The process of digestion and metabolism helps us absorb nutrients and generate energy in our bodies. These chemical reactions and processes lead to the production of toxic substances. These toxic substances, if kept inside the body for a long time, will start affecting it. Hence, they need to be thrown out regularly. The process of excretion makes sure that the body gets rid of the toxic substances from time to time.
There are various modes of excretion. Depending upon the morphology of the organism, the habitat where it lives, or the kind of waste products generated, excretion takes place in various ways by various means.
Excretion can be carried out by lungs, skin, kidneys, etc.
As for fishes, they have abundant availability of water, hence the waste generated in fishes is ammonia, which is highly soluble in water. Humans generate urea as a waste product which is fairly soluble in water. That is also why we are suggested to drink plenty of water, so that elimination of waste from our body takes place smoothly. In the case of birds, they hardly have the availability of water, and the waste generated in their bodies is uric acid, which requires less water to keep the process of excretion going.
A dedicated system of organs that removes waste products from the human body is called the human excretory system. The major waste product generated in the human body is urea. Along with which some other toxins are also generated. Urea is eliminated by kidneys by the process of urination and solid wastes are removed from the body by the intestines.
The main organs of the human excretory system are-
These organs work together to remove nitrogenous waste- urea from our bodies.
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Kidneys are a pair of bean-shaped organs primarily responsible for the production of urine. Kidneys collect waste from our bloodstream and turn that waste into a simpler form (urine) which is then eliminated from the body. Kidneys are present in the abdomen, just below the diaphragm, and are protected by the ribs. Each of an adult kidney weighs around 120g to 170 g and are 10-12 cm long and 5-7 cm wide and roughly the size of a fist.
Kidneys are made up of a number of subunits called nephrons.
The kidney is divided into two major structures-
a) Outer renal cortex
The outer part of the kidneys which consist of the glomerulus and convoluted tubules from the outer cortex. The outer cortex is surrounded by the renal capsule. The renal cortex provides the space for renal artery and veins and glomerular capillaries. Apart from providing a protective layer it also manufactures hormone Erythropoietin, which is necessary for the synthesis of new blood cells.
b) Inner renal medulla
The inner smooth part of the kidney which consists of the Loop of Henle and renal pyramids forms the Renal Medulla.
Nephrons are the functional units of the kidneys. Each of our kidneys has as many as 1 million nephrons in each human kidney. The basic functions of nephrons are to carry out secretion as well as the process of excretion. Each nephron is made up of a Malpighian body which has two parts-
Glomerulus- Glomerulus is a mass of capillaries which absorbs protein from the blood that travels through the Malpighian Body.
Renal Tubule- Renal Tubule is composed of a Proximal convoluted tubule which lies in the cortex and absorbs water, sodium, and glucose back into the blood, a distal convoluted tubule which reabsorbs sodium into the blood and absorbs potassium and acid from blood and Loop of Henle.
Loop of Henle- The long, U shaped tubule which is primarily involved in the recovery of water and sodium chloride from urine is Loop of Henle. It is ‘U’ shaped with an ascending and descending limb and it continues as the distal convoluted tubule. The Loop of Henle produces very concentrated urine. Therefore, desert animals have a highly efficient Loop of Henle to carry out excretion with very less amount of water. The Loop of Henle lies in the medulla region.
Nephrons are of two types, namely-
Cortical Nephrons- The nephrons which are at a higher position in the cortex with a short Loop of Henle which does not penetrate into the medulla are called Cortical Nephrons.
Juxtamedullary Nephrons- Juxtamedullary nephrons have a longer Loop of Henle which runs deeper into the Renal Medulla.
Each kidney is connected to the renal pelvis by a thin and muscular tube. This tube is called the Ureter. The function of the ureter is to propel urine from the kidneys to the urinary bladder from time to time.
