associated with a blood circulatory system, which becomes apparent in this group. Segmented worms, such as the earthworm, have openings or nephrostomes which collect fluids from the cavities in the segments. These fluids pass along a tubule, into another segment, from there into an enlargement or bladder and then out through an external opening or nephridiopore. This tubule complex that collects fluids in one segment, then passes into a bladder in the next segment and empties to the outside through the nephridiopore, is called a nephridium. There are two of these per segment and together they are called nephridia. Cross-section of a segmented worm Biology 30 98 Lesson 6 The tubule and bladder are intertwined closely with blood capillaries. Waste substances, such as salts, water and the especially harmful nitrogen byproducts, pass from the blood and into the tubule and bladder. Useful fluids may be partially reabsorbed into the blood. Solid wastes make their way through the open digestive tube and out at the anus. The remaining organisms described in this lesson have excretory systems which are relatively similar in the manner of processing and egesting solid wastes. Solid matter is passed through digestive tracts where nutrients are broken down and then absorbed. Remaining solids may have additional water removed and cell or tissue remains and excess body solids added before being egested from the body through an anus. Due to the similarities in solid waste removal, there will be limited mention made of this in the remainder of the lesson. Instead, the emphasis will be on the manners in which soluble nitrogenous wastes are treated by various organisms. Arthropods Some crustaceans show the presence of green glands or antennule glands which have enlargements or sacs extending into a blood sinus. Nitrogen wastes and excess water are removed from the blood by a sac and transferred along a duct or tube to a "bladder" where they collect. Egestion from the body occurs through an excretory pore which opens near the base of an antenna. Biology 30 99 Lesson 6 Arachnids and insects have tubules extending out from a portion of the intestine. These Malpighian tubules extend into adjacent blood sinuses where they remove salts, excess water and nitrogen wastes. The wastes join and mix with the other waste material in the intestine, before being egested. In many insects the nitrogen wastes are converted to insoluble uric acid crystals inside the Malpighian tubules and intestines. Vertebrates Vertebrates demonstrate a number of ways of excreting and egesting waste materials. Most solids are eliminated through an anal opening. Some waste carbon-dioxide can leave blood capillaries and diffuse through moist skin surfaces and into the external environments (amphibians), through gill filaments into water (fish) or, through moist membranes into lung aveoli before expulsion from lungs. Lungs can also be a means of removing some of the excess water which is formed by the breakdown of nutrients during cellular respiration. Biology 30 100 Lesson 6 The Water–Salt Balance Lungs in land animals may give off water vapor as an excess product of metabolism. In saltwater fish and some reptiles, water may be lost through gills or other body tissues. In these last two situations, however, water is not an excess. Organisms lose it through osmosis, as seawater has a higher salt concentration than body tissues. To make up for losses of water, saltwater organisms must constantly be "drinking" water to replace that which they lose. In doing so, they are taking in extra amounts of salt. To prevent the loss of too much water from tissues, this salt must be continually removed. Kidneys can remove some salt from the blood by active transport to form very concentrated urine. Much of the salt is removed by another means. Special salt glands in the gills of fish, near the nostrils of seabirds and near the eyes of crocodiles and some turtles, return salt to the external surface. When some of the latter organisms are on land, losses of the very salty fluids from the corners of the eyes give them an appearance of "weeping". Freshwater organisms face an opposite situation in that excess water moves into the body through the gills. This situation develops as body tissues have a higher salt concentration than the water. Freshwater organisms must excrete and egest extra amounts of water. This is accomplished mainly by the kidneys, which absorb water from the blood to form urine. Body salts are replaced largely by inward diffusion through the gills. Some organisms can make adjustments as they go from one type of environment to another. Why do some salmon die after spawning? Atlantic salmon can make spawning runs into freshwater and then return to continue life in the sea. On the other hand, Pacific salmon have a body physiology which cannot make a final adjustment. A lot of energy is used up during spawning. These fish simply do not have enough energy left to drive blood through the kidneys and to carry out active transport to remove excess water. Not being able to restore and maintain a proper water and salt balance in their tissues, these