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Mechanism of Urine Formation

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  • Last Updated : 23 Jul, 2022
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In many metabolic reactions, the body produces a variety of hazardous compounds. Among other things, it contains uric acid, carbon dioxide, water, ions, and ammonia. In higher animals, there are three main categories of nitrogenous wastes: urea, uric acid, and ammonia. One of the most hazardous nitrogenous wastes is ammonia among these animals known as ammonotelic are those that discharge NH4 as nitrogenous waste, including the majority of aquatic amphibians and bony fish. Diffusion allows for the excretion of ammonia. Urea is a nitrogenous waste that is excreted by ureotelic organisms. Mammalian, Examples include amphibians and other ureotelic creatures. Those creatures known as uricotelic organisms excrete nitrogenous waste which is called uric acid including birds and reptiles.

Excretion

Excretion is the process through which an organism gets rid of waste substances. This is mostly done by the skin, kidneys, and lungs in vertebrates. In contrast, when a material is secreted, it may carry out specified functions after exiting the cell. All types of life require the process of excretion.

Formation of the urine

It involves three steps: filtration, reabsorption, and secretion

Urine Formation

 

Glomerular Filteration

Glomerular filtration starts whenever blood would enter the glomerulus by an afferent arteriole. In contrast to proteins leaving through the efferent arteriole, waste products and the water would enter the glomerulus and blood cells. The renal filters usually between 1100 to twelve hundred ml of blood per minute in the glomerulus Three layers of capillary blood pressure force the blood to separate the initial endothelium, which encircles the glomerular blood vessels, is a layer. The following Bowman’s capsule epithelium is the layer, and a basement membrane separates it from the

 The Bowman’s capsule’s epithelial cells, known as podocytes, are organized in a complex pattern to leave behind certain tiny openings called slit pores or filtering slits. The quantity of filtrate the kidneys generate each minute would be called filtration of the glomerulus.

Both beneficial and toxic compounds are carried in the blood that the afferent arterioles deliver to the glomerular capsule. The beneficial elements include glucose, amino acids, vitamins, hormones, electrolytes, and ions, whereas the detrimental elements include metabolic wastes including urea, uric acid, creatinine, and ions.
Efferent arterioles have a smaller diameter than afferent arterioles. The hydrostatic pressure that results from blood exiting the glomerulus is caused by this variation in artery diameter.

Blood is filtered as a result of the glomerular hydrostatic pressure forcing blood out of the glomerulus. In the glomerulus, a capillary hydrostatic pressure of around 7.3 kPa (55 mmHg) develops. However, this pressure is countered by the blood’s osmotic pressure, which is around 4 kPa (30 mmHg) and is mostly provided by plasma proteins, as well as the filtrate hydrostatic pressure, which is about 2 kPa (15 mmHg) in the glomerular capsule. As a result, the net filtration pressure is 55-(30 + 15) = 10 mmHg.

Reabsorption in Tubule

The process of absorbing essential molecules, such as glucose, amino acids, etc., or ions, such as sodium ions, etc., is known as tubular reabsorption. While some compounds are absorbed passively, others are aggressively absorbed. Sugar and Water are passively absorbed, whereas amino acids would be actively absorbed.

Useful components, such as some water, electrolytes, and organic nutrients like glucose, amino acids, vitamins, hormones, etc. are preferentially recycled back from the filtration back to the bloodstream in the proximal convoluted tubule as it travels to the renal tubules. While certain chemicals are actively moved, others undergo passive absorption. Osmosis allows a large fraction of water to be reabsorbed.
Only 60–70% of the filtrate makes it to the Henle loop. Only 15 to 20 percent of the initial filtration enters the distal convoluted tubule because a large portion of this, particularly water, sodium, and chloride, is taken up in the loop. The filtrate that enters the collecting ducts is really rather diluted because more electrolytes, particularly sodium, are reabsorbed at this location. The collecting ducts’ primary job is to reabsorb as much fluid as the body requires. Active transport helps nutrients like glucose, amino acids, and vitamins to be reabsorbed. Negatively charged ions are often reabsorbed through passive transport, although positively charged ions can also be reabsorbed through active transport. Osmosis is used to reabsorb water, whereas pinocytosis is used to reabsorb tiny proteins.

Tubular secretion

Urine Secretion

Secretion is the last stage in the production of urine. To keep the ionic and acidity equilibrium of the bodily fluids, potassium, hydrogen, and ammonia ions are released.

