Download Proximal convoluted tubule

Nephron
It is basic functional unit of the kidney and capable of forming urine.
There are about million nephrons in each kidney in human body. Kidneys
cannot regenerate new nephrons and their number decrease with aging.
Each Nephron Consist of:
1. Glomeruli forming from:
a) Bowman’s capsule is the invaginated end of the tubule that covered
the glomerulus.
b) Glomerulus (branching capillaries).
The pressure in the glomerular capillaries is higher than that in other
capillary beds. The glomerular capillaries membrane is different from
other capillary membrane by having three layers instead of two. These
three layers are Endothelial layer, Basement membrane, and layer of
Epithelial cells.
Despite the number of layers, the permeability of the glomerular
membrane is from 100-500 times as great as that the usual capillary
……………..why?
Due to presence of thousands of small holes which are called
fenestrate in the endothelial cells, presence of large spaces in the
basement membrane and incontinuity of cells that form the epithelial
layer.
2. Tubular system consist of:
a) Proximal convoluted tubule
b) Loops of Henle
c) Distal convoluted tubule
d) Collecting tubules and ducts
A. Proximal convoluted tubule (PCT):
They lie in the renal cortex along with the glomerulus. The epithelial cells
of them are highly metabolic cells, with large number of mitochondria
to support rapid active transport process. It contains a brush border due
to the presence of microvilli to increase the surface area of reabsorption.
Reabsorption in the proximal tubule is isotonic; osmolality of fluid in all
parts of the proximal tubule is that of plasma.
Basic mechanism of absorption and secretion of PCT:
Normally about 65% of filtered Na, water, and lower percentage of Cl,
almost all the filtered glucose, amino acid, organic acids, small amount of
protein, much of K, Ca, phosphate and urea are reabsorbed here.
1. Na ions are reabsorbed by passive diffusion, facilitated diffusion or
by co-transport with glucose, amino acids, and by counter-transport in an
exchange with H ions. All are depending primarily on the electrochemical
gradient of Na ions created by active Na-K ATPase pump.
2. Cl ions are reabsorbed by passive and co-transport mechanisms.
3. Water is reabsorbed by osmosis.
4. Proteins are reabsorbed through the brush border of the proximal
tubule by process of pinocytosis.
The concentration of Na and the total osmolarity remains relatively
constant (isotonic) because water permeability of the proximal tubules is
so great that water reabsorption proportional to Na reabsorption.
The proximal tubules is also the site for
secretion of organic acids
and bases (bile salts, oxalate, urate, catecholamines), drugs and
toxins.