The first topic that was covered by this chapter was the excretion of wastes
by the Renal system. The first step that occurs in the kidney deals with the
nephron, which is the functional unit of the kidney. In the glomerulus the
formation of urine begins with the passive filtration of plasma through the
pores that are found in the glomerulus. The plasma is forced through these pores
by hydrostatic pressure. The only things that determines if a molecule will pass
through the pores of the glomerulus is it's molecular weight. The lower the
molecular weight, the easier it will pass through the pores. Another
determining factor will be if a molecule is bound to a large molecule. If this
is true then passage through the pores will be hindered by the size of the
of the many ions, minerals and other nutrients that escaped in
the glomerular filtrate will need to be recovered.. Reabsorption
begins in the
tubules of the nephron. Anywhere from 65% to 90% of reabsorption
these structures. Active reabsortion is used to recapture glucose, proteins,
amino acids and other nutrients. Water and chloride ions are passively
reabsorbed by the establishment of osmotic and electrochemical gradients. Both
the Loop of Henley and collecting duct are used to establish these osmolar
gradients. The tubule has a brush border that will absorb proteins and
polypeptides through pinocytosis. These molecules are sometimes catabolised and
converted into amino acids. and returned to the blood. Sometimes the
accumulation of these proteins can lead to renal toxicity
A second process that occurs in the tubules is tubular secretion. This is
another mechanism used to excrete solutes. Secretion may be either passive or
active. Secretions include organic bases, which occur in the pars recta of the
proximal tubule. Secretions of weak bases and two weak acids occur passively.
Other mechanisms... [continues]
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