fish die soon after spawning. Biology 30 101 Lesson 6 Nitrogen Wastes Organisms consume a lot of protein. More, it is said, than what is required. Unlike other macromolecules, proteins cannot be stored in large amounts. The reason for this is that protein molecules are made up of amino acids that contain nitrogen in the form called the amino group (-NH2). This amino group containing the nitrogen must be removed from the amino acid. This process is called deamination and occurs in the liver. The by-product from the deamination process is ammonia (NH3). Ammonia is toxic and is also water soluble. In smaller aquatic organisms without transport systems, ammonia can be diffused through and completely out of the bodies. Organisms with gill structures can eliminate ammonia by diffusion from the blood capillaries running through the filaments. Terrestrial organisms have the nitrogen wastes eventually reaching the liver. The liver works to get rid of the toxic ammonia. Two molecules of ammonia combine with another waste product, carbon dioxide, to form urea - CO(NH2)2 and/or another substance - uric acid – C5H4N4O3. (You don’t need to know these chemical formulas; they are given for your information only). Urea is quite toxic (only slightly less toxic than ammonia) and remains soluble. The substance, uric acid is almost an insoluble crystal and is not very toxic in this form. Each species has a different way of handling these nitrogen wastes. Those species that convert the amino groups and ammonia into uric acid are: Insects form uric acid in their Malpighian tubules and intestines. Birds and reptiles convert some of the amino groups and ammonia into uric acid. Uric acid crystals can move through the circulatory systems of birds and reptiles until they reach the kidneys. The crystals are filtered out in the kidneys and sent along tube-like ureters. Some reptiles have a small urinary bladder located just before the cloaca, to collect uric acid and some fluids. The cloaca is a common chamber for receiving and storing solid wastes from the intestine and uric acid or urine from the kidneys. At certain times of the year, a cloaca may also be a temporary storage area for eggs and sperm from reproductive organs. Biology 30 102 Lesson 6 Other reptiles, as well as birds, lack urinary bladders and the uric acid is transferred directly to the cloaca by the ureters. Uric acid mixes with the solid wastes from the intestine and the mixture is eventually eliminated through the rectum. The feces are darker-colored while the uric acid is in a white, paste-like form. Before elimination, most of what little water needed to transport the uric acid is reabsorbed from the cloaca. Transporting nitrogenous wastes as uric acid requires far less water than needed for urine removal by amphibians and especially mammals. Reptiles and birds need far less water and therefore do not need to drink as much (possibly receiving all they need from food alone) and do not necessarily need bladders or extensively developed kidneys. This is particularly useful to birds in helping to reduce their body weight for flying. Those species that convert the amino groups and ammonia into urea are: The livers of amphibians and mammals convert their nitrogen wastes into urea. Remember urea is quite toxic and remains soluble. Its solubility means that a fair amount of water is needed to move it through the excretory system. Urea is transported in the blood to the kidneys. In the kidneys it is filtered and processed to form urine. The urine then moves through ureters to a bladder. In amphibians, a bladder empties into a cloaca. In mammals, a bladder has its own separate opening out of the body through a tube called the urethra. When the bladder is full, a muscular sphincter at its juncture with the urethra relaxes and allows urine to flow to the outside as the bladder contracts. The Functioning Kidney In most vertebrates, kidney functioning is usually one of a number of different ways in which wastes are removed from bodies. However, they are the most important and any serious impairment could lead to death. The human kidney will be used to illustrate the nature of this organ in its structure and functioning. Biology 30 103 Lesson 6 Kidney Structure Adult human kidneys exist in pairs in the dorsal area of the lower back, just above the hip area. Each fist-sized organ has a renal artery leading in and a renal vein coming out. Also coming out of each is a tube-like ureter leading to the bladder. Blood carries wastes via the aorta Filtered blood returns to the heart via the interior vena cava Wastes are passed to the kidneys Urea is filtered from the blood Filtered blood leaves the kidneys Bladder Ureter carries wastes to the bladder Urethra The Human Excretory System The internal structure of the organ shows three relatively distinct areas: an outer, darker cortex; a lighter-colored medulla next; and, an inner chamber or cavity called the pelvis. to collecting tube cortex glomerulus medulla Bowman’s capsule renal vein renal pelvis renal artery collecting tube Loop of Henle ureter Kidney Biology 30 Nephron 104 to pelvis of kidney Lesson 