Urinary bladder is the organ that controls the passage of urine. It is a muscular sac-like structure present above the pubic bone that stores urine. The average capacity of a human bladder is 400-600 ml. It holds urine until its capacity and expels it by the process of micturition when it is full. The bladder is lined by a muscular tissue, which squeezes during micturition allowing the urine to flow out.
The urethra is a tube made up of fibro-muscular tissue that arises from the lower opening of the bladder and extends through urogenital and pelvic diaphragms and opens out of the body through an external urethral orifice. It helps expel urine from the body. In males it also the tubes which carry sperms. The Urethra is guarded by the sphincter which relaxes during urination.
The main function of the human excretory system is to eliminate wastes from the body which are generated as the bi-products of metabolism. The excretory system works in order to maintain homeostasis. Homeostasis is maintaining an equilibrium in the body by keeping the temperature and fluid balance stable. Apart from the metabolic wastes, there are other chemical and hormonal wastes too which are generated in the body and are excreted by various processes. The urinary system, however, filters the blood of nitrogenous waste- urea, salts, and excess water and excrete them out of the body in the form of urination. Kidneys also help in reabsorption of useful substances.
The filtration of blood occurs in the nephrons in kidneys. Millions of nephrons work together to absorb minerals like glucose, amino acid, salts and let the pure blood flow to other parts of the body. Urea and excess water are converted into urine which is passed to the bladder through the ureters and expelled from the body through the urethra. The Urinary bladder holds the urine for some time, till it's full and receives signals from the brain. Upon receiving the signal, the sphincters at the opening of the bladder relax and urine is expelled.
The urine in nephrons is formed in three steps
The first step of urine formation occurs in the glomerulus of the kidneys. The blood enters the glomerular capillaries by afferent arteriole and leaves by an efferent arteriole. In this process excess water, ions, glucose, and waste products are eliminated from the blood into the urine collection which is then eliminated from the body.
The glomerular filtrate consists of water, glucose, ions along with urea. Therefore, these necessary substances need to be reabsorbed into the body. It happens by tubular reabsorption.
The rate at which glomerulus filters blood and produces a filtrate is called the glomerular filtration rate.
It is the second step of urine formation and is a very important step, as it makes sure that essential substances and ions are excreted from our bodies. It also ensures less water loss. There are two types of transport taking place during tubular reabsorption.
1. Active transport- Glucose and Ions are absorbed by active transport.
2. Passive transport- Water is absorbed by passive transport.
Various tubules that carry out the process of reabsorption are as follows.
a. Proximal Convoluted Tubule (PCT)- The PCT reabsorbs ion, water, and nutrients from the filtrate. It also removes toxins from the filtrate thus maintaining the pH of the filtrate.
b. Ascending loop of Henle- Allows the reabsorption of sodium and chloride ions from the filtrate.
c. Descending Loop of Henle- Allows the reabsorption of water,
d. Distal Tubule- Reabsorbs selective ions from the filtrate including sodium chloride ions.
e. Collecting duct- Connect the nephrons to the minor calyx or renal pelvis and reabsorbs solutes and water from the filtrate.
Along with the formation of urine, the waste ions like hydrogen and potassium, and ammonia are secreted out which are then mixed with the urine and expelled from the body. This is called secretion.
After a series of these processes, the bladder gets filled with urine. Upon signaling from the brain, the bladder relaxes and releases urine from the body by the process of micturition or urination.
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Q1. What are the functions of the Human Excretory System?
The main functions of the human excretory system are eliminating waste products formed as a result of metabolic processes, prevention of waste accumulation in the body, and maintaining homeostasis inside the body so that all the organs can work smoothly.
Q2. What are the nitrogenous wastes expelled by living organisms?
Aamnonotelics, most fishes excrete nitrogenous waste in the form of Ammonia. Ureotelic, mostly mammals excrete urea and Uricotelic like birds excrete Uric acid. Different organisms excrete different forms of nitrogenous wastes, depending upon factors like habitat, surrounding, morphology, etc.
Q3. Name the various steps involved in urine formation?
Various steps involved in urine formation are as follows: