Segments kidney. The walls of the large and small
connecting tubules (a and b respectively), the circular structures, are formed by simple columnar epithelium (a) and simple cuboidal epithelium (b). The segments of the system are as follows:
Connecting tubule With respect to the
renal corpuscle, the
connecting tubule (
CNT, or
junctional tubule, or
arcuate renal tubule) is the most proximal part of the collecting duct system. It is adjacent to the
distal convoluted tubule, the most distal segment of the
renal tubule. Connecting tubules from several adjacent nephrons merge to form cortical collecting tubules, and these may join to form cortical collecting ducts (CCD). Connecting tubules of some
juxtamedullary nephrons may arch upward, forming an arcade. It is this "arcuate" feature which gives the tubule its alternate name. The connecting tubule derives from the
metanephric blastema, but the rest of the system derives from the
ureteric bud. Because of this, some sources group the connecting tubule as part of the
nephron, rather than grouping it with the collecting duct system. The initial collecting tubule is a segment with a constitution similar as the collecting duct, but before the convergence with other tubules. The "cortical collecting ducts" receive filtrate from multiple initial collecting tubules and descend into the
renal medulla to form medullary collecting ducts. It participates in the regulation of
water and
electrolytes, including
sodium, and
chloride. The CNT is sensitive to both
isoprotenerol (more so than the cortical collecting ducts) and
antidiuretic hormone (less so than the cortical collecting ducts), the latter largely determining its function in water reabsorption.
Medullary collecting duct "Medullary collecting ducts" are divided into outer and inner segments, the latter reaching more deeply into the medulla. The variable reabsorption of water and, depending on fluid balances and hormonal influences, the reabsorption or secretion of sodium, potassium, hydrogen and bicarbonate ion continues here. Urea passively transports out of duct here and creates 500mOsm gradient. The outer segment of the medullary collecting duct follows the cortical collecting duct. It reaches the level of the renal medulla where the
thin descending limb of loop of Henle borders with the
thick ascending limb of loop of Henle The inner segment is the part of the collecting duct system between the outer segment and the papillary ducts.
Papillary duct Papillary (collecting) ducts are anatomical structures of the
kidneys, previously known as the
ducts of Bellini. Papillary ducts represent the most
distal portion of the
collecting duct. They receive
renal filtrate (precursor to
urine) from several
medullary collecting ducts and empty into a
minor calyx. Papillary ducts continue the work of water reabsorption and electrolyte balance initiated in the
collecting tubules.
Medullary collecting ducts converge to form a central (papillary) duct near the apex of each
renal pyramid. This "papillary duct" exits the renal pyramid at the
renal papillae. The
renal filtrate it carries drains into a
minor calyx as
urine. The cells that comprise the duct itself are similar to rest of the collecting system. The duct is lined by a layer of
simple columnar epithelium resting on a thin
basement membrane. The epithelium is composed primarily of
principal cells and α-intercalated cells. The
simple columnar epithelium of the collecting duct system transitions into
urothelium near the junction of a papillary duct and a minor calyx.
Cells Each component of the collecting duct system contains two cell types,
intercalated cells and a segment-specific cell type: • For the connecting tubules, this specific cell type is the
connecting tubule cell • For the collecting ducts, it is the
principal cell. The inner medullary collecting ducts contain an additional cell type, called the
inner medullary collecting duct cell.
Principal cells The principal cell mediates the collecting duct's influence on sodium and potassium balance via
sodium channels and
potassium channels located on the cell's
apical membrane.
Aldosterone determines expression of sodium channels (especially the
ENaC on the collecting tubule). Increases in aldosterone increase expression of luminal sodium channels. Aldosterone also increases the number of
Na⁺/K⁺-ATPase pumps that allow increased sodium reabsorption and potassium excretion. Together, aldosterone and vasopressin let the principal cell control the quantity of water that is reabsorbed.
Intercalated cells Intercalated cells come in α, β, and non-α non-β varieties and participate in
acid–base homeostasis. For their contribution to acid–base homeostasis, the intercalated cells play important roles in the kidney's response to
acidosis and
alkalosis. Damage to the α-intercalated cell's ability to secrete acid can result in
distal renal tubular acidosis (RTA type I, classical RTA)(reference). The intercalated cell population is also extensively modified in response to chronic lithium treatment, including the addition of a largely uncharacterized cell type which expressed markers for both intercalated and principal cells. ==Function==