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Biology for Secondary Schools

Finally, the filtrate moves to the collecting Removal of materials
duct where more water is reabsorbed. In the distal convoluted tubule, water is
reabsorbed into the blood by osmosis.
Reabsorption involves both diffusion and The tubule is highly coiled to increase
active transport. Active reabsorption is the the surface area for reabsorption. The
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transport against concentration gradient. glomerular filtrate then flows into the
This process uses energy in the form of ATP. collecting tubules where more water is
The waste-containing fluid that remains reabsorbed. At this stage the filtrate forms
after reabsorption is urine. Tubular secretion urine which flows into the collecting duct,
is another way of getting waste materials then into the pelvis of the kidney. It then
into the urine. It refers to the passage of flows into the ureter and is temporarily
certain substances out of the capillaries stored in the bladder. When the bladder is
directly into the renal tubules. For example, full, impulses are sent to the brain to create
drugs such as penicillin are secreted into an awareness of the presence of urine in
the renal tubules from the capillaries. Urea the bladder. A person then feels the need
and uric acid that may have been reabsorbed to urinate. During urination, the bladder
are also secreted. Excess potassium ions muscles contract, sphincter muscles relax,
are also secreted into the urine. Tubular and the urine is expelled via the urethra.
secretions maintain the pH of the blood. Urine formation is essential for maintaining

homeostasis which is the ability of the body
to maintain constant internal environment.
Bowman’s capsule
Potassium and
Hydrogen ions
Drugs and poisonous material Distal Urea
Proximal convoluted
convoluted tubule tubule

Glomerulus
Water
Cortex Glucose Amino acid Calcium Salt Water

Medulla Ascending loop
Descending loop of Henle
of Henle Water

Water Salts Collecting
duct

Figure 4.4: Urine formation

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