7.21B - 7.22B - Urea and kidney failure

7.22B - Following urea from liver to urine

The kidneys remove urea, but they do not make it. Urea is produced in the liver when
excess amino acids are broken down.

Urea dissolves in blood plasma and is carried to the kidneys in the renal arteries. Because
urea is a small dissolved molecule, it enters the filtrate from glomerular blood. It is then
excreted in urine, which flows through the collecting ducts, ureter, bladder and urethra.

In the route below, the renal artery carries blood into the kidney. The glomerulus is a knot of capillaries where small dissolved substances enter the nephron fluid; this fluid eventually forms urine. A ureter carries urine to the bladder, and the urethra carries it outside.

The complete source-to-exit route is:

excess amino acids -> liver produces urea -> urea carried in blood plasma -> renal artery
-> glomerulus -> nephron filtrate -> collecting duct -> ureter -> bladder -> urethra ->
outside the body

7.21B - Treating kidney failure

Kidney failure means the kidneys can no longer remove enough waste and excess water from
the blood. Two treatments in this specification are kidney dialysis and a kidney
transplant using a donated organ.

Dialysis

In haemodialysis, blood is taken from the patient and pumped through an external filter
called a dialyser. Inside it, blood and dialysis fluid are kept separate by a partially
permeable membrane; they do not mix. Small waste molecules such as urea diffuse from their
higher concentration in the blood to their lower concentration in the dialysis fluid.
Fresh dialysis fluid is continually supplied so urea does not accumulate beside the blood; this maintains a concentration gradient for net diffusion. Its useful-solute concentrations are controlled to avoid unnecessary glucose loss. Blood cells and large plasma proteins remain in the blood, excess water can also be
removed, and the treated blood is returned to the patient.

Dialysis can be used without waiting for a donor and avoids transplant surgery. However,
it replaces only part of kidney function, must be carried out regularly and places limits
on a person's time, food and fluid intake.

Organ donation and kidney transplant

A healthy kidney donated by a living or deceased donor can be surgically transplanted into
a person with kidney failure. If it functions well, it continuously filters the recipient's
blood, so regular dialysis may no longer be needed.