Formation of Urine - Nephron Function, Animation.

The kidneys filter blood plasma,  removing metabolic wastes, toxins, and excrete them in urine. During this  process, they also maintain CONSTANT volume and composition of the blood, or homeostasis. Blood enters the kidney via the RENAL ARTERY, which divides into SMALLER  arteries, then arterioles.

The arterioles get into contact with functional  units of the kidney called the NEPHRONS. This is where blood filtration  and urine formation take place. The FILTERED BLOOD then passes through a series  of veins and exits the kidney via the RENAL VEIN.

The URINE is collected in collecting ducts  and leaves the kidney via the URETERS. Each kidney contains over a million nephrons.  A nephron consists of 2 major parts: a glomerular capsule, or Bowman’s  capsule; and a LONG renal tubule.

Renal tubules of several nephrons  connect to a common collecting duct. There are 3 steps in the formation of urine:  glomerular filtration; tubular REabsorption and secretion; and water conservation. Blood enters the Bowman’s capsule via the Afferent arteriole, passes through a  ball of capillaries called the GLOMERULUS, then leaves via the Efferent arteriole.

The  Afferent arteriole is significantly LARGER than the Efferent arteriole, creating a blood flow  with a LARGE INLET and SMALL OUTLET. As a result, the blood hydrostatic PRESSURE in these  capillaries is MUCH HIGHER than usual. Hydrostatic and osmotic pressures DRIVE water and  solutes FROM blood plasma, through a FILTRATION MEMBRANE, INTO the capsular space of the nephron. 

The filtration membrane acts like a sieve, allowing ONLY SMALL molecules to pass through.  These include water, inorganic ions, glucose, amino acids and various metabolic wastes such as  urea and creatinine; and make up the GLOMERULAR FILTRATE. The amount of filtrate produced per  minute is called glomerular FILTRATION RATE, GFR.

GFR is kept at a STABLE value by several  FEEDBACK mechanisms within the kidneys, known as RENAL AUTOregulation. GFR is also under  SYMPATHETIC and HORMONAL control. GFR regulation is generally achieved by CONSTRICTION  or DILATION of the Afferent arteriole, which causes the GLOMERULAR blood  pressure to FALL or RISE, respectively.

In a healthy person, the total filtrate volume  amounts to between 150 and 180 litters a day. However, only about 1% of this is excreted  as urine, the remaining 99% is REabsorbed BACK to the blood as the filtrate  flows through the LONG renal tubule. This is possible because the Efferent arteriole,  after exiting the Bowman’s capsule, BRANCHES OUT to form a network of so-called PERItubular  capillaries, which SURROUND the renal tubule.

The FIRST part of the tubule, the PROXIMAL  convoluted tubule, REabsorbs about two thirds of the filtrate. In this process, water and solutes  are driven through the EPITHELIAL cells that LINE the tubule, into the extracellular space. They  are then taken up by the peritubular capillaries.

SODIUM REabsorption is most important, as it  creates OSMOTIC pressure that DRIVES WATER; and ELECTRICAL gradient that drives NEGATIVELY charged  ions. Sodium level INSIDE the epithelial cells is kept LOW thanks to the sodium-potassium pumps  that constantly PUMP sodium ions OUT into the extracellular space. This creates a CONCENTRATION  GRADIENT that favors sodium diffusion FROM tubular fluid INTO the cells.

Sodium is absorbed  by SYMPORT proteins that ALSO bind GLUCOSE and some other solutes. Nearly ALL glucose  and amino acids are REabsorbed at this stage. About half of nitrogenous wastes also REabsorb  back into the bloodstream.

The kidneys REDUCE the blood levels of metabolic wastes to a SAFE amount,  but do NOT completely eliminate them. Some of the REabsorption also occurs by the paracellular route  through TIGHT junctions BETWEEN epithelial cells. At the same time, TUBULAR SECRETION,  where ADDITIONAL wastes and other solutes LEAVE the bloodstream to join  the tubular fluid, also takes place.

The processes of REabsorption and  secretion continue in the nephron loop, the loop of Henle, and the DISTAL convoluted  tubule. These parts of the tubule also have some OTHER important functions. The main function of the loop of Henle is to CREATE and maintain an OSMOLARITY GRADIENT in  the medulla that enables the collecting ducts to CONCENTRATE urine at a later stage.

The ASCENDING  limb of the loop ACTIVELY pumps sodium OUT, making the medulla “SALTY”. The DESCENDING limb  is permeable to water but MUCH LESS to sodium. As the water exits the tubule by osmosis, the  filtrate becomes more and more CONCENTRATED as it reaches the bottom.

The ASCENDING limb,  on the other hand, is permeable to ions but NOT water. As a result, the filtrate LOSES  sodium as it goes UP and becomes more DILUTED at the TOP of the loop. The medulla is in  EQUILIBRIUM with the loop and hence has the SAME SALINITY GRADIENT - SALTIER at the bottom.

REabsorption and secretion in the DISTAL convoluted tubule are under control of  various hormones. This is how the kidneys respond to the body’s needs and adjust  the composition of urine accordingly. The COLLECTING DUCT receives tubular fluid  from several nephrons.

The main function of the collecting duct is to CONCENTRATE URINE and  therefore CONSERVE WATER. This is made possible by the osmolarity gradient generated by the loop  of Henle. As it gets SALTIER deep in the medulla, the filtrate LOSES water as it  flows DOWN the collecting duct.

The collecting duct is also under HORMONAL CONTROL  so it can adjust the amount of REabsorbed water according to the body’s state of HYDRATION.  For example, when the body is DEHYDRATED, MORE water is REabsorbed back to the blood  and the excreted urine is more concentrated.