Page 96 - Biology_Form_3

Excretion

The process of urine formation reabsorption (selective reabsorption) and
Activity 4.3: secretion or removal of materials.

Demonstrating the process of urine Glomerular filtration(ultrafiltration)
formation This is the first step in the formation of urine
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Materials from the blood. Ultrafiltration occurs in the
Nephrone model, funnel, filter paper, glomerulus (plural: glomeruli). The lumen of
beaker, coloured water the afferent arteriole is significantly smaller

Procedure than the efferent arteriole. As a result, a
1. Place a filter paper inside a Bowman’s very high hydrostatic pressure is set in the
capsule of the nephrone model. glomerulus when blood passes through
it. This pressure forces plasma through
2. Pour colored water into the funnel the capillary walls in the glomerulus. The
and observe how it filters into the plasma contains water, glucose, amino
beaker. acids, urea, salts, uric acid, vitamins,

3. Compare the liquid obtained in a hormones, white blood cells, blood platelets
beaker with the poured liquid in the and plasma. Red blood cells and proteins
bowman’s capsule. are retained in the blood capillaries because
the cells and proteins are too large to pass
Safety precaution
through the wall. This process is called
(a) Ensure the beaker is properly placed ultrafiltration and the fluid which passes
to avoid water spillage. into Bowman’s capsule is known as the
(b) All safety gears should be properly glomerular filtrate. The glomerular filtrate
used. collects in the capsule and enters the renal
Questions tubule at the proximal convoluted tubule
1. How do nephrone structures facilitate which is coiled to increase the surface area
their functions? for reabsorption of substances.
2. Why is selective reabsorption Reabsorption (selective reabsorption)
necessary for body homeostasis? As glomerular filtrate moves along

the tubules, most of the substances are
The blood from afferent arteriole which reabsorbed back into the blood. When the
enters Bowman’s capsule is rich in glucose, blood passes in proximal convoluted tubule,
fatty acids, amino acids, vitamins, proteins, glucose, amino acids and some amount of
urea and excess salts. The formation of water are reabsorbed. As filtrate moves to
urine from blood involves three steps, the descending limb of loop of Henle, most
which all occur in the nephrones of the of the salts are reabsorbed then the filtrate
kidneys as shown in Figure 4.4. The steps moves to the distal convoluted tubule
are glomerular filtration (ultrafiltration), where more salts and water are reabsorbed.

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