15. Excretion and Osmoregulation part 03 - Urine : formation,Ultrafiltration,Concentration and Composition of Urine

 

15. Excretion and Osmoregulation part 03 - Urine : formation,Ultrafiltration,Concentration and Composition of Urine

Urine formation :

• Process of urine formation is completed in three successive steps as-

1.  Ultrafiltration / Glomerular filtration
2. Selective reabsorption
3. Tubular secretion / Augmentation.

Ultrafiltration / Glomerular filtration :

• Diameter of afferent arteriole is greater than efferent arteriole. 
• Diameter of capillaries is still smaller than both arterioles. 
  
• Due to such difference in diameter, blood flows with greater pressure through glomerulus. This is called as glomerular hydrostatic pressure (GHP).
• Normally GHP is about 55 mm Hg. 
• This pressure is opposed by osmotic pressure of blood (normally, about 30 mm Hg) and capsular pressure (normally, about 15 mm Hg). 
• Hence net / effective filtration pressure (EFP) is 10 mm Hg. 
• Walls of capillaries are extremely thin. 
• Under the effect of high pressure, walls become permeable to major components of blood (except blood cells and macromolecules like protein). 
• Thus plasma except proteins oozes out through wall of capillaries. 
• About 600 ml blood passes through each kidney per minute.
• The blood (plasma) flowing through kidney (glomeruli) is filtered as glomerular filtrate - at a rate of 125 ml / min. (180 L/d). 
• Glomerular filtrate / deproteinized plasma/primary urine is alkaline, contains urea, amino acids, glucose, pigments, and inorganic ions. 
• Glomerular filtrate passes through filtration slits into capsular space and then reaches the proximal convoluted tubule.

Selective reabsorption :

• PCT is place of reabsorption. 
• It is highly coiled so that glomerular filtrate passes through it very slowly. 
  
• Columnar cells of PCT are provided with microvilli due to which absorptive area increases enormously.
• This makes the process of reabsorption very effective.
• These cells perform active (ATP mediated) and passive (simple diffusion) reabsorption.
• Substances with considerable importance (high threshold) like - glucose, amino acids, Vit.C, Ca++, K+, Na+, Cl¯ are absorbed actively, against concentration gradient. 
• Low threshold substances like water, sulphates, nitrates, etc. are absorbed passively. 
• In this way, about 99% of glomerular filtrate is reabsorbed in PCT and DCT.

Tubular secretion / Augmentation :

• Finally filtrate reaches the distal convoluted tubule via loop of Henle.
• Peritubular capillaries surround DCT. 
• Cells of distal convoluted tubule and collecting tubule actively absorb the wastes like creatinine and ions like K+, H+ from peritubular capillaries.
• Also secrete into lumen of DCT and CT, thereby augmenting the concentration of urine and changing its pH from alkaline to acidic.
• Secretion of H+ ions in DCT and CT is an important homeostatic mechanism for pH regulation of blood. This process is called as tubular secretion or augmentation.
• Tubular secretion is only mode of excretion in marine bony fishes and desert amphibians.

Concentration of urine :

• Under the conditions like low water intake or high water loss due to sweating, human can produce concentrated urine. 
• It can be almost four times concentrated i.e. 1200 mOsm/L than the blood (300 mOsm/L). 
• For this purpose, a mechanism called countercurrent mechanism is operated in human kidneys.
• Limbs of Henle’s loop of juxtamedullary nephrons and vasa recta operate countercurrent mechanism as follows-

Mechanism :

•  involves the passage of fluid from descending to ascending limb of Henle’s loop. 
• Flow of tubular fluid is in opposite direction through both limbs; hence the name counter (opposite) current (flow). 
• In case of vasa recate, blood flows from ascending to descending parts of itself.
• Wall of descending limb is thin and permeable to water where as that of ascending limb is thick and impermeable to water. 
• In the region of descending limb, water diffuses from tubular fluid into tissue fluid due to which, tubular fluid becomes concentrated. 
• Ascending limb of Henle’s loop is thick walled and its cells can reabsorb Na+ and Cl¯ from tubular fluid and release into tissue fluid.
• Due to this, tissue fluid around descending limb becomes concentrated. 
• This makes the more water to move out from descending limb into tissue fluid by osmosis.
• Thus, as tubular fluid passes down through descending limb, its osmolarity (concentration) increases gradually due to water loss.
• On the other hand, progressively decreases due to Na+ & Cl¯ secretion as it flows up through ascending limb.
• Besides, whenever water retention is necessary, pituitary secretes ADH.
• ADH makes the cells in the wall of collecting ducts permeable to water. 
• Due to this, water moves from tubular fluid into tissue fluid, making the urine concentrated.
• Cells in the wall of deep medullar part of collecting ducts are permeable to urea.
• As concentrated urine flows through it, urea diffuses from urine into tissue fluid and from tissue fluid into the tubular fluid flowing through thin ascending limb of Henle’s loop. 
• This urea cannot pass out from tubular fluid while flowing through thick segment of ascending limb, DCT and cortical portion of collecting duct due to impermeability for it in these regions. 
• However, while flowing through collecting duct, water reabsorption is operated under the influence ofADH. 
• Due to this, urea concentration increases in tubular fluid and same urea again diffuses into tissue fluid in deep medullar region. 
• Thus, same urea is transferred between segments of renal tubule and tissue fluid of inner medulla. This is called urea recycling.
• Urea recycling operated for more and more water reabsorption from tubular fluid and thereby excreting small volumes of concentrated urine.
• Osmotic gradient is essential in the renal medulla for water reabsorption by countercurrent multiplier system. 
• This osmotic gradient is maintained by vasa recta by operating countercurrent exchange system.
• Vasa recta also have descending and ascending limbs. 
• Blood that enters the descending limb of the vasa recta has normal osmolarity of about 300 mOsm/L. 
• As it flows down in the region of renal medulla where tissue fluid becomes increasingly concentrated, Na+, Cl¯ and urea molecules diffuse from tissue fluid into blood and water diffuse from blood into tissue fluid.
• Due to this, blood becomes more concentrated which now flows through ascending part of vasa recta. 
• This part runs through such region of medulla where tissue fluid is less concentrated.
• Due to this, Na+, Cl¯ and urea molecules diffuse from blood to tissue fluid and water from tissue fluid to blood. This mechanism helps to maintain the osmotic gradient.
• So as to reabsorb water to maximum capacity, loops of Henle are longer in desert mammals like camel. Due to this, camel excretes concentrated urine.

