Page 708 - Atlas of Histology with Functional Correlations

nephrons passes through the uriniferous and collecting tubules of the

kidneys, it undergoes significant changes in its content and volume
producing concentrated urine, containing metabolic waste products. The cells
of the proximal convoluted tubules show deep infoldings of the basal cell
membrane, between which are located elongated mitochondria and lateral
membrane interdigitations with neighboring cells. These features are

characteristic of cells involved in active transport of molecules and
electrolytes from the filtrate across the cell membrane into the interstitium.
The mitochondria supply the necessary ATP (energy) for active transport of

+
+
sodium by Na /K ATPase (sodium pump) that is located in the basolateral
regions of the cell membrane.
Reabsorption that is both active and passive of most of the substances

from the glomerular filtrate takes place in the proximal convoluted tubules,
which receive the glomerular ultrafiltrate from the capsular (urinary) space of
the Bowman capsule. As the glomerular filtrate enters the proximal
convoluted tubules, all glucose, proteins, and amino acids; almost all
carbohydrates; and about 75% to 85% of water and sodium chloride ions are

absorbed from the glomerular filtrate into the surrounding interstitium and
peritubular capillaries. The presence of long and closely spaced microvilli
(brush border) on proximal convoluted tubule cells greatly increases the

surface area and facilitates absorption of the filtered material. In addition, the
proximal convoluted tubules secrete certain metabolites, hydrogen, ammonia,
dyes, and drugs such as penicillin from the body into the glomerular filtrate.
The metabolic waste products urea and uric acid remain in the filtrate of the
proximal convoluted tubules and are eliminated from the body in the urine.

Loops of Henle

The descending and ascending loops of Henle of the juxtaglomerular
nephrons are long, extend deep into the medulla, and have different
permeabilities and different functions. As a result, hypertonic urine is

produced in the tubules by an osmotic gradient in the surrounding
interstitium from the cortex of the kidney to the tips of the renal papillae.
Sodium chloride and urea are transported and concentrated in the interstitial

tissue of the kidney medulla by means of a complex countercurrent
multiplier system, which creates a high interstitial osmolarity deep in the
medulla. The descending loop of Henle is permeable to water but much less
to sodium chloride, whereas the thin ascending limb is permeable to sodium

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