Page 192 - 84 human physiolofy part-1

Chapter 10

Unlike the descending limb, the ascending limb of Henle's loop is impermeable to water, a
critical feature of the countercurrent exchange mechanism employed by the loop. The ascending
limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives
countercurrent exchange. In passing through the ascending limb, the filtrate grows hypotonic
since it has lost much of its sodium content. This hypotonic filtrate is passed to the distal
convoluted tubule in the renal cortex.

Distal Convoluted Tubule (DCT)

The distal convoluted tubule is similar to the proximal convoluted tubule in structure and function.
Cells lining the tubule have numerous mitochondria, enabling active transport to take place by the
energy supplied by ATP. Much of the ion transport taking place in the distal convoluted tubule is
regulated by the endocrine system. In the presence of parathyroid hormone, the distal convoluted tubule
reabsorbs more calcium and excretes more phosphate. When aldosterone is present, more sodium is
reabsorbed and more potassium excreted. Atrial natriuretic peptide causes the distal convoluted tubule
to excrete more sodium. In addition, the tubule also secretes hydrogen and ammonium to regulate pH.
After traveling the length of the distal convoluted tubule, only 3% of water remains, and the remaining
salt content is negligible. 97.9% of the water in the glomerular filtrate enters the convoluted tubules and
collecting ducts by osmosis.

Collecting ducts

Each distal convoluted tubule delivers its filtrate to a system of collecting ducts, the first segment
of which is the connecting tubule. The collecting duct system begins in the renal cortex and extends
deep into the medulla. As the urine travels down the collecting duct system, it passes by the medullary
interstitium which has a high sodium concentration as a result of the loop of Henle's countercurrent
multiplier system. Though the collecting duct is normally impermeable to water, it becomes permeable
in the presence of antidiuretic hormone (ADH). As much as three-fourths of the water from urine can
be reabsorbed as it leaves the collecting duct by osmosis. Thus the levels of ADH determine whether
urine will be concentrated or dilute. Dehydration results in an increase in ADH, while water sufficiency
results in low ADH allowing for diluted urine. Lower portions of the collecting duct are also permeable
to urea, allowing some of it to enter the medulla of the kidney, thus maintaining its high ion
concentration (which is very important for the nephron).

Urine leaves the medullary collecting ducts through the renal papilla, emptying into the renal
calyces, the renal pelvis, and finally into the bladder via the ureter. Because it has a different embryonic
origin than the rest of the nephron (the collecting duct is from endoderm whereas the nephron is from
mesoderm), the collecting duct is usually not considered a part of the nephron proper.

Renal Hormones

1. Vitamin D- Becomes metabolically active in the kidney. Patients with renal disease have
symptoms of disturbed calcium and phosphate balance.
2. Erythropoietin- Released by the kidneys in response to decreased tissue oxygen levels
(hypoxia).
3. Natriuretic Hormone- Released from cardiocyte granules located in the right atria of the
heart in response to increased atrial stretch. It inhibits ADH secretions which can contribute to
the loss of sodium and water.

192 | Human Physiology