- 1 Excretion in Plants and Animals, Human Excretory System
Evolutionary Biology Topics allow us to trace the history of life on Earth.
Excretion in Plants and Animals, Human Excretory System
All the organisms (plants and animals) are made up of cells. These cells work all the time for sustaining the life of the organism. Most of the work of the cells is in the form of biochemical reactions which they carry out all the time. The biochemical reactions taking place in the cells of an organism may produce toxic wastes (poisonous wastes) in the body. The accumulation of toxic wastes in the body harms an organism. So, for an organism to lead a normal life, the toxic wastes being produced in its body must be removed continuously. The process of removal of toxic wastes from the body of an organism is called excretion. Excretion takes place in plants as well as in animals.
Excretion in Plants -Transpiration and Excretory Products
Like animals, plants also produce a number of waste products during their life processes. Compared to animals, plants produce waste products very slowly and in very small amounts. Plants have no special organs for waste removal like animals. The plants remove their waste products by different methods. Some of the important plant wastes and the methods by which they are removed are described below.
The main waste products produced by plants are carbon dioxide, water vapour, and oxygen. Carbon dioxide and water vapour are produced as wastes during respiration by plants whereas oxygen is produced as waste during photosynthesis. The gaseous wastes of respiration and photosynthesis in plants (carbon dioxide, water vapour, and oxygen) are removed through the ‘stomata’ in leaves and ‘lenticels’ in stems and released to the air.
The plants excrete carbon dioxide produced as waste during respiration only at night time. This is because the carbon dioxide produced during respiration in the day time is all used up by the plant itself in photosynthesis. The plants excrete oxygen as waste only during the daytime (because oxygen is produced by photosynthesis only during the daytime when the sunlight is there). Water vapour produced as waste by respiration is, however, excreted by plants all the time (day as well as night). This wastewater is got rid of by transpiration.
The plants also store some of the waste products in their body parts. For example, some of the waste products are collected in the leaves, bark, and fruits of the plants (or trees). The plants get rid. of these wastes by shedding of leaves, peeling of bark, and felling fruits. So, when the dead leaves, bark, and ripe fruits fall off a tree, then the waste products contained in them are got rid of. Some of the plant wastes get stored in the fruits of the plant in the form of solid bodies called raphides. These wastes are removed when the fruits get detached from the plant.
For example, the fruit called ‘yam’ (zamikand) has needle-shaped raphides on its surface. The plants secrete their wastes in the form of gum and resins from their stems and branches. The plants also excrete some waste substances into the soil around them.
From the above discussion, we conclude that the various methods used by the plants to get rid of their waste products are the following:
- The plants get rid of gaseous waste products through stomata in leaves and lenticels in stems.
- The plants get rid of stored solid and liquid wastes by the shedding of leaves, peeling of bark, and felling of fruits.
- The plants get rid of wastes by secreting them in the form of gums and resins.
- Plants also excrete some waste substances into the soil around them.
Excretion in Animals
Different animals have different arrangements (or organs) for excretion, which depend on the constitution of the animal. For example:
- In Amoeba (and other single-celled animals), the waste material carbon dioxide is removed by diffusion through the cell membrane, but nitrogenous wastes (like ammonia) and excess water are removed by the contractile vacuole.
- In earthworms, the tubular structures called nephridia are the excretory organs. In addition to nephridia, the moist skin of earthworms also acts as an excretory organ.
- In human beings, the microscopic thin tubules form a nephron, which functions as an excretory unit. About 1 million nephrons taken together form the excretory organ of human beings called the kidney.
This cow is excreting carbon dioxide, and urea (in the form of urine), as waste products. Carbon dioxide is excreted by the lungs of the cow (while ‘exhaling’) and urea is excreted by the kidneys in the form of urine.
Removal of Waste Products in Humans
All the time (even when we are asleep), our body produces waste substances. The major wastes produced by the human body are Carbon dioxide and Urea. Carbon dioxide is produced as waste by the oxidation of food during the process of respiration. Urea is produced as waste by the decomposition of unused proteins in the liver. Our body must get rid of these waste materials because their accumulation in the body is poisonous and harms us. Waste removal is called excretion.
The human body has different organs for the removal of wastes from the body. These are our lungs and kidneys. Our lungs excrete carbon dioxide. Our kidneys excrete urea. The kidneys are the main excretory organs of the human body. So, the main excretory system in human beings involves the kidneys. We will first describe how lungs excrete carbon dioxide and then study the main excretory system of the human body.
The lungs remove respiratory waste carbon dioxide. This happens as follows: Carbon dioxide is produced as a waste product in the body by the oxidation of food during respiration. This carbon dioxide enters from the body tissues into the bloodstream by diffusion. Blood carries this carbon dioxide to the lungs. When we breathe out, then the lungs excrete carbon dioxide which goes into the air through the nostrils. Thus, our lungs act as the excretory organs for removing the waste product carbon dioxide from the body.
Human Excretion: Excretory System, Kidneys
The excretory system of human beings collects the liquid wastes of the body and helps it get rid of them. The excretory system of human beings consists of the following main organs: Two kidneys, Two ureters, Bladder and Urethra. The kidneys are bean-shaped organs towards the back of our body just above the waist. Every person has two kidneys. The blood in our body is constantly passing through our kidneys. The renal artery (or kidney artery) brings in dirty blood (containing waste substances) into the kidneys.
