After completing this section, you should know:
- the parts of the urinary system
- the structure and function of a kidney
- the structure and function of a kidney tubule or nephron
- the processes of filtration, reabsorption, secretion and concentration that convert blood to urine in the kidney tubule
- the function of antidiuretic hormone in producing concentrated urine
- the composition, storage and voiding of normal urine
- abnormal constituents of urine and their significance
- the functions of the kidney in excreting nitrogenous waste, controlling water levels and regulating salt concentrations and acid-base balance
- that birds do not have a bladder
The cells of an animal can only remain healthy if the conditions are just right. The processes that take place in them are upset if the temperature is too high or too low, or if the fluid around or inside them is too acid or alkaline. Homeostasis is the name given to the processes that help keep the internal conditions constant even when external conditions change. The word means, “staying the same”.
There are a number of organs in the body that play a part in maintaining homeostasis. For example, the skin helps keep the internal temperature of bird and mammals bodies within a narrow range even when the outside temperatures change (see Chapters 5 and 16); the lungs control the amount of carbon dioxide in the blood (see Chapters 8 and 16); the liver and pancreas work together to keep the amount of glucose in the blood within narrow limits (see Chapter 5) and the kidneys regulate the acidity and the concentration of water and salt in the blood (also see Chapter 16). How the kidneys do this will be described later in this chapter.
Hormones are chemicals that carry messages around the body in the blood and are central to many of the homeostatic processes mentioned above. Their role will be described in more detail in Chapter 16.
Water In The BodyEdit
Water is essential for living things to survive because all the chemical reactions within a body take place in a solution of water. An animal’s body consists of up to 80% water. The exact proportion depends on the type of animal, its age, sex, health and whether or not it has had sufficient to drink. Generally animals do not survive a loss of more than 15% of their body water.
In vertebrates almost 2/3rd of this water is in the cells (intracellular fluid). The rest is outside the cells (extracellular fluid) where it is found in the spaces around the cells (tissue fluid), as well as in the blood and lymph.
Diagram 12.1 - Water in the body
Maintaining Water BalanceEdit
Animals lose water through their skin and lungs, in the faeces and urine. These losses must be made up by water in food and drink and from the water that is a by-product of chemical reactions. If the animal does not manage to compensate for water loss the dissolved substances in the blood may become so concentrated they become lethal. To prevent this happening various mechanisms come into play as soon as the concentration of the blood increases. A part of the brain called the hypothalamus is in charge of these homeostatic processes. The most important is the feeling of thirst that is triggered by an increase in blood concentration. This stimulates an animal to find water and drink it.
The kidneys are also involved in maintaining water balance as various hormones instruct them to produce more concentrated urine and so retain some of the water that would otherwise be lost (see later in this Chapter and Chapter 16).
Coping with water loss is a particular problem for animals that live in dry conditions. Some, like the camel, have developed great tolerance for dehydration. For example, under some conditions, camels can withstand the loss of one third of their body mass as water. They can also survive wide daily changes in temperature. This means they do not have to use large quantities of water in sweat to cool the body by evaporation.
Smaller animals are more able than large ones to avoid extremes of temperature or dry conditions by resting in sheltered more humid situations during the day and being active only at night.
The kangaroo rat is able to survive without access to any drinking water at all because it does not sweat and produces extremely concentrated urine. Water from its food and from chemical processes is sufficient to supply all its requirements.
Animals need to excrete because they take in substances that are excess to the body’s requirements and many of the chemical reactions in the body produce waste products. If these substances were not removed they would poison cells or slow down metabolism. All animals therefore have some means of getting rid of these wastes.
The major waste products in mammals are carbon dioxide that is removed by the lungs, and urea that is produced when excess amino acids (from proteins) are broken down. Urea is filtered from the blood by the kidneys.
Diagram 12.2 - The position of the organs of the urinary system in a dog
The Kidneys And Urinary SystemEdit
The kidneys in mammals are bean-shaped organs that lie in the abdominal cavity attached to the dorsal wall on either side of the spine (see diagram 12.2). An artery from the dorsal aorta called the renal artery supplies blood to them and the renal vein drains them.
