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.