kidneys

the kidney

• Two functions: osmoregulation and excretion

• Achieves this by filtering 20% of the water and solutes from the blood plasma and then selectively absorbing the substances that the body requires

• The basic functional unit of the kidney is the nephron: a tube with a wall consisting of one layer of cells - an epithelium

osmoregulation

• Osmoregulation is keeping osmotic concentrations of the body within it's limits, part of homeostasis

• Osmotic concentration is the overall concentration of solutes in a fluid that can affect the movement of water by osmosis

• The kidneys carry out osmoregulation by varying the amounts of water and salt removed from the body in urine

excretion

• Removal of toxic waste (nitrogen-containing compounds) and substances not used by the body (drugs, pigments)

structure & function

• Bowman’s capsule: a cup-shaped structure with a highly porous inner wall that collects the fluid filtered from the blood

• Glomerulus: a knot-like, high-pressure capillary tuft that is the site of blood filtratioon

• Loop of Henle: hairpin shaped tube that consists of a descending and an ascending limb 

• Proximal convoluted tubule: highly twisted section of the nephron with cells containing mitochondria and microvilli projecting into the lumen of the tube 

• Distal convoluted tubule: highly twisted section, but with less mitochondria and microvilli 

• Collecting duct: a wider tube that carries the filtrate to the renal pelvis 

ultrafiltration (overview)

• Ultrafiltration is the first stage in urine production. It is carried out by the glomerulus inside the Bowman’s capsule (about a million in each kidney).

Structure of the Glomerulus

• The glomerulus is a ball-shaped network of blood capilaries. The high permeability of the capillary is due to the presence of pores called fenestrations that allow fluid to pass through. 

• Blood flows into the glomerulus through an afferent arteriole and leaves the glomerulus through an efferent arteriole. 

Causes of High Pressure in the Glomerulus

• Two factors that cause high pressure of blood in the glomerulus.

  • The efferent arteriole is narrower than the afferent arteriole

  • The contorted that blood must follow to pass through the glomerulus

Structure of Bowman’s Capsule (Podocytes)

• The Bowman’s capsule is comprised of an inner surface of cells called podocytes 

• Podocytes have cellular extensions called foot processes that wrap around the blood vessels of the glomerulus. Narrow gaps between adjacent foot processes prevent small molecules from being filtered out of the glomerulus. 

Basement Membrane (Filtration Barrier)

• Between the podocytes and the glomerulus is a glycoportein matrix called the basement membrane that filters the blood

• The basement membrane is size selective and restricts the passage of large proteins and blood cells

• Hence when the blood is filtered, the filtrate (fluid forced out of the blood plasma) formed does not contain any blood cells, platlets , or plasma membrane. 

Convoluted tubules 

• Once the glomerular filtrate is formed, it will flow to the next part of the nephron, the proximal convoluted tubule

• This is where selective reabsorption of substances occurs

• The proximal convoluted tubule has a microvilli cell lining to increase the surface area for material absorption from the filtrate 

• There are also a large number of mitochondria within these tubule cells, as reabsorption involves active transport the tubules reabsorb all glucose, amino acids, with most of the mineral ions and water (80%)

  • Ions (sodium and chloride) are actively transported by protein pumps and carried

  • Glucose is cotransported across the apical membrane with sodium (symport)

  • Water follows the movement of mineral ions passively via osmosis

Adaptations of the proximal convoluted tubule

• The wall of the proximal convoluted tubule is only one cell thick

• High number of channel proteins and pump proteins to ensure that only substances required bythe body are reabsorbed

• The apical membrane contains a large number of microvilli to increase the SA

• The basal membrane has infoldings (invaginations) which also increase the SA available

loop of henle

• The kidney has 2 main regions: an outer cortex and an inner medulla

• The cortex contains the Bowman’s capsule and the convoluted tubules, the medulla contains the loop of henle (and collecting duct)

• The role of the loop of henle is to establish & maintain a salt gradient  (hypertonicity) in the medulla

• Sodium ions are pumped out of the filtrate to the interstital fluid by protein pumps creating an osmotic gradient

• As filtrate flows down the descending limb (permeable to water) the increased solute concentration of interstitial fluid in the medulla causes the water to be drawn out of the filtrate

• The walls of the ascending limb are impermeable to water so water is retained in the filtrate despite the interstitial fluid being hypertonic compared to the filtrate 

• This system of raising solute concentration is an example of countercurrent system because of the flows of fluid in opposite directions

osmoregulation

• The osmotic concentration of the blood is monitored in the hypothalamus

• ADH (antidiuretic hormone) is released from the pituitary gland in response to dehydration (detected by osmoreceptors in the hypothalamus)

• ADH increases the permeability of collecting duct to water, by upregulating production of aquaporins (water channels)

• This means that less water remains in the filtrate, urine becomes concentrated and the individual urinates less