Proximal convoluted tubules

The role of tubules Simple cuboidal brush boundary epithelium in proximal convoluted tubules (PCT) offers a bigger space for reabsorption surfaces. Most of the electrolytes and water in PCT are reabsorbed. It helps to keep the body’s pH and ionic balance in check. by secreting hydrogen ions, potassium ions, and ammonium ions from fluids Into filtrate. To ensure that wastes like creatine or excessive Hydrogen ion or excessive Potassium ions are effectively released into the filtrate to be expelled, tubular secretion occurs first from the blood inside the peritubular capillaries towards the filtration in the renal tubules.
In exchange for the Na+ ions being reabsorbed, more Potassium ions are secreted in the tubules, which leads to the clinical situation known as hyperkalemia. Hydrogen ion (H+) production from the tubules is crucial for preserving a normal blood pH. Due to the brief time that substances like spend in the glomerulus, medications like penicillin and aspirin may not be completely removed from the blood. Such chemicals are eliminated via secretion into the filtration inside the convoluted tubules first from peritubular capillaries.

Henle’s Loop 

This mechanism is essential for preserving the fluid’s osmolarity. Reabsorption in the ascending limb is extremely little. It is electrolyte-permeable yet impervious to water. The vast bulk of the water is absorbed by the descending limb, The filtrate is concentrated. Nearly all substances cannot pass through the descending limb-electrolytes. As a result, Henle’s loop’s various components absorb differently.

Distal Convoluted Tubule (DCT) 

The DCT secretes potassium ions and hydrogen ions while absorbing H2o, Na ions, and HCO3 ions to maintain the fluid’s ionic balance.

Collecting Ducts

A lot of water is reabsorbed by the collecting duct. Urine concentration. Along with potassium ions, it also would secrete the ions of hydrogen ions. It maintains the pH and ionic equilibrium of the blood.

Conceptual Questions 

Question 1: Describe the differences between glomerular filtrate and urine in terms of composition?

Answer:

It can be extrapolated that 99 percent of the filtrate must be recycled back by the renal tubules when the quantity of the filtrate is compared to that of the urine produced each day, which is 180 liters and 1.5 liters, respectively. This process is known as reabsorption. Because they are rapidly recycled back in the filtrate, substances including amino acids, Na+, and glucose are not present in urine.

Question 2: What part does tubular secretion play in preserving the ionic and acid-base balance of bodily fluids?

Answer:

During the production of urine, the tubular cells release ammonia, Hydrogen ion, and Potassium ions into the filtrate. Urine generation requires tubular secretion, which also aids in preserving the ionic and acid-base equilibrium in bodily fluids. Ammonium, H+, and K+ are selectively secreted into the filtrate by the PCT to assist in this process. The DCT is also capable of secreting potassium, hydrogen, and ammonia ions with sufficient selectivity to maintain the pH balance of sodium and potassium in the blood. By releasing just H+ and K+ ions, another structure aids in preserving the pH and osmotic equilibrium of the blood.

Question 3: Why does the glomerular filtrate in the Henle loop become concentrated in the descending limbs and dilute in the ascending limbs?

Answer:

The Henle’s loop’s thin wall is sensitive to water but not to solutes. The higher osmolarity of the medulla interstitium, from which the limbs extend, causes exosmosis, which causes the tonicity tubular fluid to gradually lose water as it travels down the limb. The filtrate consequently has a tendency of becoming hypertonic to the plasma. The ascending limb of the Henle loop is impervious to water and permeable to the cations Potassium ions, Na+, and Chloride ions as well as urea to a lesser extent. Calcium, Magnesium, Chlorine, potassium, and Sodium are thereby reabsorbed in the thick ascending limb of the loop of Henle, resulting in the filtrate being less concentrated and hypotonic to blood plasma than in the descending limb.

Question 4: Why formation of urine is important?

Answer:

Various processes of life are carried out by our bodies. Each of these processes causes the body to produce a large amount of waste. These wastes must be eliminated by our body or they could become hazardous to the body. Kidneys excrete these wastes through the production of urine. The kidneys create urine by filtering the blood to remove waste.

Question 5: What would be the reason urine is more concentrated in the case of summer?

Answer:

Urine that is dark yellow implies that waste materials are excreted in lesser amounts of water and that the urine is concentrated. We perspire more in the summer to keep our body temperatures stable. As a result, our body loses more water. We have a higher risk of excreting yellowish urine in the heat if we don’t drink enough water.

Question 6: Different stages in case of formation of urine?

Answer:

  • Filtration of glomerulus: Glomerular filtration starts whenever blood would enter the glomerulus by an afferent arteriole. In contrast to proteins leaving through the efferent arteriole, waste products and the water would enter the glomerulus and blood cells.
  • Reabsorption of tubule: The process of absorbing essential molecules, such as glucose, amino acids, etc., or ions, such as sodium ions, etc., is known as tubular reabsorption. While some compounds are absorbed passively, others are aggressively absorbed. Sugar and Water are passively absorbed, whereas amino acids would be actively absorbed.
  • Tubular secretion: Urine Secretion: Secretion is the last stage in the production of urine. To keep the ionic and acidity equilibrium of the bodily fluids, potassium, hydrogen, and ammonia ions are released.

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