URINE FORMATION
Introduction :-
URINE FORMATION – The production of urine serves to cleanse the blood. Normally, the kidneys receive 1,300 mL of blood, or 26% of cardiac output. Urine is the product of the kidneys excreting waste materials and water from the blood. The average amount of urine produced in a 24-hour in a healthy adult ranges from 600 to 2500 milliliters. Usually, it is determined by the subject’s (a) water intake, (b) food, (c) environmental temperature, (d) mental state, and (e) physical circumstances. Urine production during sleep is about half that of exercise.
The skin and kidneys have a unique interaction in which these two organs play a significant role in the excretion of water. Consequently, there is an inverse relationship between sweat and urine production.
Steps of Urine Formation :-
The Bowman capsule is where the plasma is filtered out of the blood as it goes through glomerular capillaries. We refer to this procedure as glomerular filtration.
The Bowman capsule filtrate enters the nephron through the tubular section. Both in quantity and quality, the filtrate varies during its passage through the tubule. The tubules reabsorb a large number of desired substances, including glucose, amino acids, water, and electrolytes. We use the term this process as tubular reabsorption.
Additionally, peritubular blood veins leak certain undesirable chemicals into the tubule. This procedure is known as excretion or tubular secretion.
Thus, three steps are involved in producing of urine:
1. Glomerular filtration
2. Tubular reabsorption
3. Tubular secretion.
Filtration is one of the three processes that the glomerulus does. The tubular part of the nephron is responsible for reabsorption and secretion.
1. Glomerular filtration :-
The kidneys’ initial phase in the process of producing urine is called glomerular filtration. It takes place in the glomerulus and Bowman’s capsule that make up the renal corpuscle. The process that occurs when blood is filtered by a filtration membrane as it navigates glomerular capillaries is known as glomerular filtration. The filtration membrane’s structure is ideal for filtering.
RBF normal ranges from 1.2 to 1.3 L/min, or 20–25% of what the heart pumps at rest.
Whenever blood passes via the glomeruli, approximately ten percent of the RBF filters off into Bowman’s capsule.
The filtrate and plasma remain identical in the following respects:
(1) pH, osmolality;
(2) electrical conductivity;
(3) concentration of smaller organic molecules, such as glucose, urea, and creatinine, and electrolyte concentrations.
1. Glomerular filtration Rate :-
The total amount of glomerular filtrate produced by all of the the renal cells in both kidneys per minute is referred to as GFR 125 mL/min = 170-180 L/day is the normal value. Women’s GFR values are 10% lower than men’s. The kidneys filter the following volume of fluid in a single day at a rate of 125 mL/min:
2. Mechanism of Glomerular filtration :-
The same mechanism that controls filtration through all other bodily capillaries also governs filtration through glomerular capillaries. namely:
1. the difference in hydrostatic pressure along the capillary wall
2. the capillary wall’s osmotic pressure gradient
3. the capillaries’ permeability; and
4. the capillary bed’s dimensions.
3. Filtration Coefficient :-
The filtration coefficient is GFR presented as net filtration pressure. In terms of effective filtration pressure per millibar Hg, it is the GFR. Like if the total filtration pressure is 20 mm Hg and the GFR is 125 ml/min. The kidney’s glomerular filtration rate (GFR), an important indication of renal function, is strongly dependent on the filtration parameter. A lower coefficient may suggest a decline in filtration effectiveness, which could be brought on by a number of kidney or filtration membrane disorders, whereas a higher coefficient often implies a larger filtering capacity.
Factors Affecting GFR :-
Glomerular filtration rate (GFR) is a measure of how quickly the kidneys filter blood and reflects renal function. Several factors can affect GFR, which can be broadly divided into physiological, pathological, and pharmacological factors. The breakdown of the main factors that control or affect GFR is as follows:
1. Renal Blood Flow (RBF) and Renal Perfusion Pressure : It is the component that glomerular filtration depends on the greatest. Renal blood flow is closely associated with GFR. Both kidneys have an average blood flow rate of 1,300 mL/min. The blood flow inside the kidneys is controlled by renal autoregulation. For more information, see the chapter that comes before this one. Renal perfusion pressure: The pressure gradient that controls blood flow via the kidney. It is affected by both systemic blood pressure and the vascular resistance of the renal artery.