Diagram 12.3 - The urinary system
To the naked eye kidneys seem simple enough organs. They are covered by a fibrous coat or capsule and if cut in half lengthways (longitudinally) two distinct regions can be seen - an inner region or medulla and the outer cortex. A cavity within the kidney called the pelvis collects the urine and carries it to the ureter, which connects with the bladder where the urine is stored temporarily. Rings of muscle (sphincters) control the release of urine from the bladder and the urine leaves the body through the urethra (see diagrams 12.3 and 12.4).
Diagram 12.4 - The dissected kidney
Kidney Tubules Or NephronsEdit
It is only when you examine kidneys under the microscope that you find that their structure is not simple at all. The cortex and medulla are seen to be composed of masses of tiny tubes. These are called kidney tubules or nephrons (see diagrams 12.5 and 12.6). A human kidney consists of over a million of them.
Diagram 12.5 - Several kidney tubules or nephrons
Diagram 12.6 - A kidney tubule or nephron
At one end of each nephron, in the cortex of the kidney, is a cup shaped structure called the (Bowman’s or renal) capsule. It surrounds a tuft of capillaries called the glomerulus that carries high-pressure blood. Together the glomerulus and capsule act as a blood-filtering device (see diagram 12.7). The holes in the filter allow most of the contents of the blood through except the red and white cells and large protein molecules. The fluid flowing from the capsule into the rest of the kidney tubule is therefore very similar to blood plasma and contains many useful substances like water, glucose, salt and amino acids. It also contains waste products like urea.
Processes Occurring In The NephronEdit
After entering the glomerulus the filtered fluid flows along a coiled part of the tubule (the proximal convoluted tubule) to a looped portion (the Loop of Henle) and then to the collecting tube via a second length of coiled tube (the distal convoluted tubule) (see diagram 12.6). From the collecting ducts the urine flows into the renal pelvis and enters the ureter.
Note that the glomerulus, capsule and both coiled parts of the tubule are all situated in the cortex of the kidney while the loops of Henle and collecting ducts make up the medulla (see diagram 12.5).
As the fluid flows along the proximal convoluted tubule useful substances like glucose, water, salts, potassium ions, calcium ions and amino acids are reabsorbed into the blood capillaries that form a network around the tubules. Many of these substances are transported by active transport and energy is required.
Diagram 12.7 - Filtration in the glomerulus and capsule
In a separate process, some substances, particularly potassium, ammonium and hydrogen ions, and drugs like penicillin, are actively secreted into the distal convoluted tubule.
By the time the fluid has reached the collecting ducts these processes of absorption and secretion have changed the fluid originally filtered into the Bowman’s capsule into urine. The main function of the collecting ducts is then to remove more water from the urine if necessary. These processes are summarised in diagram 12.8.
Normal urine consists of water, in which waste products such as urea and salts such as sodium chloride are dissolved. Pigments from the breakdown of red blood cells give urine its yellow colour.
The Production Of Concentrated UrineEdit
Because of the high pressure of the blood in the glomerulus and the large size of the pores in the glomerulus/capsule-filtering device, an enormous volume of fluid passes into the kidney tubules. If this fluid were left as it is, the animal’s body would be drained dry in 30 minutes. In fact, as the fluid flows down the tubule, over 90% of the water in it is reabsorbed. The main part of this reabsorption takes place in the collecting tubes.
The amount of water removed from the collecting ducts is controlled by a hormone called antidiuretic hormone (ADH) produced by the pituitary gland, situated at the base of the brain. When the blood becomes more concentrated, as happens when an animal is deprived of water, ADH is secreted and causes more water to be absorbed from the collecting ducts so that concentrated urine is produced. When the animal has drunk plenty of water and the blood is dilute, no ADH is secreted and no or little water is absorbed from the collecting ducts, so dilute urine is produced. In this way the concentration of the blood is controlled precisely.