6 The basic unit of a kidney is a nephron, of which there are about one million in each organ. Arteries in a kidney divide into smaller branches. A small branch then leads to, and becomes, a twisted, folded, clump of capillaries which make up a glomerulus. A blood capillary leads out of this mass and then divides and re-divides around a tubule, before finally combining into a vein. A collecting tubule forms a double-walled chamber or Bowman's capsule around the glomerulus. The tubule straightens out to form Henle's loop which extends into the medulla (note the characteristic U shape as seen in the diagram on the previous page). The capillary masses or glomeruli and the rest of the nephron are in the cortex, which accounts for its darker color. The loop of Henle ends in another series of coils which joins with other tubules leading to the pelvis. Urine Formation Three types of actions occur in a nephron unit: filtration, reabsorption, and secretion. Major Processes in the formation of urine in a nephron. Biology 30 105 Lesson 6 Filtration At the site of the glomerulus and Bowman's capsule, blood pressure (as a continuation along the arterial system from the heartbeat) produces filtration. This first action results in water and other molecules passing through capillary walls and into Bowman's capsule. Only the larger proteins and red blood cells remain to continue their way along the capillary. The initial fluid or urine in the first part of the collecting tube is really like the intercellular fluid or lymph in other body areas. All human blood is completely filtered about once every four minutes. This filtering results in anywhere from 160 to 190 litres of the total blood volume entering the tubules during the course of a day. Resorption Survival would not long be possible if a body eliminated or egested the fluids and other matter at the same rates that they are filtered. Most of the initially filtered material is too valuable to the body. Instead, the capillary network in which the collecting tubule (loop of Henle) lies, carries out resorption (or, reabsorption) of most of the matter which entered the tube. The length of the loop of Henle and the amount of the capillary network surrounding it in a particular organism can be an indicator of how much reabsorption occurs. These can also be indicators of where some organisms live. A short, almost non-existent loop and a limited surrounding capillary network suggest that little reabsorption of water is necessary or actually occurs. This is a common situation with fresh water organisms, such as fish, which must remove a lot of water from their bodies. Humans do have a fairly well-developed Henle's loop and surrounding capillary network. In a human, approximately 80 to 85% of the water initially filtered out re-enters the blood to maintain proper blood volume. Also reabsorbed, usually by active transport, are glucose, amino acids, lipids, vitamins and mineral ions such as sodium, potassium and calcium. Even some of the waste materials make their way back, although most of these do eventually make up the final urine. Of the initial 160 to 190 L of filtrate, only 1 to 2 litres is eliminated each day. For animals living in deserts, such as camels, nature has provided for them to get the water they need. The Loop of Henle and surrounding capillaries allow them to remove more moisture from the urine to be taken back into the body. Biology 30 106 Lesson 6 Secretion Secretion occurs in a nephron unit . In a way, this is a form of selective absorption. Cells lining the collecting tubule have an ability to draw particular substances from the blood and carry them across, or secrete them, into the waste fluid or urine in the tubule. The secretions of particular ions control the acidity-alkalinity or pH levels of the blood. Excess salts, vitamins and even foreign matter can be removed from the body by this form of active transport. The final composition of the urine eliminated from a body could be variable. A major part of it, approximately 95%, is water. Urea, the slightly less toxic form of ammonia combined with carbon, makes up about 2 to 3%. The remaining wastes consist of excess mineral salts, blood pigments or other broken down cell parts and excess materials, such as vitamins. Regulation of Kidney Functioning The last two actions of absorption and secretion, described in urine formation, are important not only in waste elimination but also in body regulation or homeostasis. Maintaining suitable levels of water, mineral ions, glucose and other substances in body fluids can be just as critical to proper cell functioning as getting rid of wastes. The hypothalamus area of the brain is normally sensitive to the state or condition of the blood in the circulatory system. Detecting variations, it can stimulate or inhibit the pituitary gland in its production of a certain antidiuretic hormone, or ADH. This hormone, sometimes called vasopressin, increases the permeability of the cell walls in the collecting tubules. This causes kidneys to reabsorb or retain more water in the circulatory system (resulting in a lower volume and more concentrated