TOPIC 4: EXCRETION
Concept of Excretion
The Concept of Excretion
Explain the concept of excretion
Chemical reactions occur in the cells of living organisms all the time to carry out the life processes. The sum of these reactions is called metabolism. Metabolism produces useful products as well as toxic (poisonous) by-products.
These toxic substances have to be removed as they are harmful if allowed to accumulate. The removal of metabolic waste products from the body of an organism is known as excretion.
The major excretory products are carbon dioxide, excess water, and nitrogenous compounds like ammonia, urea, uric acid, etc. Carbon dioxide and water are produced in the process of tissue respiration. Nitrogenous compounds are formed from the breakdown of proteins and amino acids. Water and salts in excess of the body’s needs are also excreted.
Other excretory products include chemicals from medicines, toxic substances, and circulating hormones that have already served their purpose. We will learn how metabolic wastes get eliminated.
In concise, excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism.
Examples of Excretory Products Eliminated by Organisms
Give examples of excretory products eliminated by organisms
Living organisms excrete various excretory products of diverse chemical nature. The following are examples of excretory products excreted by living organisms:
- Carbon dioxide:This is a by-product of respiration of both plants and animals. It is excreted through the pores of the stomata in plants (some of the carbon dioxide produced by respiration is used in photosynthesis). In man, carbon dioxide is eliminated from the body by lungs.
- Water:The concentration of water in cells must be kept within narrow limits. Too little or too much water can have a negative effect on the osmotic condition in and around the cell. Therefore, it has to be regulated. Plant cells are protected from bursting by their cell walls. Animals do not have cell walls, and will burst if they have too much water. Excess water is lost from the surface of gaseous exchange in both plants and animals. In mammals, water is also lost through sweat and through osmoregulation controlled by the kidneys.
- Urea:This is a compound produced in mammals from the breakdown of excess amino acids. Amino acids cannot be stored because their accumulation is toxic. They are therefore converted into a less toxic substance. This process occurs in the liver and is called de-amination. Ammonia is converted to urea by the liver. Urea is transported by blood to the kidneys where they are excreted. The kidneys are also used to remove uric acid, water, excess salts, excess hormones and bile pigments.
- Calcium oxalate: This is a waste material produced by plants and is stored as an insoluble crystalline structure in the cells. Calcium oxalate is stored in aging leaves, stems and roots, flowers or fruits.
- Oxygen:Through the process of photosynthesis, oxygen is produced as a by-product. Some of the oxygen is used for respiration, and the remainder is excreted through the stomata of the leaves.In plants, some waste substances are stored in parts of the plant that are dead. Examples of this are the tannin in the bark of trees such as mangroves and the dyes in the heartwood of trees such as log wood. The purpose of the storage of waste material ranges from protection to a decreased risk of being consumed.
Excretion in Human
The main excretory system in humans is the urinary system. The skin also acts as an organ of excretion by removing water and small amounts of urea and salts (as sweat). The urinary system includes a pair of bean-shaped kidneys located in the back of the abdominal cavity.
Excretory Organs in Human Being
Mention excretory organs in human being
There are special organs concerned with removal of excretory products from the body. Such organs are called excretory organs. The excretory organs in an animals body are the kidney, the skin the lungs and the liver. The following table shows the excretory organs and the products they excrete.
The table below summarizes excretory products in humans and the organs concerned with excretion of the product and the mode of excretion.
|Excretory product||Organ||Mode of excretion|
|Urea and excess mineral salts||Kidneys||Urine|
|Bile pigments (from breakdown of haemoglobin)||Liver||Faeces|
A comparison between different excretory products can be made. Following is the comparison between different types of excretory products:
|Ammonia||Produced with little energy||Toxic in concentrated solution. Excretion take place in lot of water||Water||Marine and fresh water invertebrates, bony fishes, amphibians|
|Urea||Less toxic than ammonia. Less water is needed to excrete it||Requires more energy to produce it||Land, Sea||Adult amphibians, turtles, mammals and bony fishes|
|Uric Acid||Very little water is used for its excretion||Requires considerable energy to produce it||Land||Reptiles, birds, insects, gastropods (snails and slugs)|
|Guanine||Relatively non toxic. Less water is used to excrete it||More energy is needed||Arid habitat||Scorpions, Spiders|
The excretory organs in humans beings may conventionally be put into two groups namely, primary and accessory organs:
Primary excretory organs
- Kidneys:Kidneys are bean-shaped organs of a reddish brown colour that are found in the sides of the vertebral column. Once the body has extracted wastes from food, it sends the wastes to the kidneys. The kidneys filter the wastes, including urea, salt and excess water, which are flushed out of the body as urine.