Diagram 12.8 - Summary of the processes involved in the formation of urine
Water Balance In Fish And Marine AnimalsEdit
Fresh Water FishEdit
Although the skin of fish is more or less waterproof, the gills are very porous. The body fluids of fish that live in fresh water have a higher concentration of dissolved substances than the water in which they swim. In other words the body fluids of fresh water fish are hypertonic to the water (see chapter 3). Water therefore flows into the body by osmosis. To stop the body fluids being constantly diluted fresh water fish produce large quantities of dilute urine.
Marine fish like the sharks and dogfish have body fluids that have the same concentration of dissolved substances as the water (isotonic) have little problem with water balance. However, marine bony fish like red cod, snapper and sole, have body fluids with a lower concentration of dissolved substances than seawater (they are hypotonic to seawater). This means that water tends to flow out of their bodies by osmosis. To make up this fluid loss they drink seawater and get rid of the excess salt by excreting it from the gills.
Marine birds that eat marine fish take in large quantities of salt and some only have access to seawater for drinking. Bird’s kidneys are unable to produce very concentrated urine, so they have developed a salt gland. This excretes a concentrated salt solution into the nose to get rid of the excess salt.
Diabetes And The KidneyEdit
There are two types of diabetes. The most common is called sugar diabetes or diabetes mellitus and is common in cats and dogs especially if they are overweight. It is caused by the pancreas secreting insufficient insulin, the hormone that controls the amount of glucose in the blood. If insulin secretion is inadequate, the concentration of glucose in the blood increases. Any increase in the glucose in the blood automatically leads to an increase in glucose in the fluid filtered into the kidney tubule. Normally the kidney removes all the glucose filtered into it, but these high concentrations swamp this removal mechanism and urine containing glucose is produced. The main symptoms of this type of diabetes are the production of large amounts of dilute urine containing glucose, and excessive thirst.
The second type of diabetes is called diabetes insipidus. The name comes from the main symptom, which is the production of large amounts of very dilute and “tasteless” urine. It occurs when the pituitary gland produces insufficient ADH, the hormone that stimulates water re-absorption from the kidney tubule. When this hormone is lacking, water is not absorbed and large amounts of dilute urine are produced. Because so much water is lost in the urine, animals with this form of diabetes can die if deprived of water for only a day or so.
Other Functions Of The KidneyEdit
The excretion of urea from the body and the maintenance of water balance, as described above, are the main functions of the kidney. However, the kidneys have other roles in keeping conditions in the body stable i.e. in maintaining homeostasis. These include:
- controlling the concentration of salt ions (Na+, K+, Cl-) in the blood by adjusting how much is excreted or retained;
- maintaining the correct acidity of the blood. Excess acid is constantly being produced by the normal chemical reactions in the body and the kidney eliminates this.
Normal urine consists of water (95%), urea, salts (mostly sodium chloride) and pigments (mostly from bile) that give it its characteristic colour.
Abnormal Ingredients Of UrineEdit
If the body is not working properly, small amounts of substances not normally present may be found in the urine or substances normally present may appear in abnormal amounts.
- The presence of glucose may indicate diabetes (see above).
- Urine with red blood cells in it is called haematuria, and may indicate inflammation of the kidney,or urinary tract, cancer or a blow to the kidneys.
- Sometimes free haemoglobin is found in the urine. This indicates that the red blood cells in the blood have haemolysed (the membrane has broken down) and the haemoglobin has passed into the kidney tubules.
- The presence of white blood cells in the urine indicates there is an infection in the kidney or urinary tract.
- Protein molecules are usually too large to pass into the kidney tubule so no or only small amounts of proteins like albumin is normally found in urine. Large quantities of albumin indicate that the kidney tubules have been injured or the kidney has become diseased. High blood pressure also pushes proteins from the blood into the tubules.
- Casts are tiny cylinders of material that have been shed from the lining of the tubules and flushed out into the urine.
