Urine Formation

Urine formation is one of the key types of methods of excretion observed in various animals including humans. Urine is a liquid or semi-solid metabolic by-product that contains toxic materials that the excretory organs withdraw from the circulatory fluids (blood) to expel from the body. In humans, the process of urine formation is carried out in the nephrons which are units of the kidneys. The mechanism of urine formation consists of mainly three processes- glomerular filtration, tubular reabsorption and tubular secretion, which takes place in different parts of the nephrons. Expelling urine from the body is important to maintain the homeostasis of the body. 

Key Takeaways - Urine, urine formation, the chemical composition of urine, human urinary system.

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What is Excretion?

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Excretion is a biological process in which metabolic waste products are eliminated from an organism. In both plants and animals during hundreds of metabolic activities, a lot of waste products including toxic substances are produced which are needed to be eliminated to maintain the body's homeostasis. In unicellular organisms, the excretory materials are secreted directly through the body surface. In the case of multicellular organisms, plant excretion usually takes place through a process called transpiration while in animals the waste products are excreted as urine, sweat, tears, faeces etc. especially in vertebrates the process is carried out by multiple organs such as lungs, kidney and skin.

Excretion

Ammonia, uric acid and urea are the main nitrogenous substances excreted by the different animals. Here, ammonia is the most toxic substance and requires a greater amount of water for excretion. Uric acid is the least toxic material here and needs a minimum amount of water for elimination. The animals such as bony fishes, aquatic amphibians and insects excrete ammonia. Thus, they are called ammonotelic organisms. The animals, like many terrestrial organisms, excrete urea and are thus called ureotelic organisms. Last but not the least, birds, reptiles, snails excrete uric acid as their waste material and they are named uricotelic organisms.

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What is Urine?

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Urine is a liquid or semi-solid metabolic by-product in several animals including humans. It is a waste product that includes toxic materials that the excretory organs withdraw from the circulatory fluids (blood) to expel from the body. The composition and the density of the urine may vary from organism to organism. For example, in the case of freshwater organisms usually excrete very dilute urine while marine organisms tend to secrete more concentrated urine as they have to battle extra water loss in the saline environment. Terrestrial animals also tend to expel high-density urine depending on their habitat due to differential retention of the water. In mammals including humans, the formation of urine takes place in the kidneys.

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Urine Formation in Different Organisms

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In vertebrates such as mammals’ urine formation usually starts in the specialized cells- nephrons of the kidneys by filtering the blood plasma. The fluid inside the nephron is essentially the same as the blood plasma, however, without the macromolecules such as proteins. As the fluid is passed through the nephron tube water and several other important components are reabsorbed into the bloodstream leaving a concentrated yellowish filtrate called urine.

In the case of other vertebrates such as reptiles, birds and terrestrial insects the excretory product is uric acid and the urine colour of birds and reptiles are whitish. Here, the suspension of the uric acid crystals is passed into the cloaca and mixed with faecal substances before being excreted. The terrestrial insects have been found to secrete solid urine. Animals like amphibians and some fishes expel solutions of urea; as their excretory organs do not absorb large amounts of water the urine remains very dilute.

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Structure of Human Excretory System

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The human excretion system is composed of a pair of kidneys, a urinary bladder and a urethra. The kidneys are bean-shaped structures, situated between the last thoracic and third lumbar vertebra in the abdominal cavity. There is a notch-like concave region towards the centre of the kidney called the hilum through which the ureter, nerves and blood vessels enter. Inside the kidney, there are two zones- the outer cortex and the inner medulla. The medulla is divided into conical masses which are called medullary pyramids. The cortex extends through the medullary pyramids as renal columns called the Columns of Bertini.

Each kidney is made of millions of tubular cells called nephrons which are the actual sites of urine formation. Each nephron has two parts- glomerulus and renal tubules.

The Glomerulus is a tuft of afferent renal arterioles. Blood from here is carried away by a bunch of efferent arterioles.

The renal tubule starts with a double-walled cuplike structure named Bowman’s capsule that encloses the glomerulus. Glomerulus along with the Bowman’s capsule is called Malpighian corpuscle. The renal tubule continues to form a highly convoluted network of tubules- proximal convoluted tubule (PCT), thereafter a hairpin shaped Henle’s loop with descending and ascending limb. The ascending limb continues as another coiled tubular structure called collecting duct, many of such ducts converge and open into the renal pelvis through medullary pyramids.

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Formation of Urine in Humans: The Mechanism

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The process of urine formation consists of mainly three processes- glomerular filtration, reabsorption and secretion, takes place in different parts of the nephrons.

Formation of Urine in Humans Mechanism

Glomerular Filtration: The process of glomerular filtration occurs in the glomerular capillaries. The filtration process leads to the formation of an ultrafiltrate. The blood goes into these capillaries with high pressure and gets filtered through the thin capillary walls. Here, everything else except the blood cells and important proteins is pushed into the capsular space of Bowman’s capsule. The glomerular filtration rate or GFR is 125ml/min or 180litres/day.

Tubular Reabsorption: During the process of glomerular filtration, all substances except some proteins and blood cells are pushed out. At the level of the proximal convoluted tubule(PCT), some of the substances are reabsorbed from the filtrate. These substances include NaCl, potassium, amino acids, glucose molecules, bicarbonates and 75% of water. Some of the substances are reabsorbed through passive absorption while some are actively transported while others are co-transported. Selective absorption is observed in the distal convoluted tubule (DST). The reabsorbed substances and water are then taken up by the peritubular capillaries to be returned to the blood.

Tubular Secretion: The peritubular capillaries actively secrete substances like H+ and K+ ions. Whenever an excess level of K+ ion is secreted into the filtrate, Na+ ions are actively reabsorbed. This helps to maintain Na-K balance in the system. Some drugs are not filtered in the glomerulus but are actively secreted into the filtrate during the tubular secretion phase.

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Composition of Urine

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Urine is a pale-yellow waste product, produced in kidneys having a slightly ammoniacal smell. It is a clear liquid with a pH of 4-8. Chemically, urine is mainly composed of urea, sodium chloride, potassium ions, creatinine, ammonia products, several proteins and other metabolic substances. Certain abnormalities can change the composition of urine. These abnormalities are Haematuria (blood in urine), Pyuria (presence of pus cells in urine), Glycosuria (presence of glucose molecules in urine), Proteinuria (presence of proteins in the urine) etc.

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Things to Remember

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• Excretion is a biological process of eliminating metabolic waste products from the body of an organism.
• Urine is a liquid or semi-solid metabolic by-product in several animals including humans. It contains waste products that include toxic materials that the excretory organs withdraw from the circulatory fluids to expel from the body.
• Chemically, urine is composed of urea, sodium chloride, potassium ions, creatinine, ammonia products, several proteins and other metabolic substances.
• The process of urine formation consists of mainly three processes- glomerular filtration, reabsorption and secretion, taking part in different parts of the nephrons.
• Glomerular filtration is the process that leads to ultrafiltration. It takes place in the glomerular capillaries. Here, all except the blood cells and important proteins are pushed into the capsular space of Bowman’s capsule.
• At the level of the proximal convoluted tubule (PCT), some of the substances like NaCl, potassium, amino acids, glucose molecules, bicarbonates and 75% of water are reabsorbed from the filtrate. This is known as tubular reabsorption.
• To maintain Na-K balance the peritubular capillaries actively secrete substances like H+ and K+ ions. Whenever an excess level of K+ ion is secreted into the filtrate, Na+ ions are actively reabsorbed. This is the tubular secretion process.
• After its formation, the urine comes to the urinary bladder where it gets released through the urethra after some time.