- Skin:The skin performs its excretory function via the sweat glands. These are coiled tubules surrounded by blood capillaries. These glands produce sweat that contains mineral salts, excess oils, water, and traces of urea and lactic acid from the surrounding blood vessels which are then excreted out of the body through sweat pores. The tubules extend into sweat ducts which open on the surface of the skin as pores. Sweating also helps to cool the body during evaporation.
- Lungs:The lungs are very important excretory organs as they expel carbon dioxide from the body via exhalation. The lungs use cells known as alveoli to remove the carbon dioxide from our blood. Otherwise, the carbon dioxide would accumulate and have a detrimental effect to our body.
Accessory excretory organs
- Liver:Although considered a secondary or accessory excretory organ, the liver plays a vital part in keeping the body clean. Amino acids are the end-product of proteins. Excess amino acids cannot be stored in the body. They are taken from the gut into the liver by the hepatic portal vein. The liver converts them into useful carbohydrates (stored in the form of glycogen) and ammonia. Ammonia is very poisonous and must be removed from the body. The liver converts ammonia into a harmless compound called urea. Urea dissolves in the blood and is transported into the kidney via the hepatic vein where it is removed as component of urine.Harmful poisons and chemicals that are either produced in the body or consumed are broken down and detoxified by the liver.
- Gall bladder:Although the gallbladder does not have a highly significant role to play in the excretory system, it does have a function that assists the overall process. Bile, a liquid produced by the liver to break down waste, is first stored in the gall bladder. When needed, it is discharged into the small intestine whose role is to break down fats, ethanol and other acidic wastes.
- Urinary bladder:The waste fluid that is created in the liver and collected in the kidney is transferred into the urinary bladder where it is temporarily stored until the individual urinates. The urinary bladder provides a short term solution for storing urine in the body until it is ultimately discharged.
- Ureters:The ureters tubes of smooth muscle fibre transfer liquid waste from the kidneys into the urinary bladder. The urine is moved with peristaltic movements which force the urine away from the kidneys. The ureters also have ureterovesical valves which ensure the waste fluid does not travel back into the kidney.
- Urethra:The urethra runs through the penis in males, and serves as a carrier of semen as well as urine for their ultimate discharge out of the body. The urethra tube is shorter in females and is just above the vaginal opening.
- Large intestine:Food particles are absorbed into the blood stream via the small intestine. The undigested substances are transferred to the large intestine which essentially serves as a storage organ for the excretory products. The descending, ascending and transverse colons also facilitate the absorption of leftover vitamins, water and salt. The distal straight section (known as the rectum) is used for the storage of waste products (feces) before they are excreted from the body via the anal canal with the help of internal and external sphincters.
The Urinary System and its Adaptive Features
Describe the urinary system and its adaptive features
The urinary system is a system concerned with production, storage and removal of urine. In humans, it is made up of the two kidneys, ureters, blood vessels that join them, urinary bladder and urethra.
Each kidney is connected to a urinary bladder by a tube called the ureter. As urine is formed, it passes through the ureter to the urinary bladder where it is stored temporarily. Below the bladder is a tube called urethra which is surrounded by a ring of muscles called sphincter muscles. The urethra is a canal that carries urine from the bladder and expels it outside the body. The urethra passes urine when sphincter muscles relax.
The kidneys:Kidneys are bean-shaped organs that are deep red in colour located on the dorsal part of the abdominal cavity. Mammals have one pair of kidneys. Kidneys are the principle organs of the urinary system.
Functions of the kidneys
- They filter blood to remove wastes and reabsorb useful substances such as water and salts.
- They maintain appropriate water-salt balance in the blood.
- They regulate the acid-base balance such that the pH of blood remains at about 7.4 for proper functioning of the body.
- They are important in regulation of blood pressure.
- They produce erythropoietin, calcitriol and an enzyme called rennin, which is involved in the formation of the hormone, angiotensin, which raises blood pressure. Erythropoietin is a hormone that speeds up the process of the formation of red blood cells in the bone marrow. Calcitriol is an active form of vitamin D which increases the rate of calcium reabsorption for bone formation.
Structure of the kidney
Each kidney is enclosed in a thin, fibrous covering called the capsule. The kidney has three distinct regions, namely the cortex, medulla and pelvis. The cortex is the outermost layer of the kidney. The medulla is the middle layer of the kidney, normally red in colour. The pelvis is the space inside the kidney which collects the urine and leads it to the ureter. The ureter passes urine to the urinary bladder where it is stored before it is released out through the urethra in the genitals.
Each kidney is supplied with oxygen-rich blood from the renal artery. Deoxygenated blood is returned to the circulatory system from the kidneys via the renal vein. Blood entering the kidneys from the renal artery are rich in waste materials, and blood exiting the kidneys from the renal vein is deficient in these waste materials.
Each kidney is made up of numerous, coiled excretory tubules known as nephrons, and collecting ducts associated with tiny blood vessels.
A nephron consists of a long, coiled tubule, and the Malpighian corpuscle. The tubule of the nephron is differentiated into the proximal convoluted tubule, loop of Henle and the distal convoluted tubule. The distal tubule opens into the collecting duct.
At the proximal end of the nephron is the Malpighian corpuscle, which consists of Bowman’s capsule and the glomerulus. Bowman’s capsule is a double-walled, cup-like structure which surrounds the dense network of blood capillaries called the glomerulus.
There is a branch of the renal artery,the afferent arteriole, entering the small cup-like space of theBowman’s capsuleas a network of blood capillaries. This network is known as theglomerulus. Emerging from this network, the capillaries re-unite to form a small arteriole, known asthe efferent arteriole. As the efferent arteriole continues it twines around the proximal and distal convoluted tubule. The efferent arteriole divides into capillaries at several points along the length of the tubules, absorbing various substances. These capillaries eventually reunite to drain into the renal vein. The efferent arteriole is smaller than the afferent arteriole. This difference in diameter helps to raise the glomerular pressure and aids in ultra filtration.
Some animals do not have a well developed kidney; they may have structures called nephridia. Animals such as earthworms that are simple tube-like structures have nephridia that have the same role as the more complex nephrons in the kidneys.
The Process of Urine Formation
Explain the process of urine formation
The blood from the afferent arteriole, which enters the Bowman’s capsule, is rich in nutrients such as glucose, fatty acids, amino acids, vitamins, proteins, urea and excess salts. A nephron is the structural and functional unit of a kidney which is concerned with excretion and haemostasis.
The three (3) distinct stages of urine formation are: (1) glomerular filtration, (2) tubular reabsorption and secretion, and (3) water conservation
- Glomerular filtration:Filtration occurs in the glomerulus (plural; glomeruli). Because the lumen of the afferent arteriole is significantly smaller than the efferent arteriole there is a large amount of pressure in the glomerulus. This pressure forces forces plasma, the liquid portion of the blood, through the capillary walls in the glomerulus. The plasma contains water, glucose, amino acids, urea, salts, uric acid, vitamins, and hormones. Red blood cells and proteins are retained in the blood capillaries because they are too large to pass through the wall. This process is called ultra-filtration and the fluid which passes into the Bowman’s capsule is known as the glomerular filtrate.The glomerular filtrate collects in the capsule and enters the renal tubule at the proximal convoluted tubule which is coiled to increase the surface are for reabsorption of substances.
- Reabsorption and secretion:The efferent arteriole branches into capillaries which surround the proximal convoluted tubule. There, glucose, amino acids, vitamins, hormones and about 80% of sodium chloride and water are reabsorbed into the blood capillaries. Only the substances that are useful to the body are reabsorbed. This is referred to as selective reabsorption. Normally 100% of glucose is reabsorbed. Blood cells are not reabsorbed due to their large molecular size. Reabsorption involves both diffusion and active transport (against concentration gradient), which uses energy in the form of ATP. The waste-containing fluid that remains after reabsorption is urine. Tubular secretion is the passage of certain substances out of the capillaries directly into the renal tubules. Tubular secretion is another way of getting waste materials into the urine. For example, drugs such as penicillin and phenobarbital are secreted into the renal tubules from the capillaries. Urea and uric acid that may have been reabsorbed are secreted. Excess potassium ions are also secreted into the urine. Tubular secretions also maintain the pH of the blood.
- Water conservation:In the distal convoluted tubule, water is reabsorbed into the blood by osmosis. The tubule is also highly coiled to increase the surface area for reabsorption. The glomerular filtrate then flows into the collecting tubules where more water is reabsorbed. The filtrate now called urine flows into the collecting duct then into the pelvis of the kidney. It then flows into the ureter and is temporarily stored in the bladder. When it gets full, impulses are sent to the brain to create an awareness of the presence of urine in the bladder. A person then fells the need to urinate and the bladder muscles contract, sphincter muscles relax and the urine is expelled via the urethra.Urine formation is essential for maintaining homeostasis, which is the ability of the body to maintain constant internal environment.
Adaptations of the urinary system to its functions
- The urinary system has a large afferent arteriole, and narrow efferent arteriole, which allows build up of pressure, thus facilitating ultra-filtration.
- The glomerulus capillaries are highly coiled and semi permeable, causing a build up of pressure in the glomerulus hence ultrafiltration.
- The glomerular capillaries are semi permeable (tubule also semi permeable) to allow selective movement of materials in and out of the nephron (selective reabsorption).
- The tubules’ epithelium is thin (1 cell thick) to reduce diffusion distance for faster passage and hence reabsorption of materials; and they are and leakier than normal capillaries.
- It is connected to a collecting duct, which channels the filtrate (urine) out of the nephron to the ureter to allow for continous functioning of the nephron.
- The proximal convoluted tubule and the distal convoluted tubule are coiled so as to increase the nephrons’s length and hence more surface area for efficient reabsorption to take place.
- The Bowman's capsule is cup-shaped to provide maximum surface area for filtration.
- The tubule is supplied with a network of blood capillaries for maximum reabsorption.
- The nephrons are numerous in number for efficient excretion of waste products.
Complications and Disorders of the Excretory System
Common Complications and Disorders of the Excretory System
Mention common complications and disorders of the excretory system
A large number of medical conditions of varying intensity are associated with the excretory system. If these are not addressed immediately, they may even prove to be fatal and claim one’s life. We need to be well acquainted with the most commonly occurring diseases of excretory system along with their description, symptoms and effective treatment measures. The table below shows some of the common complications and disorders of the excretory system, their causes, symptoms, effects and preventive measures.
The Causes, Symptoms, Effects and Control Measures of Common Complications and Disorders of the Excretory System
Explain the causes, symptoms, effects and control measures of common complications and disorders of the excretory system
Disorders of the Excretory System
|Kidney (renal) failure – failure of the kidney to function adequately due to partial or entire destruction of nephrons||Bone damage, muscle paralysis, abnormal heart rhythm, loss of memory, pain in the back or side and if not treated earlier, can lead to death if it involves both kidneys|
|Urinary tract infections (UTIs)|
|Liver cirrhosis: a condition in which liver cells degenerate and are replaced by scar tissue, causing the liver to shrink, harden, become fibrous and fail to function normally|
|Hepatitis:Inflammation of the liver|
Excretion in Plants
Types of Excretory Products Eliminated by Plants
Mention types of excretory products eliminated by plants
Excretion occurs in plants and animals as they both have waste products to be removed from their bodies. Compared to animals, plants do not have a well-developed excretory system to throw out nitrogenous waste materials. This is because of the differences in their physiology. Therefore, plants use different strategies for excretion.
Plants eliminate some waste through diffusion. During the day, excess oxygen gas produced by photosynthesis is released through the stomata. Carbon dioxide produced by respiration is normally used up during photosynthesis. At night, however, as photosynthesis slows down, carbon dioxide is not used up as fast as it is produced, and it is released as a waste product. Plants also produce oxygen as a by-product of photosynthesis. The oxygen that is not used for respiration is also excreted through the stomata.
Excess water evaporates mostly from stomata and also from the outer surface of the stem, fruits, etc., throughout the day. This process of getting rid of excess water is called transpiration.
Many plants store organic waste products in their permanent tissues that have dead cells, e.g., in heartwood. Plants also store waste within their leaves or barks. These wastes are periodically removed as the leaves and barks fall off.
Some of the waste products are stored in special cells or cellular vacuoles. Various waste products such as tannins, essential oils, gums, resins, etc., are produced during catabolic processes. Tea leaves contain tannin. Tannins are found also in the barks of trees.
The leaves of many plants, like Eucalyptus, lemon, etc., contain essential oils. The coating of oranges and lemons and the petals of flowers like rose and jasmine also contain oils. Some plant wastes are stored as a thick, white fluid. You may have seen white fluid ooze out when you pluck a papaya or a fig or other plants. This white fluid is called latex.
