Page 191 - 84 human physiolofy part-1

The Urinary System

Any proteins that are roughly 30 kilodaltons or under can pass freely through the membrane.
Although, there is some extra hindrance for negatively charged molecules due to the negative charge of
the basement membrane and the podocytes. Any small molecules such as water, glucose, salt (NaCl),
amino acids, and urea pass freely into Bowman's space, but cells, platelets and large proteins do not. As
a result, the filtrate leaving the Bowman's capsule is very similar to blood plasma in composition as it
passes into the proximal convoluted tubule. Together, the glomerulus and Bowman's capsule are called
the renal corpuscle.

Proximal Convoluted Tubule (PCT)

The proximal tubule can be anatomically divided into two segments: the proximal convoluted
tubule and the proximal straight tubule. The proximal convoluted tubule can be divided further into S1
and S2 segments based on the histological appearance of it's cells. Following this naming convention,
the proximal straight tubule is commonly called the S3 segment. The proximal convoluted tubule has
one layer of cuboidal cells in the lumen. This is the only place in the nephron that contains cuboidal
cells. These cells are covered with millions of microvilli. The microvilli serve to increase surface area
for reabsorption.

Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular
capillaries, including approximately two-thirds of the filtered salt and water and all filtered organic
solutes (primarily glucose and amino acids). This is driven by sodium transport from the lumen into the
blood by the Na+/K+ ATPase in the basolateral membrane of the epithelial cells. Much of the mass
movement of water and solutes occurs in between the cells through the tight junctions, which in this
case are not selective.

The solutes are absorbed isotonically, in that the osmotic potential of the fluid leaving the proximal
tubule is the same as that of the initial glomerular filtrate. However, glucose, amino acids, inorganic
phosphate, and some other solutes are reabsorbed via secondary active transport through cotransport
channels driven by the sodium gradient out of the nephron.

Loop of the Nephron or Loop of Henle

The loop of Henle (sometimes known as the nephron loop) is a U-shaped tube that consists of a
descending limb and ascending limb. It begins in the cortex, receiving filtrate from the proximal
convoluted tubule, extends into the medulla, and then returns to the cortex to empty into the distal
convoluted tubule. Its primary role is to concentrate the salt in the interstitium, the tissue surrounding
the loop.

Descending limb
Its descending limb is permeable to water but completely impermeable to salt, and thus only
indirectly contributes to the concentration of the interstitium. As the filtrate descends deeper into
the hypertonic interstitium of the renal medulla, water flows freely out of the descending limb by
osmosis until the tonicity of the filtrate and interstitium equilibrate. Longer descending limbs
allow more time for water to flow out of the filtrate, so longer limbs make the filtrate more
hypertonic than shorter limbs.

Ascending limb

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