Composition of Urine :

• Normal urine is pale yellow coloured transparent liquid. 
• This colour is due to pigment urochrome. 
• Composition of urine depends upon food and fluid consumed by the individual.
• There are two ways. 

1. One by regulating water reabsorption through ADH 
2. Other by electrolyte reabsorpion though RAAS.

• Hypothalamus in midbrain has special receptors called osmoreceptors. 
• They can detect change in osmolarity (measure of total number of dissolved particles per litre of solution) of blood.
• If osmolarity of blood increases due to any reason such as after eating namkeen or due to sweating. 
• Due to water loss from the body, osmoreceptors trigger release of Antidiuretic hormone (ADH) from neurohypophysis. (posterior pituitary). 
• ADH stimulates reabsorption of water from last part of DCT and entire collecting duct by increasing the permeability of cells. 
• This leads to reduction in urine volume and decrease in osmolarity of blood. 
• Once the osmolarity of blood comes to normal, activity of osmoreceptor cells decreases leading to decrease in ADH secretion. This is called negative feedback.
• In case of haemorrhage or severe dehydration too, osmoreceptors stimulate ADH secretion. 
• ADH is important in regulating water balance through kidneys. 
• In absence of ADH, diuresis (dilution of urine) takes place and person tends to excrete large amount of dilute urine. This condition called as diabetes insipidus.
• Another regulatory mechanism is RAAS (Renin Angiotensin Aldosterone system) by Juxta Glomerular Apparatus (JGA).
• Whenever blood supply (due to change in blood pressure or blood volume) to afferent arteriole decreases (e.g. low BP/dehydration), JGA cells release Renin. 
• Renin converts angiotensinogen secreted by hepatocytes in liver to Angiotensin I. 
• Angiotensin converting enzyme further modifies Angiotensin I to Angiotensin II, the active form of hormone.

Angiotensin II has triple function :

1. It constricts arterioles in kidney thereby reducing blood flow and increasing blood pressure.
2. Stimulates PCT cells to enhance reabsorption of Na+, Cl- and water.
3. It stimulates adrenal cortex to release another hormone called aldosterone that stimulates DCT and collecting ducts to reabsorb more Na+ and water, thereby increasing blood volume and pressure.

• Both ADH and RAAS are essential. 
• Only ADH can lower blood-Na+ concentration by way of water reabsorption in DCT and collecting duct, whereas RAAS stimulates Na+ reabsorption, thus maintains osmolarity of body fluid. 
• Action of ADH and RAAS leads to increase in blood volume and osmolarity. 
• A large increase in blood volume and pressure stimulates atrial wall to produce atrial natriuretic peptide (ANP). 
• ANP inhibits Na+ and Cl- reabsorption from collecting ducts inhibits release of renin, reduces aldosterone and ADH release too. This leads to a condition called Natriuresis (increased excretion of Na+ in urine) and diuresis.
• Kidneys participate in synthesis of calcitriol, the active form of Vitamin D which is needed for absorption of dietary calcium.
• Deficiency of calcitriol can lead to brittle bones.

Something Interesting :

• Vampire bat from south America is a nocturnal sanguivorous mammal.
• It feeds on blood of large birds and mammals. 
• It has to fly long distances to locate suitable prey.
• Once found, it can even consume blood to an extent of more than half it's body mass.
• In such a case, the body of bat becomes too heavy to fly. 
• To compensate for this, while the bat is feeding, it's kidneys excrete large amount of dilute urine (upto 24% of it's body mass). 
• Now bat can fly back to the cave/tree where it can spend the day. 
• During day time, it cannot go to drink water. 
• At the same time diet being high on proteins, large amount of nitrogenous waste is produced. 
• Instead of diluting waste, kidneys resort to concentrating urine in order to conserve water. 
• This capacity to rapidly change the osmolarity of urine is a classic example of adaptation