Download The kidney`s principal function is to filter the blood in order to remove

Structure of THE Excretory (URINARY) SYSTEM
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Two fist-sized kidneys are located in the area of the lower back on either side of the spine.
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A large cushion of fat usually surrounds the kidneys and provides some protection for them.
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The kidney’s principal function is to filter the blood in order to remove cellular waste products from the body,
specifically from the blood.
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The kidneys release urine into the ureters (~28 cm long muscular tubes)
that carry it into the urinary bladder.
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The bladder temporarily stores urine and moves it by the peristaltic actions
of its smooth muscle tissue.
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Drainage from the bladder is controlled by two sphincter muscles – the
innermost sphincter is involuntarily controlled by the brain, whereas the
other sphincter one learns to voluntarily control relaxation during
childhood.
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Both sphincters must relax before urine can drain from the bladder.
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Urine exits the bladder and body through the urethra.
Efferent
arteriole
Afferent
arteriole
Renal
Renal
Peritubular
 The functional units of the kidneys are known as NEPHRONS, which act as tiny filters. Extending between the
cortex and medulla of each kidney are about one million microscopic nephrons.
 Blood enters the cavity of BOWMAN’S CAPSULE through a network of porous, thin-walled capillaries called the
GLOMERULUS. Under the influence of blood pressure, some blood plasma and small particles are forced out of
the glomerulus and into the surrounding Bowman’s capsule.
 The fluid inside the Bowman’s capsule is called the GLOMERULAR FILTRATE and it is pushed out of the capsule,
under high pressure, and into the tubes (PROXIMAL TUBULE, LOOP OF HENLE, & DISTAL TUBULE) of the
nephron.
 The tubes of the nephron are surrounded by cells, and a network of blood vessels (PERITUBULAR CAPILLARIES).
Any material that leaves the nephron enters the surrounding cells and eventually returns to the bloodstream
through the network of blood vessels.
 By controlling what leaves and what remains in the nephrons, the kidneys keep the levels of water, ions and
other materials in the body nearly constant and within the limits necessary to maintain homeostasis.
 The filtrate the remains in the COLLECTING DUCT is now called URINE. It is ~1% of the original filtrate volume and
~4X more concentrated with waste molecules as the extracellular fluid.
High Pressure
URINE FORMATION in the nephron
The formation of urine depends on 3 processes:
1.
FILTRATION – the movement of water, salts, nutrient molecules, waste molecules and other dissolved materials
from the blood into Bowman’s capsule (nephron), through a selectively permeable membrane
formed by the capillaries of the glomerulus (contain pores) and the cells of Bowman’s capsule.
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Blood in the glomerulus is under high pressure, and this drives the filterable contents into Bowman’s
capsule.
Blood cells, platelets, and plasma proteins are too large to pass through and thus, remain in the blood.
2. REABSORPTION – materials required by the body are removed from the filtrate and returned to the
bloodstream, via the interstitial fluid around the nephron.
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The inner walls of the tubule are covered with microvilli which increase the total surface area available
for reabsorption of solutes.
~65% of the filtrate that passes through the entire length of the proximal tubule is reabsorbed and
returned to the body (blood). This is done via both passive and active transport.
LOCATION
proximal tubule
descending loop
of Henle
ascending loop of
Henle
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distal tubule
collecting duct
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DESCRIPTION OF REABSORPTION
nutrients (eg. glucose, amino acids, Na+, K+) removed from filtrate via active transport.
negatively charged ions (eg. Cl-) are passively reabsorbed by electrical attraction.
removal of water from filtrate via osmosis
removal of water from filtrate via osmosis (aided by increasing [Na+] as move deeper into the
medulla).
descending loop is permeable to water and only slightly permeable to ions (eg. Na+, Cl-)
Na+ concentration in tubule reaches its maximum at bottom of loop, as water leaves filtrate.
thin portion of ascending loop is impermeable to water and concentrated Na+ move by passive
transport into medulla and then into blood.
thick portion of ascending loop towards top allows for active transport of Na+ out of the filtrate
–  responsible for the increasing [Na+] in medulla &  filtrate becomes less concentrated than
cortex tissue and surrounding blood.
Cl- ions follow Na+ due to charge attraction.
active transport of Na+ and Cl- and other molecules out of nephron (depending on body’s needs)
passive reabsorption of water (normally under control of ADH – Anti-diuretic hormone)
passive reabsorption of water
removal of more water passively concentrates the urine (normally under control of ADH)
urea diffuse from nephron into interstitial fluid, and then blood (less reabsorbed than originally
filtered).
3. SECRETION – the passive and active transport of substances out of the blood and
interstitial fluid and into the tubule (proximal and distal). These
substance include ammonia, many water-soluble drugs and their
metabolites (eg. penicillin and other medications), excess H+ ions etc.
URINE OUTPUT and WATER BALANCE
 increased water intake = increased urine output
 decreased water intake = decreased urine output OR (increased exercise leading to increased water loss)
 The permeability of the distal tubule and collecting duct is controlled by a hormone called
ANTI-DIURETIC HORMONE (ADH)
 ADH
 kidney reabsorption of water
[ie.  removal of water from filtrate and returned into bloodstream]
 concentration of urine
[ie. less water is excreted in the urine]
This is a response to the body’s need to conserve H2O
 ADH
 kidney reabsorption of water
[ie.  removal of water from filtrate and returned into bloodstream]
 concentration of urine
[ie. more water is excreted in the urine]
This is a response to the body’s need to eliminate excess H2O
 The majority of water filtered into nephron is reabsorbed in proximal tubule and loop of Henle back into
bloodstream.
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The remaining water filtered into nephron is reabsorbed under the control of ADH in the distal tubule and
collecting duct.