urine). "Watery" blood would result in less vasopressin being produced and less reabsorption into the circulatory system. Tubule walls would be more impermeable to water. This results in a higher volume of less concentrated urine. Irregularities In Kidney Functioning The homeostatic ability of kidneys could be upset in a number of ways. Defective functionings of glands can affect the levels of a number of hormones which control absorptions within the organs. Too little or no antidiuretic hormone or vasopressin can bring on a form of diabetes. Failure by the kidneys to reabsorb much of the water and glucose causes victims to eliminate large amounts of watery urine. Drinking of fresh water offers no lasting relief to constant thirst. Kidney stones may present difficulties for some people. Mineral salts or ions can begin forming solid deposits around clumps of bacterial cells, bits of degenerated body tissues or small blood clots. Once a stone is started, the process continues. Difficulties occur if these stones should block kidney tubules leading into the pelvis or even lodge and block the ureter itself. Chemicals or sound vibrations have been used in attempts to reduce the size or to shatter the stones. Actual removal by some forms of operations are used when initial attempts are unsuccessful. Biology 30 107 Lesson 6 Injuries, poisonings or diseases may lead to severe or complete kidney failure. If one kidney is affected, the other may enlarge somewhat and be able to do the work of both. Failure of both kidneys may require the use of an artificial kidney machine or a dialysis unit to purify the blood on a regular basis. An individual's blood circulates through the unit so that waste molecules can pass from the blood through a membrane and into a fluid medium. This fluid medium is carefully controlled so that it has the proper concentrations of glucose, mineral and other molecules to prevent the loss of necessary molecules from the blood. An alternative to dialysis treatments, which require long periods of time, is a kidney transplant. However, immune rejection is a problem when tissue types do not match up closely. The Skin Some of the same materials removed by the kidneys from the blood, are also eliminated by sweat glands in the skin. Water, salts and urea are absorbed from nearby capillaries by coiled sweat glands in the dermis. These wastes are transported through a duct to the skin surface and then eliminated through a pore. An accumulation of wastes on the skin surface could result in corresponding increases of certain kinds of bacteria which use them as nutrients. The decomposing actions of these bacteria on the "nutrients" lead to body odor. Sweat glands carry out another important function in some organisms. Evaporation of any liquid requires energy to change it to vapor form. Sweat, evaporating from a skin surface, derives heat energy from the skin itself. This produces a cooling effect on the body. The highest concentration of sweat glands in humans occurs on the hands, followed by the feet, forehead and then other areas of the body. Biology 30 108 Lesson 6 Summary Removal of metabolic wastes or substances which may be in excess amounts in cells or bodies is just one of a number of important actions which are part of homeostasis. In smaller, less complex organisms, removal of wastes from cells may occur by diffusion or osmosis to a nearby external environment. The same processes occur in plants, although in larger, multicellular varieties, wastes are usually left in vacuoles or moved only short distances to dead cells where they are deposited. Animals begin to show a variety of excretory systems adapted to body complexities and life styles. These include flame cells, nephridia, Malpighian tubules and nephrons (in kidneys). As complexities in bodies increase, organisms may rely on more than one organ or method of removing wastes. In mammals, the lungs, sweat glands, kidneys and the liver all have some part in the general process, in addition to the removal of solid wastes from the digestive tract or large intestine. To regard excretory systems only as adaptations in removing wastes from cells or bodies would be omitting some of their other important roles. Some "wastes" can be regarded as substances or byproducts having no value, or even being harmful, to cells or tissues. In other instances, certain substances could be wastes at one particular time and nutrients at another. Excretory systems become especially important in their abilities not only to eliminate, but at other times to retain certain matter. This becomes even more complex if one realizes that some of the adjustments occur within very precise or narrow limits. Excretory systems make these adjustments as they maintain certain water-salt levels in tissue fluids, mineral ion concentrations and even help to maintain body temperatures at certain levels such as our own sweat glands do. Any abnormalities or disruptions to excretory systems, which in turn lead to abnormalities in concentration levels or conditions within bodies, could have just as serious consequences as defects in any other systems. Biology 30 109 Lesson 6