Gums are a group of sticky, water- soluble wastes found in the common gum tree. Resins are another group of wastes found commonly in the stems of conifers (e.g., pine, fir).
Alkaloids are a group of toxic waste products. But some of these are useful to us. Quinine and morphine are medicines derived from alkaloids stored in Cinchona bark and opium poppy flowers respectively. Caffeine found in coffee seeds and nicotine in tobacco leaves is also alkaloids.
Plants also eliminate waste by the accumulation of waste in the vacuoles of the aging leave cells. These leaves will eventually die and fall off, removing waste in the process. This process is called abscission.
Organic acids, which might prove harmful to plants, often combine with excess cations and precipitate out as insoluble crystals that can be safely stored in plant cells. Calcium oxalate crystals accumulate in some tubers like yam.
Aquatic plants lose most of their metabolic wastes by direct diffusion into the water surrounding them. Terrestrial plants excrete some waste into the soil around them.
In plants, breakdown of substances is much slower than in animals. Plants do not have specialized excretory organs for the removal of metabolic wastes.
Plants do not need a specialized excretory system like in animals because of the following reasons:
- The rate of producing waste products in plants is much lower.
- Most excretory products from plants are removed by diffusion through the stomata or lenticels. Lenticels are areas in woody stems where the cells are loosely packed allowing gaseous exchange and removal of waste products.
- The major excretory products of plants are carbon dioxide, oxygen and water. These products are recycled. For example, the oxygen released as a by product during photosynthesis, is used in respiration by animals and plants. The carbon dioxide released during respiration is used for photosynthesis by the plants. Water is released into the atmosphere where it enters the water cycle.
- Plants produce less poisonous substances compared to the nitrogenous wastes produced by animals.
- Plants have large vacuoles which store waste substances often accumulating at concentrations that lead to crystal formation in form of oil droplets or granules.
- Plants can also store the waste products in organs that are destined to fall or die off. So they can be excreted via abscission. Other waste material such as resins, saps and latexes are forced out from the interior of some plants by hydrostatic pressure inside the plant.
The Importance of Common Excretory Products of Plants
Explain the importance of common excretory products of plants
Have you ever noticed sticky, milky or oily substances being oozed from the bark of trees? These are excretory products and may be resins, gums, latex and or other excretory products. These products may be poisonous. However, many have found use in everyday life of humans, such as latex which is used to produce gloves and clothing. The table below shows functions of some plant waste products.
|Product (waste)||Plant source||Use and effects|
|1. Tannins||Dead tissues of plants such as acacia, conifers, coffee and mangroves.|
|2. Alkaloids: Are nitrogenous excretory products in plants and occur in various forms- Common alkaloids include:|
|(a) Caffeine and theophilinet||-Coffee fruits and tea leaves||-Mild stimulants to increase mental activity and reduce fatigue.|
|(b)Quinine||- Bark of cinchona tree.||-A drug for the treatment of malaria.-An additive in drinks to act as a stimulant.|
|(c) Cocaine||- Leaves of coca plant|
|(d) Cannabis||-Fruits, flowers and leaves of cannabis sativa (bhang or marijuana)|
|(e) Opiates(morphine)||-Opium poppy||Manufacture of drugs like morphine and codeine both of which are effective painkillers, muscle relaxant, cough suppressants, and anti-diarrhoeal.|
|(f) Nicotine||-Tobacco leaves.|
|(g) Papain||-Epicarp of pawpaw fruits (especially raw).||-Has proteolytic activity hence used as a meat tenderizer.|
|(h) Colchicine||-Roots of the crocus plant.|
|(i) Pyrethrins||-Flowers of pyrethrum||Making of insecticides|
|(j) Khat (miraa) mirungi)||-Leaves and twigs of the “khat” plat||-Used as a stimulant.|
|4. Gums||Different plants such as Arabic ghath and carob, acacia tree, etc.|
|5. Anthocyanins||-Petals and leaves of various plants, and are mostly red, blue or purple.|
|6.Digitalis glycosides||-Foxglove||Manufacture of drugs used for treatment of heart diseases such as digitoxin.|
|7. Rennin||-Certain tree stems like the casuarina tree||-Manufacture of varnish and gum.|
|8. Oil||-Flowers or leaves of certain trees.||Manufacture of perfume and ointment for insect bites.|