- Mucus is not usually found in the urine of healthy animals but is a normal constituent of horses’ urine, giving it a characteristic cloudy appearance.
Tests can be carried out to identify any abnormal ingredients of urine. These tests are normally done by “stix”, which are small plastic strips with absorbent ends impregnated with various chemicals. A colour change occurs in the presence of an abnormal ingredient.
Excretion In BirdsEdit
Birds’ high body temperature and level of activity means that they need to conserve water. Birds therefore do not have a bladder and instead of excreting urea, which needs to be dissolved in large amounts of water, birds produce uric acid that can be discharged as a thick paste along with the feces. This is the white chalky part of the bird droppings that land on you or your car.
- The excretory system consists of paired kidneys and associated blood supply. Ureters transport urine from the kidneys to the bladder and the urethra with associated sphincter muscles controls the release of urine.
- The kidneys have an important role in maintaining homeostasis in the body. They excrete the waste product urea, control the concentrations of water and salt in the body fluids, and regulate the acidity of the blood.
- A kidney consists of an outer region or cortex, inner medulla and a cavity called the pelvis that collects the urine and carries it to the ureter.
- The tissue of a kidney is composed of masses of tiny tubes called kidney tubules or nephrons. These are the structures that make the urine.
- High-pressure blood is supplied to the nephron via a tuft of capillaries called the glomerulus. Most of the contents of the blood except the cells and large protein molecules filter from the glomerulus into the (Bowmans) capsule. This fluid flows down a coiled part of the tubule (proximal convoluted tubule) where useful substances like glucose, amino acids and various ions are reabsorbed. The fluid flows to a looped portion of the tubule called the Loop of Henle where water is reabsorbed and then to another coiled part of the tubule (distal convoluted tubule) where more reabsorbtion and secretion takes place. Finally the fluid passes down the collecting duct where water is reabsorbed to form concentrated urine.
Use this Excretory System Worksheet to help you learn the parts of the urinary system, the kidney and kidney tubule and their functions.
The Urinary System Test Yourself can then be used to see if you understand this rather complex system.
1. Add the following labels to the diagram of the excretory system shown below. Bladder | ureter | urethra | kidney | dorsal aorta | vena cava | renal artery | vein
2. Using the words/phrases in the list below fill in the blanks in the following statements.
- | cortex | amino acids | renal | glucose | water reabsorption | large proteins |
- | bowman’s capsule | diabetes mellitus | secreted | antidiuretic hormone (ADH) | blood cells |
- | glomerulus | concentration of the urine | medulla | nephron |
a) Blood enters the kidney via the ......................... artery.
b) When cut across the kidney is seen to consist of two regions, the outer.............. and the inner..............
c) Another word for the kidney tubule is the...............................
d) Filtration of the blood occurs in the..............................
e) The filtered fluid (filtrate) enters the.............................
f) The filtrate entering the e) above is similar to blood but does not contain.................. or....................
g) As the fluid passes along the first coiled part of the kidney tubule.................. and.................... are removed.
h) The main function of the loop of Henle is.............................................................
i) Hydrogen and potassium ions are.............................. into the second coiled part of the tubule.
j) The main function of the collecting tube is..................................................
k) The hormone...................................... is responsible for controlling water reabsorption in the collecting tube.
l) When the pancreas secretes inadequate amounts of the hormone insulin the condition known as............................... results. This is most easily diagnosed by testing for................................ in the urine.
- http://www.biologycorner.com/bio3/nephron.html Biology Corner. A fabulous drawing of the kidney and nephron to print off, label and colour in with clear explanation of function.
- http://health.howstuffworks.com/adam-200032.htm How Stuff Works. This animation traces the full process of urine formation and reabsorption in the kidneys, its path down the ureter to the bladder, and its excretion via the urethra. Needs Shockwave.
- http://en.wikipedia.org/wiki/Nephron Wikipedia. A bit more detail than you need but still good clear explanations and lots of information.