The function of the kidneys is to remove the poisonous substance urea, other waste salts, and excess water from the blood and excrete them in the form of a yellowish liquid called urine. Thus, kidneys clean our blood by filtering it to remove unwanted substances present in it. The cleaned blood is carried away from the kidneys by the renal vein (or kidney vein). The ureters (or excretory tubes), one from each kidney, opens into the urinary bladder. Ureters are the tubes that carry urine from the kidneys to the bladder. Urine is stored in the bladder. The bladder is a bag that stores urine till the time we go to the toilet. The urethra is a tube. The urine collected in the bladder is passed out from the body through the urethra. We will now describe a kidney in detail.
The figure shows a kidney that has been cut open to show the inner structure. Each kidney is made up of a large number of excretory units called nephrons (We have shown only one nephron to make things simple). The nephron has a cup-shaped bag at its upper end which is called Bowman’s capsule. The lower, end of Bowman’s capsule is tube-shaped and it is called a tubule. The Bowman’s capsule and the tubule taken together make a nephron.
One end of the tubule is connected to the Bowman’s capsule and its other end is connected to a urine-collecting duct of the kidney. The Bowman’s capsule contains a bundle of blood capillaries which is called glomerulus (plural glomeruli). One end of the glomerulus is attached to the renal artery which brings the dirty blood containing the urea waste into it. The other end of the glomerulus comes out of Bowman’s capsule as a blood capillary, surrounds the tubule of the nephron, and finally joins a renal vein (putting urea-free clean blood into it).
The function of the glomerulus is to filter the blood passing through it. Only the small molecules of substances present in blood like glucose, amino acids, salts, urea, water, etc., pass through the glomerulus and collect as filtrate in the Bowman’s capsule. Large molecules like proteins and blood cells cannot pass out through the glomerulus capillaries and hence remain behind in the blood. The function of the tubule of the nephron is to allow the selective reabsorption of useful substances like glucose, amino acids, salts, and water into the blood capillaries (which surround it). But the waste material like urea remains behind in the tubule. It does not get reabsorbed into blood capillaries. We will now describe the working of the excretory system of humans.
In order to understand the working of the excretory system of humans, we will use a highly magnified diagram of a nephron shown in Figure. The dirty blood containing waste like urea (brought by the renal artery) enters the glomerulus. The glomerulus filters this blood. During filtration, the substances like glucose, amino acids, salts, water, urea, etc., present in the blood pass into Bowman’s capsule and then enter the tubule of the nephron. When the filtrate containing useful substances as well as the waste substances passes through the tubule, then the useful substances like all glucose, all amino acids, most salts, and most water, etc., are reabsorbed into the blood through blood capillaries surrounding the tubule.
Only the waste substances urea, some unwanted salts, and excess water remain behind in the tubule. The liquid left behind in the tubule of the nephron is urine. The nephron carries this urine into the collecting duct of the kidney from where it is carried to the ureter. From the ureter, urine passes into the urinary bladder. Urine is stored in the bladder for some time and ultimately passed out of the body through the urethra. Please note that human urine contains water, some salts, and nitrogenous substances, most of which is urea (and some uric acid).
Renal Failure (Kidney Failure) and the Technology for Survival
Sometimes, a person’s kidneys may stop working. An infection in the kidneys, an injury to the kidneys, very high blood pressure, very high blood sugar, or restricted blood flow to the kidneys, can damage the kidneys due to which they stop working. Complete failure of the kidneys allows the urea and other waste products to build up in the blood. Even the amount of water in the body is not regulated. This will cause death if the patient is not given immediate treatment.
The best long-term solution for kidney failure is a kidney transplant. The damaged kidney is removed and a matching kidney donated by a healthy person is transplanted in its place by a surgical operation. If a kidney transplant is not possible due to some reasons, then the patient with kidney failure is treated periodically on a kidney machine (by a procedure called dialysis). This is because a kidney machine can do the work of damaged kidneys. The kidney machine is sometimes called an ‘artificial kidney’. An artificial kidney is a device to remove nitrogenous waste products from the blood through dialysis.
A kidney is being prepared for a transplant operation.
This woman having damaged kidneys has been put on a kidney dialysis machine to filter her blood and remove urea.
How Does a Dialysis Machine Work?
The blood of a person having kidney failure can be cleaned regularly by using a kidney machine (or dialysis machine). The procedure used for cleaning the blood of a person by separating the waste substance (urea) from it is called dialysis. The principle of dialysis is explained below.
The blood from an artery in the patient’s arm is made to flow into the dialyzer of a dialysis machine made of long tubes of selectively permeable membrane (like cellulose) which are coiled in a tank containing dialyzing solution. The dialyzing solution contains water, glucose, and salts in similar concentrations to those in normal blood. As the patient’s blood passes through the dialyzing solution, most of the wastes like urea present in it pass through the selectively permeable cellulose tubes into the dialyzing solution. The clean blood is pumped back into a vein of the patient’s arm.