Page 351 - Biology_F5

Biology for Advanced Level Secondary Schools

that is pressure filtration. This pressure as glucose are taken back to the blood
is known as hydrostatic pressure, and it but not lost in the urine. The cells of the
stems from blood pressure. Thus, the blood proximal convoluted tubule are adapted
brought into the glomerular capillaries by for re-absorption as follows:
the afferent arteriole is pumped by the (a) They have numerous microvilli and
heart at high pressure; which rises as the basal channels which increase the
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blood leaves the wide arteriole and enters surface area for the absorption process.
the narrow capillaries. (b) They have numerous mitochondria

to supply energy, which is constantly
The structures of the glomerulus and needed for active uptake of substances
Bowman’s capsule is adapted for filtration from the filtrate.
in that, endothelial layers of glomerulus
are very thin and have many pores to (c) Their basement membranes are very
allow the passage of a glomerular filtrate. close to the endothelial lining of the
A network of fibres such as collagen blood capillaries to ensure fast uptake
fibres, with spaces between them, allows and efficient transportation of the
the passage of small solute molecules in reabsorbed substances back into the
the filtrate. They restrict the passage of body.
blood cells that are larger than the pores. (d) Channel proteins are present for
Large molecules such as proteins are also transportation of substances like
restricted from passing through and are amino acids and ions from the cells
repelled by the negatively charged fibres. of the proximal convoluted tubule to
Moreover, the epithelial layer of the the spaces between these cells and the
Bowman’s capsule contains podocytes, basal channels.
the highly modified filtration cells. These (e) At the base of the microvilli, small
have extensions which interact to form proteins from the renal fluid are
filtration slits or pores to allow the passage removed by the process of pinocytosis.
of the glomerular filtrate. The proteins are then enclosed in
pinocytic vesicles and are finally
The proximal convoluted tubules broken down by hydrolytic enzymes
are mainly concerned with selective from the lysosomes (Figure 7.8).
reabsorption so that organic nutrients such

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