This document summarizes the anatomy and physiology of the nephron, specifically the collecting tubules and juxtaglomerular apparatus. The collecting tubule is the final segment of the nephron before entering the collecting duct system. It can be divided into the cortical and medullary collecting tubules. The cortical collecting tubule reabsorbs sodium, potassium, and water. The medullary collecting tubule is responsible for acidification of urine and is permeable to water and urea. The juxtaglomerular apparatus is formed by modified cells in the afferent arteriole and macula densa. It detects changes in chloride concentration and blood pressure, triggering the release of renin and regulating the

1. ANATOMY &
PHYSIOLOGY OF
NEPHRON: THE
COLLECTING
TUBULES &
JUXTAGLOMERULAR
APPARATUS Dr.Verdah Sabih
PGT Anesthesia
Holy family Hosp.Rwp

2. THE COLLECTING TUBULE
 Nephron:basic structural & functional unit of kidney.
 Collecting tubule:final segment of the tubule before
it enters the collecting duct system.
 Because of different embryonic origin(endoderm
rather than mesoderm),not considered a part of
nephron proper
 variably active portion of the Nephron
 last segment to save water for the body

4.  Each distal convoluted tubule delivers its filtrate to a
collecting duct, most of which begin in the renal
cortex and extend deep into the medulla
 collecting tubule can be divided into:
• cortical
• Medullary
 Together, they normally account for the
reabsorption of 5–7% of the filtered sodium load.

5. A. CORTICAL COLLECTING TUBULE
 consists of two cell types:
i. principal cells (P cells), :
 which primarily secrete K and participate in aldosterone-
stimulated Na- absorption& water from lumen
 (sites of action of the K-sparing diuretics)
 Reabsorption of Na+ generates a negative luminal voltage, which
provides the driving force for reabsorption of Cl- across the
paracellular pathway
ii. intercalated cells (I cells)
 which are responsible for acid–base regulation.
 Reabsorb K ions and secrete H+ ions into the tubular lumen
 Reabsorption of K+ is mediated by an H+,K+-ATPase located in
the apical cell membrane
 some I cells are capable of secreting bicarbonate ion in response
to large alkaline loads.


6. lumen
blood

8. B. MEDULLARY COLLECTING TUBULE
 courses down from the cortex through the
hypertonic medulla before joining collecting tubules
from other nephrons to form a single ureter in each
kidney
 principal site of action for antidiuretic hormone
(ADH), also called arginine vasopressin (AVP)
which stimulates the expression of a water channel
protein, aquaporin-2, in the cell membrane
 The permeability of the luminal membrane to water
is entirely dependent on the presence of ADH

10.  Dehydration increases ADH secretion whereas
adequate hydration suppresses ADH secretion
 This part of the nephron is responsible for
acidifying urine(secreting H+ against a large
concentration gradient-key role in regulating acid-
base balance)
 Unlike the cortical collecting tubule, the medullary
collecting duct is permeable to urea (special urea
transporters that facilitate urea diffusion across the
luminal and basolateral membranes)

11. ROLE OF THE COLLECTING TUBULE IN
MAINTAINING A HYPERTONIC MEDULLA
 Differences in permeability to urea in the cortical
and medullary collecting tubules account for up to
half the hypertonicity of the renal medulla
 Cortical collecting tubules are impermeable to urea,
whereas medullary collecting tubules are normally
permeable.
 In the presence of ADH, the innermost part of the
medullary collecting tubules becomes even more
permeable to urea
 when ADH is secreted, water moves out of the
collecting tubules and the urea becomes highly
concentrated

12.  Urea can then diffuse out deeply into the medullary
interstitium, increasing its tonicity.

14. THE JUXTAGLOMERULAR APPARATUS
 It is small organ formed of modified:
I. Macula densa cells.
II. Juxta- glomerular granular cells(segment of afferent
arteriole)
III. Extra-glomerular mesengial cells

15.  Juxtaglomerular cells contain the enzyme renin and
are innervated by the sympathetic nervous system.
 Release of renin depends on β 1 -adrenergic
sympathetic stimulation, changes in afferent
arteriolar wall pressure, and changes in chloride
flow past the macula densa.
 Renin released into the bloodstream catalyzes the
conversion of angiotensinogen, a protein
synthesized by the liver, to angiotensin 1