Download The Nephron as a Unit of Kidney Function

The Nephron as a Unit of Kidney Function
Objectives – What you will need to know from this section
 Outline the structure & associated blood supply & draw a diagram.
 Explain urine formation, including: Bowman's capsule/glomerulus/ proximal convoluted
tubule/Loop of Henle/distal convoluted tubule/pelvis/bladder
 Outline the sites & action of reabsorbing glucose/amino acids/salts/water
 State that reabsorbing water occurs in the collecting duct & is under the influence of ADH.
The Nephron as a Unit of Kidney Function
 The structural and functional unit of the kidney is the nephron.
1. Filtration
 The renal artery divides into afferent arterioles and then into a capillary network
(glomerulus) at the top of each nephron.
 A cup-shaped funnel (Bowman’s capsule) surrounds each glomerulus and it is here that
smaller molecules in the blood are forced, under pressure, out of the plasma and into the
lumen of Bowman’s capsule, forming the glomerular filtrate.
 The blood pressure is high because the efferent arteriole is narrower than the afferent
arteriole, so force-filtering the plasma.
 Everything except large proteins and blood cells gets filtered.
2. Reabsorption
 The body cannot afford to lose useful chemicals like food and water, so as the glomerular
filtrate passes from the Bowman's capsule into the proximal convoluted tubule, glucose,
amino acids, some salts and water are reabsorbed back into the blood.
 The food molecules, including most of the salt ions, are taken back by active transport
(against the concentration gradient, so energy is needed for this). Most of the water is
reabsorbed by osmosis—from the Loop of Henle and convoluted tubules.
 Urea and other wastes, along with some water, are not reabsorbed. They pass, as urine, into
the pelvis of the kidney and to the bladder for storage. Of the 180L of blood filtered each
day, about 99 % of the filtrate is reabsorbed.
The kidney regulates the amount of water in the body by varying the amount of urine produced.
 This is known as osmoregulation, and it is an example of homeostasis.
ADH [Anti-diuretic hormone] controls whether the distal tubule and collecting ducts reabsorb water
or not.
When the body is low on water
 ADH is secreted from the pituitary gland into the blood.
 Distal tubal and collecting duct walls become more permeable so
 More water is reabsorbed
 and only a small volume of urine is produced.
If you drink a great deal of water
 the hypothalamus in the brain detects the diluted blood and turns off ADH production in the
pituitary.
 Less water is reabsorbed, so more water is allowed to escape to the bladder,
 and a larger volume of dilute urine is produced.
If a person’s kidneys are not functioning correctly they make be put on dialysis machine which filters
the their blood for them and then returns it to the body.
Blood should not contain blood cell and proteins (should not get filtered) or glucose (should be
reabsorbed.