The kidneys produce urine through ultrafiltration and selective reabsorption. During ultrafiltration in the glomerulus, water and small molecules are filtered from the blood into the renal capsule. Then in the nephron tubules, useful materials like water, salts, glucose and amino acids are reabsorbed back into the blood while waste products remain and pass into the urine. The composition of urine is affected by diet and conditions like diabetes, as well as environmental factors such as temperature.

1. URINE PRODUCTION
THE ROLE OF NEPHRON
Learning Objective:
Outline the function of kidney tubules with reference to ultra-filtration
and selective reabsorption in the production of urine.

2. URINE
FORMATION
• Involves two main
processes:
• Ultrafiltration of very
small molecules
from the blood (in
Malpighian
corpuscle)
• Selective
reabsorption of
useful materials
(along the tubules
from the renal
capsule to the
collecting duct)

3. ULTRAFILTRATION
• Blood passes from branches of the renal artery into
the glomeruli in the Malpighian corpuscles.
• The afferent arteriole that brings blood into the
glomerulus is wider than the efferent arteriole that
carries blood away from the glomerulus.
• This creates a high blood pressure in the glomerulus.
• The hydrostatic blood pressure provides the main
force required for the filtration process.

4. ULTRAFILTRATION
• In the Malpighian corpuscle, mechanical filtration
occurs in each glomerulus.
• Most of the blood plasma is forced out of the
glomerular blood capillaries into the Bowman’s
capsule (renal capsule).
• A partially permeable basement membrane wraps
around the glomerular blood capillaries.
• Only water and very small molecules can pass
through the very small pores in the basement
membrane.

5. ULTRAFILTRATION
Blood cells, platelets and
large molecules such as fats
and proteins are retained in
the glomerular capillaries.
Water, glucose, amino
acids, mineral salts
and nitrogenous waste
products are forced
out of the glomerulus
into the renal capsule,
forming the filtrate.

6. SELECTIVE REABSORPTION
• At the proximal convoluted tubule,
• Most water,
• Most mineral salts,
• All glucose,
• All amino acids
• are reabsorbed through the walls of the tubule into
the surrounding blood capillaries.
• The solutes are reabsorbed by diffusion and active
transport.
• The water is reabsorbed by osmosis.

7. SELECTIVE REABSORPTION
• At the loop of Henle,
• Some water,
• is reabsorbed through the walls of the tubule into
the surrounding blood capillaries.
• The water is reabsorbed by osmosis.

8. SELECTIVE REABSORPTION
• At the distal convoluted tubule,
• Some water,
• Some mineral salts (sodium ions),
• are reabsorbed through the walls of the tubule into
the surrounding blood capillaries.
• The solutes are reabsorbed by diffusion and active
transport.
• The water is reabsorbed by osmosis.

9. SELECTIVE REABSORPTION
• At the collecting duct,
• Excess water
• Excess mineral salts
• Nitrogenous waste products (urea, uric acid, creatinine)
• passes through the collecting duct into the renal
pelvis as a mixture called urine..
• The water is reabsorbed by osmosis.

10. COMPOSITION OF URINE
• A protein rich diet will result in more urea being
present in the urine due to deamination of excess
amino acids.
• A water-rich diet will increase water potential of the
blood. Hence more urine is excreted .
• A salt-rich diet will result in excess salts excreted in
the urine.
• A sugar-rich diet will result in some glucose being
present in the urine.

11. HIGH AMOUNT OF GLUCOSE IN URINE
(DIABETES MELLITUS)
• Islets of Langerhans in pancreas cannot secrete
insulin.
• Excess glucose in blood cannot be converted into
glycogen.
• Glucose is filtered off in the glomerular filtrate.
• Kidney tubules are unable to reabsorb all the
glucose fast enough.
• High amount of glucose passes out in the urine.

12. AMOUNT OF URINE CAN BE AFFECTED
BY WEATHER
IN HOT WEATHER
• More sweat is secreted.
• Water potential of
blood is lower.
• Less urine is excreted.
IN COLD WEATHER
• Less sweat is secreted.
• Water potential of
blood is higher.
• More urine is excreted.