2. Surface Area and Permeability of the Filtration Membrane : The capillary membrane’s surface area and GFR are intimately correlated. The surface area for filtration is decreased when the glomerular capillary membrane is impacted, as in certain renal disorders. As a result, the GFR drops.
The permeability of the glomerular capillary membrane directly relates to GFR. Numerous anomalous circumstances arise, including low oxygen levels, inadequate blood flow, and the existence of harmful agents. The capillary membrane becomes more permeable. Under these conditions, plasma proteins are likewise filtered out of urine.
The spaces between the glomerular capillaries are home to glomerular mesangial cells. These cells contract, which lowers the capillary surface area and lowers GFR.
3. Glomerular Capillary Pressure: There is a clear correlation between glomerular filtration rate and glomerular capillary pressure. 60 mmHg is the typical glomerular capillary pressure. GFR rises with glomerular capillary pressure. Capillary pressure is influenced by arterial blood pressure and renal blood flow.
4. Constriction of Efferent and Afferent Arteriole : When the afferent arteriole narrows, blood flow to the glomerular capillaries is reduced, resulting in a reduced GFR. Due to blood stasis in the capillaries, the GFR first rises when the efferent arteriole narrows.Then, once all substances have been filtered out of this blood no further filtration takes place. This is because no new blood can reach the glomerulus for filtration since the efferent arteriole narrows, preventing blood from leaving the glomerulus.
5. Colloidal Osmotic Pressure : Oncotic Pressure Glomerular filtration rate is inversely proportional to the oncotic pressure exerted by plasma proteins in the glomerular capillary blood.
Normal oncotic pressure is 25 mmHg. When oncotic pressure increases, as in dehydration or elevated plasma protein levels, the GFR decreases. When oncotic pressure decreases, as in hypoproteinemia, the GFR increases.
6. Hydrostatic Pressure in Bowman Capsule : Hydrostatic pressure in Bowman’s capsule GFR is inversely proportional to it. It is normally 15,
mmHg. GFR is lowered by a rise in hydrostatic pressure within Bowman’s capsule. Hydrostatic pressure in Bowman’s capsule increases under conditions such as urethral obstruction and renal edema under the renal capsule.
7. Systemic Arterial Pressure : Systemic arterial pressure As long as the mean arterial pressure is between 60 and 180 mmHg, renal blood flow and GFR are not affected. Autoregulatory mechanisms. Pressure fluctuations above 180 mmHg or below 60 mmHg affect renal blood flow and GFR accordingly, because beyond this range the autoregulatory mechanisms fail.
8. Methods of autoregulatory- Myogenic mechanism: This mechanism modifies the afferent arteriole’s diameter in response to variations in blood pressure. For instance, a rise in blood pressure narrows the afferent arteriole, maintaining a steady GFR.
Tubular-glomerular feedback: The distal tubule’s macula densa cells detect variations in sodium chloride content and modify the GFR correspondingly. When sodium chloride concentrations are high, the afferent arteriole constricts, lowering GFR; when concentrations are low, the arteriole dilates, raising GFR.
9. Pathological conditions-
Diabetes: can cause changes in the glomerular membrane and increased filtration pressure.
Hypertension: Prolonged elevated blood pressure can damage the glomeruli and reduce GFR.
Kidney disease: Conditions such as glomerulonephritis, polycystic kidney disease, and chronic kidney disease can impair kidney function and reduce GFR.
2. Tubular reabsorption :-
The process that moves things like water and other materials from the renal tubule into the circulation is called tubular reabsorption. Two types of alterations take place as the glomerular filtrate passes through the tubular section of the nephron: quantitative and qualitative. The tubular epithelial cells reabsorb large amounts of chemicals, including electrolytes and more than 99% of water. The materials that have been reabsorbed enter the renal medulla’s interstitial fluid. The chemicals then enter the bloodstream through the capillaries that encircle the tubule. Tubular reabsorption is the term used to describe the entire process of chemicals from the glomerular filtrate being absorbed back into the circulation.
