Mechanism of Urine Formation

Urine formation is a vital process by the kidneys to maintain the body's fluid and electrolyte balance, eliminate waste products, and regulate blood pressure. The kidneys filter blood, remove toxins, and conserve essential substances, producing urine as a means of excretion. This process occurs through three main stages: filtration, reabsorption, and secretion.

1.0Introduction

The mechanism of urine formation involves three steps or processes : 

• Ultrafiltration or Glomerular filtration 
• Selective tubular reabsorption 
• Tubular secretion 

2.0Mechanism of Urine Formation 

Ultrafiltration or Glomerular Filtration

• The first step in urine formation is blood filtration, carried out by the glomerulus. This process is known as glomerular filtration.  
• This process occurs in the Malpighian corpuscle of the nephron.  
• The glomerular capsular membrane (Filtration membrane), through which blood is filtered, consists of three layers. 

(i) The endothelium of glomerular blood vessels. 

(ii) The epithelium of Bowman's capsule. 

(iii) A basement membrane between these two layers. 

• The epithelial cells of Bowman's capsule, called podocytes, are arranged intricately to leave some minute spaces called filtration slits or slit pores.  
• The blood is filtered so finely through these membranes that almost all the plasma constituents except the proteins pass into the lumen of the Bowman's capsule. 
• Therefore, it is considered a process of ultrafiltration.  
• The plasma fluid that filters out from glomerular capillaries is called glomerular filtrate. 
• It is protein-less plasma. About 20% of plasma fluid filters out into Bowman's capsule.

• The amount of filtration the kidneys form per minute is called glomerular filtration rate (GFR). 
• GFR in a healthy individual is approximately 125 ml/min, i.e. 180 litres per day. 
• On average, 1100-1200 ml of blood is filtered by kidneys per minute (Renal blood flow), which constitutes roughly 20-25% of the blood pumped by each ventricle of the heart in a minute (cardiac output), and of this blood is about 650 ml is the blood plasma (55%). 
• This 650 ml is called Renal plasma flow (RPF). About 20% of the blood plasma is filtered by all nephrons of both kidneys in a minute. It is 125 ml, called glomerular filtration rate (GFR). 
• Filtration fraction = GFR 125 ml /min 1 = = RPF 650ml /min 

• The effective filtration pressure that causes ultrafiltration is determined by three pressures: 

(1) Glomerular hydrostatic pressure (GHP) 

(2) Blood colloid osmotic pressure (BCOP) 

(3) Capsular hydrostatic pressure (CHP) 

• Glomerular hydrostatic pressure is the blood pressure in glomerular capillaries, due to the difference in diameter of afferent and efferent arteriole. 
• It is the main driving force for filtration. (It is 60 to 75 mm Hg) 
• Colloid osmotic pressure is the osmotic pressure created in the blood of glomerular capillaries due to plasma proteins. 
• It resists the filtration of fluid from the capillaries. (It is 30 to 32 mm Hg) 
• Capsular hydrostatic pressure is the pressure caused by fluid (filtrate) that reaches into Bowman's capsule, which resists filtration. (It is about 10 to 18 mm Hg).

Net Filtration Pressure [NFP]

NFP = Glomerular hydrostatic pressure – [Blood colloid osmotic pressure + capsular hydrostatic pressure] NFP = GHP – [BCOP + CHP] = (75 or 60) – [32 + 18] mm of Hg NFP = 10 to 25 mm Hg.

Selective Tubular Reabsorption

• A comparison of the volume of the filtrate formed per day (180 litres per day) with that of the urine released (1.5 litres) suggests that nearly 99% of the filtrate has to be reabsorbed by the renal tubules. This process is called reabsorption. 
• The tubular epithelial cells in different nephron segments perform this reabsorption by active and passive mechanisms. Substances like glucose, amino acids, Na+, etc., in the filtrate, are reabsorbed actively, whereas the nitrogenous wastes are absorbed by passive transport. 
• Reabsorption of water also occurs passively in the initial segments of the nephron.

Function of the Tubules 

1. Proximal convoluted tubule

• The Proximal Convoluted Tubule (PCT) is lined by simple cuboidal brush border epithelium, increasing the reabsorption surface area. This segment resorbs nearly all essential nutrients and 70-80% of electrolytes and water.  
• Active transport in PCT completely reabsorbed glucose, Amino, and Fatty acids.  
• Water & Cl– are reabsorbed passively. Reabsorption in this segment is maximum.

2. Henle's loop

• Reabsorption in this segment is minimum. 
• Reabsorption is minimum in its ascending limb. 
• However, this region plays a significant role in maintaining high osmolarity of medullary interstitial fluid. 
• The descending limb of the loop of Henle is permeable to water but almost impermeable to electrolytes, so water is reabsorbed passively here. This limb concentrates the filtrate as it moves down. 
• The ascending limb loop of Henle is impermeable to water but allows the transport of electrolytes actively or passively. Therefore, as the concentrated filtrate passes upward, it gets diluted due to the passage of electrolytes into the medullary fluid.

3. Distal convoluted tubule (DCT)

• This segment involves conditional reabsorption of Na+ and water. In the presence of the Aldosterone hormone, salts (Na+) are reabsorbed actively, and water is reabsorbed passively due to the Antidiuretic hormone (ADH). DCT is also capable of reabsorbing HCO3 –. 

4. Collecting duct

• This long duct extends from the cortex of the kidney to the inner parts of the medulla.  
• A large amount of water could be reabsorbed from this region to produce concentrated urine (in the presence of ADH).  
• This segment allows the passage of small amounts of urea into the medullary interstitium to keep up the osmolarity.  
• The distal part of the collecting duct is permeable to urea, so a small amount of urea is also reabsorbed from the filtrate, adding to the osmolarity of the medullary interstitium. 

Tubular Secretion

• During urine formation, the epithelial cells of renal tubules (tubular cells) secrete excretory substances like H+, K+ and ammonia (from the blood of peritubular capillary into the filtrate). 
• This process is tubular secretion, an active process that occurs in PCT, DCT, and collecting ducts.  
• PCT also helps maintain the pH and ionic balance of body fluids by selectively secreting Hydrogen ions, Ammonia, Potassium ions, creatinine, uric acid, drugs, hippuric acid, pigments, etc. into the filtrate and absorbing HCO3 from it.  
• DCT is also capable of reabsorbing HCO3 and selectively secreting Hydrogen ions, Potassium ions, and NH3 to maintain the pH and sodium-potassium balance in the blood. The collecting duct plays a role in maintaining the pH and ionic balance of blood by selectively secreting H+ and K+ ions.  
• Tubular secretion is also an important step in urine formation, as it helps maintain body fluids' ionic and acid-base balance.  
• Tubular secretion is the only method of urine formation in organisms with glomerular kidneys, such as marine teleost fish and desert amphibians.

3.0Chemical Composition of Urine

• Water is 95%, urea is 2.7%, salt is 2%, and other materials (such as drugs, hippuric acid, uric acid, Vitamin C, Dyes, etc.) are 0.3%.  
• An adult human excretes 1-1.5 liters of urine daily.  
• Urine is pale yellow due to urochrome pigment. On average, 25-30 gm of urea is excreted daily. Normal urine is slightly acidic (pH = 6.0).  
• The volume of urine produced daily will increase on a cold day due to decreased ADH secretion.