Assignment for lecture 8 (kidney and urinary system)
The functional unit of the kidney is called a nephron, and consists of a glomerulus,
proximal tubule, loop of Henle, distal tubule and collecting duct. The three basic
processes that produce urine are filtration in the glomerulus, and reabsorption and
secretion in the tubules. Different substances are handled in different ways: creatinine
(inulin would also be correct) is an example of a substance that is filtered and neither
secreted nor reabsorbed; glucose is an example of a substance that is filtered and
completely reabsorbed, while sodium and water are only partially reabsorbed.
Substances that are actively secreted include potassium and PAH (p-aminohippuric
acid).
Measurement of creatinine is a particularly useful measure of renal function. The
glomerular filtration rate (abbreviated GFR), the amount of fluid filtered through all
the glomeruli in both kidneys, is normally about 180 litres per day. It can be assessed
by measuring the concentration of creatinine left in the blood: high plasma creatinine
indicates GFR that is lower than normal, indicating poor kidney function. In contrast,
if we measure the urinary excretion of PAH (p-aminohippuric acid), which is
completely cleared from the blood in one passage through the kidneys, we get an
estimate of renal blood flow.
Urine is normally more concentrated than plasma, and this is made possible by the
loop of Henle, which produces a highly concentrated extracellular space in the
medulla of the kidney. The collecting duct passes through this highly concentrated
region as it is about to leave the kidney; urine concentration is varied by allowing
variable amounts of water to cross from the collecting duct, and the permeabillity of
the collecting duct is controlled by antidiuretic hormone (abbreviated to ADH). A
lack of this hormone results in the disease known as diabetes insipidus, which is
characterised by copious production of very dilute urine. In healthy individuals, urine
production is normally about 1-3 litres per day.