What is urine?

What is urine?     

Urine formation takes place by the following three mechanism.Formation of glomerular filtrate,re absorption
of glomerular filtrate from tubule, tubular secretion. 

Mechanism of formation of dilute urine :When the glomerular filtrate is initially formed, its osmolarity is about the same as plasma  (300 m0sm/L).To excrete excess water,it is necessary to dilute the fiirate as it passes along the tubule.This is achieved by reabsorbing solutes to a greater extent than water. Tubular fluid remains isosmotic in the proximal tubule : As fluid flowes through the proximal tubule,solutes and water are absorbed in equal proportions,so that little change in osmolarity occurs; that is the proximal tubule fluid remains isosmotic to the plasma, with an osmolarity of about 300 m0sm/L.As fluid passes down the descending loop of henle,water is reabsorbed by osmosis and the tubuler fluid reaches equilibrium equilibrium with the surrounding interstital fluid of the renal medulla,which is very hypertonic about two to four times the osmolarity of the original glomerular filtrate.Therefore,the tubular fluid becomes more concentrated as it flows into the inner medulla.Tubular fluid becoms dilute in the ascending loop of henle: In the ascending limb of the loop of henle especially in the thick segment, sodium potassium, and chloride are avidly reabsorbed.However, this protion of the tubular segment is impermeable to water, even in the presence of large amounts of ADH.Therfore, the tubular fluid becomes more dilute as it flows up the ascending  lop of henle into the early distal tubule, with the osmolarity decreasing progressively to about 100 mOsm/L by the time the fluid enters the early distal tubular segment. Thus regardless of whether ADH is present or absent ,fluid leaving the early distal tubular segment is hypo-osmoticwith an osmolarity of only about one third the osmolarity of plasma.
 
N.B. To summarize,the mechanism for forming a dilute urine is to continue reabsorbing solutes from the distal
segments of the tubular system while failing to reabsorb water.In healthy kidney, fluid leaving the ascending
loop of henle and early distal tubule is always dilute, regardless of the level of ADH.In the absense of ADH,the urin is further diluted in the late distal tubule and collecting ducts, and a large volume of dilute urine is excreted.

Concentrated urine

Mechanism of formation of concentrated urine: The ability of the kidney to from a urine that ia more concentrated than plasma is essential for survival of mammals that live on land, including humans.The basic requirements for forming  a concentrated are a level of ADH, which increases the permeability of the distal tubules and collecting ducts to water, thereby allowing these tubular segments to avidly reasorb water.A high osmolarity of the renal medullary interstitial fluid which provides the osmotic gradient necessary  for water reabsorption to occur in the presence of high levels of ADH.

Counter current mechanism :When a fluid passes through two prallel stem of `U` shaped tube in opposite direction with close proximity is called counter.current mechanism. During this counter current flow tremendous concentration of solute can be build up at the top of the loop.There are two special features of the renal medullary blood flow that contribute to the preservation of the high soluteconcentration. The medullary blood flow is low , acconting for only 1 to 2 per sent of the total renal blood flow.This
sluggish blood flow is sufficient to supply the metabolic needs of the tissues but helps to minimize solute loss from the medullary interstitium.The vasa recta serve as countercurrent exchangers minimizing washout of solutes from the medullary interstitium

The countercurrent exchange mechansim operates as follow: Blood enters and leaves the medulla by way of the vasa recta at the boundary of the cortex and renal medulla.The vasarecta , like other capillaries, are highly permeable to solutes in the blood,except for the plasma proteins.As blooddescends into the medulla toward the papillac, it becomes progressively more concentrated,party by solute entry fromthe interstitium.By the time the blood reaches the tips of the vasa recta,it has a concentration of about 1200 mOsm/L,the same as that of the medullary interstitium and was water moves into the vasa recta.Thus, although there is a large amount of fluid and solute exchange across the vasa recta, there is little net dilution of the concentration of the interstitial fluid at each level of the renal medulla because of the U shape of the vasa recta capillaries, which act as countercurrent exchangers.Thus, the vasa recta do not create the medullary hyperosmolarity,but they do prevent it from being dissipated.

Renal corpuscle and blood filtration : Each renal corpuscle is about 200 um in diameter and consists of a tuft of capillaries, the glomerulus, surrounded by a double-walled epithelial capsule called bowman's capsule. The internal layer (the visceral layer) of the capsule envelops the capillaries of the glomerulus. The external layer forms the outer limit of the renal corpuscle and is called the parietal layer of boweman's capsule.Between the two layers of bowman's capsule is the urinary space,which receives the fluid filter-
ed through the capillary wall and the visceral layer. Each renal corpuscle has a vascular pole, where the afferent arteriole enters and the efferent arteriole leaves, and a urinary pole, where the proximal convoluted tubule begins. After entering the renal corpuscle, the afferent arteriole usually divides into two to five primary branches, each subdividing into capillaries and forming the renal glomerulus. The parietal layer of bowman's capsule consists of a simple squamous epithelium supported by a basal lamina and a thin layer of
recticular fibers. At the urinary pole, epithelium changes to simple columnar epithelium characteristic of the proximal tubule. During embryonic development, the epithelium of the parietal layer remains relatively unchanged, whereas the internal, or visceral, layer is greatly modified. The cells of this internal layer, the podocytes, have a cell body from which arise several primary processes. Each primary process gives rise to numerous secondary processes, called pedicels that embrace the capillaries of the glomerulus. The
secondary processes of podocytes interdigitate, defining elongated spaces about 25 nm wide-the filtration slits. Beside endothelial cells and podocytes, the glomerular capillaries have mesangial cells adhering to their walls in places where the basal lamina forms a sheath that is shared by two or more capillaries.

Important for viva : The blood flow in the kidneys of an adult amounts to 1,2-1.3 L of blood per minute. This means that all the circulating blood in the body passes through the kidneys every 4-5 min. The glomeruli are composed of arterial capillaries in which the hydrostatic pressure- about 45 mm Hg-is higher than that found in other capillaries. The glomerular filtrate is formed in response to the hydrostatic pressure of blood, which is opposed by the osmotic (oncotic) pressure of plasma colloids (20mm Hg), and the hydrostatic pressure of the fluids in bowman's capsule (10mmHg).The net filtration pressure at the afferent end of glomerular capillar
ies is 15 mm Hg. The glomerular filtrate has a chemical composition similar that of blood plasma but contains almost no protein,because macromolecules do not readily cross the glomerular filter. The largest protein molecules that succeed in crossing theglomerular filter have a molecular mass of about 70 KDa, and small amounts of plasma albumin appear in the filtrate.                
Steps involved in causing the hyperosmotic renal medullary interstitium.  Assume that the loop of henle is filled with fluid with a concentration  of 300 mOsm/L, the same as that leaving the proximal tubule. Next, the active pump of the thick ascending limb on the loop of henle is turned on, reducing the concentration inside the tubule and raising the interstitial concentration; This pump establishes a 200-mOsm/L concentration gradient between the tubuler fluid and the interstitial fluid. The limit of the gradient is about 200 mOsm/L because paracellular diffusion of ions back into the tubule eventually counter balances transport of ions out of the lumen when the 200-mOsm/L concentration gradient is achieved. Is that the tubular fluid in the descending limb of the loop of henle and the interstitial fluid quickly reach osmotic equilibrium because of osmosis of water out of the descending limb. The interstitial osmolarity is maintained at 400 mOsm/L because of continued transport of ions out of the thick ascending loop of henle. Thus by itself, the active transport of sodium chloride out of the thick ascending limb is capable of establishing only a 200-mOsm/L concentration gradient, much less than that achieved by the countercurrent system. Is additional flow of fluid into the loop of  henle from the proximal tubule, which causes the hyperosmotic fluid previously formed in the desceding limb to flow into the ascending limb. Once this fluid is in the ascending limb, additional ions are pumbed into the interstitium,with water remaining behind, until a 200-mOsm/L osmotic gradient is established, with the interstitial fluid osmolarity rising to 500 m-Osm/L. Then, once again, the fluid in the descending limb reaches equilibrium with the hyperosmotic medullary interstitial fluid, and as the hyperosmotic  tubular fluid from the descending lmb of the loop of henle flows into the ascending limb, still more solute is continuously pumped out of the tubules deposited into the inedullary  interstitium.