regulation of urine

Where does ultrafiltration take place

In the nephrons’ renal capsule

What is ultrafiltration powered by

The high hydrostatic pressure from the blood coming through into the renal artery from the aorta then into the renal arteriole

How does blood enter the renal capsule

Through the afferent arteriol

How does the blood leave the renal capsule

Through the efferent arteriole

How does the afferent arteriole differ from the efferent arteriole and how does it aid ultrafiltration

The lumen of the efferent arteriole is much smaller than the afferent arteriole so causes a build up of hydrostatic pressure of the highly convoluted capillaries of the glomerulus inside the renal capsule

What are the three layers through which the glomerular filtrate passes through

• endothelium of the capillary

• The basement membrane

• The epithelium cells of the capsule (podocytes)

How is the endothelium of the capillary adapted for ultrafiltration of the blood

It has tiny pores (fenestrations) which are too small for cells and plasma proteins to leave the blood.

What is the role of the basement membrane

Acts as a filter

What is the limit of Mr for molecules to pass through the basement membrane

68000g/mol

What is the name of the cells that line the capsule wall

Podocytes

What is the role of podocytes and how are they adapted to their role

They have feet like extensions that form a network with tiny slits between them called filtration slits through which fluid can pass into the lumen of the renal capsule (the glomerular filtrate is very similar to blood plasma but with protein molecules missing)

What is the glomerular filtration rate

The rate at which fluid filters from the capillary to the renal capsule (125cm³/min)

What is the glomerular filtration rate determined by

The difference in water potential between the plasma in the glomerular capillaries and filtrate in Bowman’s capsule

Overall the effect of pressure outweighs the effects of solute concentration so fluid continues to move out of capillary into capsule

What happens in selective reabsorption

Some of the molecules in the glomerular filtrate are reabsorbed into the blood

Where does most reabsorption of water (and solutes) occur

The proximal convoluted tubule

How are the cells of the proximal convoluted tubule adapted to their role

• many mitochondria

• Apical microvilli on apical membrane (facing lumen) on brush border, increases SA

• Tight junctions hold cells together so fluid can’t pass between them, must go through

• May co-transporter proteins (and aquaporins) on apical membrane

• Basal membrane folded to increase SA

How does selective reabsorption work?

• basal membranes (nearest capillaries/tissue fluid) contain sodium-potassium pumps

• Pumps sodium ions out of cell, decreasing its concentration inside cells

• Na+ passively diffuses through special co-transporter proteins into cells from glomerular filtrate across apical membrane

• Co-transports glucose/amino acids etc

• Na+ conc gradient provides energy to move glucose out of filtrate against concentration gradient (secondary active transport).

What happens to glucose once it is in the cells of the proximal convoluted tubule.

Glucose diffuses down its concentration graident through a transport protein in the basal membrane and into the blood

How does a sodium-potassium pump work

• 2K+ ions actively transported into cell

• 3Na+ ions transported out of cell

• Creates an decrease of Na+ concentration inside cells

• Na+ re-enters via co-transporter proteins on other side of cell

• K+ re-exits cell by facilitated diffusion down conc gradient

• Repeats

Define osmoregulation

The control of water content in the blood (and therefore the whole body) achieved by negative feedback mechanisms

How does the body detect changes in blood water potential

hypothalamus detects changes in the water potential of blood passing through capillaries within the hypothalamus

What are the names of the cells which detect changes in water potential, and how are they affected

Osmoreceptors in the hypothalamus shrink when there is less water in the blood

Describe the journey of ADH from where it is produced to where it is secreted

• ADH made by the hypothalamus

• Travels down in neurosecretory cells

• To the posterior pituitary gland

• Stored in the vesicles at the end of the neurosecretory cells in the posterior pituitary ready to be secreted

How is ADH released into the bloodstream

• Hypothalamus detects that water potential of the blood is too low

• Sends a nervous impulse to the pituitary

• Releases the ADH into the bloodstream

Where are the target cells of ADH

In the wall of the collecting duct and te distal convoluted tubule of the kidneys

How are the cell surface membranes of the collecting duct cells adapted for reabsorption of water

contain lots of channel proteins called aquaporins

How can aquaporins be found within the cell

Either embedded in the cell surface membrane or stored in vesicles within the cell

Describe the action of ADH on target cells

• ADH binds to receptor proteins in cell surface membrane of cells in collecting duct wall

• Triggers formation of cAMP, acts as a second messenger

• Causes an enzyme cascade

• Triggers the movement of vesicles containing aquaporins towards the cell surface membrane

• Vesicles fuse to membrane and insert extra aquaporins

What happens when blood water potential returns to set point

• once water potential of the blood rises, ADH is stopped

• Membrane pinches off vesicles containing the aquaporins towards

• Water does not leave the collecting duct but instead continues to bladder

• More dilute urine, copious in volume