1. Mechanisms of Reabsorption :-
Active Transport: Active reabsorption is the movement of molecules against their electrochemical (uphill) gradient. This requires the release of energy obtained from ATP. Substances that are actively reabsorbed from the renal tubule are sodium, calcium, potassium, phosphate, sulfate, bicarbonate, glucose, amino acids, ascorbic acid, uric acid and ketone bodies.
Passive Transport: Passive reabsorption is the movement of molecules along their electrochemical (downhill) gradient. This process does not require energy. Substances that are passively absorbed are chloride, urea and water.
Cotransport and counter transport: It involves the simultaneous transport of two substances in the same direction (cotransport) or in opposite directions (counter transport). For example, the sodium-glucose cotransport mechanism in the PCT.
2. Sites of Reabsorption :-
Proximal convoluted tubule (PCT): Most of the reabsorption occurs here. Approximately 65-70% of the water and solutes filtered by the glomerulus are reabsorbed in the PCT.
Loop of Henle: Here, there is additional reabsorption of water and dissolved materials, particularly in the ascending and descending limbs (sodium and chloride reabsorption and water reabsorption).
Distal convoluted tubule (DCT): Influenced by hormonal regulation, involved in fine-tuning electrolyte and fluid balance.
Collecting duct: Final regulation of water and electrolyte balance. hormones like antidiuretic hormone (ADH) and aldosterone are also involved in regulation.
3. Regulation of Tubular Reabsorption :-
1. Glomerular tubular balance : Glomerular tubular balance is the balance between filtration and reabsorption of solutes and water in the kidney. As the GFR increases, the tubular load of solutes and water in the proximal tubule increases. This is followed by increased reabsorption of solutes and water. This mechanism facilitates the constant reabsorption of solutes from the tubule, particularly sodium and water.
Mechanism of Glomerular Canalicular Equilibrium Glomerular canalicular equilibrium results from the osmotic pressure of the peritubular capillaries. As the GFR increases, more plasma proteins accumulate in the glomerulus. As a result, the osmotic pressure in the blood increases until it reaches the efferent arteriole and the peritubular capillaries. The increased osmotic pressure in the peritubular capillaries increases the reabsorption of sodium and water from the tubule into the capillary blood.
2. Hormones:
Aldosterone – Increases sodium reabsorption (and consequently water reabsorption) in the DCT and collecting duct.
Antidiuretic hormone (ADH)-induced enhanced water reabsorption in the collecting duct is the reason urine is concentrated.
3. Nervous control: Activity of the sympathetic nervous system can affect reabsorption, especially during times of stress or hypovolemia.
3. Tubular secretion :-
Substances secreted during tubular secretion are transported from the blood into the renal tubules. It is also called tubular excretion. In addition to reabsorption from the tubules, some substances are secreted from the peritubular capillaries into the lumen by the tubular epithelial cells.
Substances excreted - :-
1. Ions:
Hydrogen ion (H⁺): Excreted into tubular fluid, plays an important role in acid-base balance by allowing reabsorption of bicarbonate and excretion of excess acid.
Potassium ion (K⁺): Regulated by aldosterone, potassium is secreted into tubular fluid, especially into the DCT and collecting duct, to maintain electrolyte balance.
Ammonium ion (NH₄⁺): Produced and secreted by glutamine metabolism in renal cells, supports acid excretion.
2. Organic acids and bases:
Creatinine: A waste product of muscle metabolism that is not reabsorbed and is often excreted into the renal tubules.
Uric acid: An end product of purine metabolism that is partially excreted by the renal tubules.
Drugs and metabolites: A variety of drugs and their metabolites are actively secreted to facilitate their elimination from the body.
In conclusion, tubular secretion plays a critical role in controlling body chemistry and waste product excretion. I It ensures that excess ions, metabolic by-products, and toxins are removed from the bloodstream, maintaining a balanced internal environment.