A. Loops of Henle

A. Loops of Henle: consist of three portions
1. Descending segment: The epithelial cells of the thin descending segment of the loop of Henle are very thin with no brush border and very few mitochondria. They are highly permeable to water but much less permeable to urea, sodium and most other ions.
2. Ascending thin segment: The epithelial cells of ascending thin segment are less permeable to water but more permeable to urea than descending portion.
3. Ascending thick segment: The epithelial cells of the ascending thick segment are similar to those of proximal tubules except that they have a rudimentary brush border and much tighter tight junction. The cells adapted for strong active transport of sodium, potassium and possibly chloride ions, impermeable to both water and urea.
Macula Densa: The thick ascending segment ascends back to the same glomerulus through angle between the afferent and efferent arterioles. The cells of this portion of thick ascending segment which are incomplete attachment with the epithelial cells of the afferent and efferent arterioles are called Macula Densa.
The specialized smooth muscle cells of afferent arterioles that come in contact with the macula densa are called Juxtaglomerular cells which contain renin granules suggesting that these cells secreting a renin toward the arterioles.
Macula densa, Juxtaglomerular cells and few granulated cells between them are known as Juxtaglomerular complex which has a dense adrenergic neural innervations.
Basic mechanism of absorption and secretion of loop of Henle:
• About 20% of filtered water is reabsorbed in the thin descending limb.
• About 25% of filtered Na, Cl and K(other ions such as Ca, HCO3, Mg) are absorbed in the thick ascending limb.
• Na ions are reabsorbed by secondary active to Na-K-2Cl co-transport and by Na-H secondary active counter-transport mechanisms.
Because the thick segment of ascending loop of Henle is impermeable to water, most of the water remain in tubule, tubular fluid becomes very dilutes as it flow toward the distal tubule (hypotonic).
B. Distal convoluted tubule (DCT):
They lie in the renal cortex. Its epithelium is similar to that of the proximal tubule but they have distinct brush border.
1. The first half of the distal tubule as the thick segment of ascending limb of the loop of Henle is absorbs most of ions but impermeable to both water and urea.
2. The second half of distal tubule and the cortical collecting tubule are similar to each other and both impermeable to urea and reabsorb sodium ions in an exchange with K ions under the effect of aldosterone, permeable to water only in the presence of antidiuretic hormone (ADH).
Basic mechanism of absorption and secretion of DCT:
1. Early part of the distal convoluted tubules:
Has characteristic of thick segment of ascending limb of loop of Henle.
It reabsorbs most of ions including Na, K, Cl, impermeable to water and urea.
2. The second half of the DCT and cortical collecting tubules are:
• Reabsorb of Na ion in an exchange with K ions (via Na-K ATPase pump) under effect of aldosterone hormone.
• Secretion of H ions by (H-ATPase pump), after being generated inside the cell by action of carbonic anhydrase on water and CO2 to form carbonic acid (H2CO3) which then dissociates into H ions and bicarbonate (HCO3) ions then HCO3 ions are reabsorbed across the basolateral membrane.
• Permeability to water is controlled by ADH (vasopressin).these segments are permeable to water in present ADH and impermeable to water in absent of this hormone.
C. Collecting tubules and ducts:
About eight distal tubules coalesce to form the collecting tubule which passes from cortex down ward into medulla. Where it becomes the collecting ducts the epithelium of the collecting duct is made up of:
1. P- cells: which are involved in Na ions reabsorption and water reabsorption under ADH stimulation.
2. Intercalated cells: which are concerned with acid secretion in exchange with bicarbonate ions.
Basic mechanism of absorption and secretion of medullary collecting ducts:
• Reabsorb of less than 10% of the filtered water and Na under the control of ADH.
• Secreting H ions against concentration gradient.
• Permeable to urea.
So it plays an important role in determining the final urine output of water and solutes.