Exam 4 Review:  Chapter 25:  The Nephron II - Renal Tubules

renal tubule - A general anatomical term referring to any portion of the tubule of the nephron or of the collection duct system of the kidney; more specifically referring to that part of the nephron distal to the renal corpuscle and glomerulus and consisting of the proximal convoluted tubule, the loop of the nephron and the distal convoluted tubule; these tubules are lined with a simple, usually cuboidal, epithelium and they use a variety of passive and active transport mechanisms to modify the composition of the plasma filtrate derived from the renal corpuscle and glomerulus converting it into urine.  For an excellent review, visit "Kidney and Urinary Tract Histology Study Guide."

proximal convoluted tubule - The first coiled segment of the renal tubule which receives the plasma filtrate from the renal corpuscle and glomerulus; it is the longest (~14 mm) and widest portion of the renal tubule; it is lined by a simple cuboidal epithelium with a brush border of microvilli which greatly increase the surface area of the inner lining of the tubule in order to maximize the number of active and passive transport proteins in the lining cells' membranes; it is the major site for tubular reabsorption (~80% of the filtrate volume) of water (obligatory water resorption), nutrients, electrolytes, small proteins, e.g., albumin, etc.; it also plays a lesser role in tubular secretion of certain electrolytes and nitrogenous wastes; its actions are regulated locally by autoregulatory mechanisms and it is responsive to a variety of hormones including aldosterone, atrial natriuretic peptide, and parathyroid hormone.

Proximal Convoluted Tubule

loop of Henle = nephron loop = loop of the nephron - The U-shaped second segment of the renal tubule which is located between the proximal and distal convoluted tubules; it may be located in the kidney cortex or it may extend into the renal pyramid in the medulla; it is the narrowest portion of the renal tubule with a smaller lumen as well as thinner lining cells, a simple squamous epithelium; it plays a role in the transport of ions and water and the concentrating of urine; it, along with the collecting ducts, are major sites of facultative water resorption; it is primarily responsive to antidiuretic hormone = ADH = vasopressin.

Loop of the Nephron

descending limb of the loop of the nephron (of Henle) - The proximal portion of the loop of the nephron which transports the filtrate from the cortex down into the medulla; its squamous cells are more permeable to water and somewhat less permeable to sodium and chloride ions; as a result, water tends to leave the descending limb by osmosis and enter the interstitial fluid of the medulla and from there into the venous capillary drainage and back to the systemic circulation.

ascending limb of the loop of the nephron (of Henle) - The distal portion of the loop of the nephron which transports the filtrate from the medulla back to the renal cortex; its squamous cells are less permeable to water and more permeable to sodium and chloride ions; as a result of the sodium-potassium ATPase pumps, sodium ions are extracted from the filtrate and replaced in the filtrate by potassium ions; the sodium ions, and chloride ions which follow the sodium ions passively, enter the interstitial fluid of the medulla and from there into the venous capillary drainage and are returned to the systemic circulation.

thin ascending limb - That more proximal portion of the ascending limb of the loop of the nephron (of Henle) which is lined by a simple squamous epithelium; it is impermeable to water at all times and insensitive to the action of antidiuretic hormone = ADH = vasopressin; however, it plays a major role in electrolyte pumping by the sodium-potassium ATPase pumps (sodium ions are extracted from the filtrate and replaced in the filtrate by potassium ions; the sodium ions, and chloride ions which follow the sodium ions passively, enter the interstitial fluid of the medulla and from there into the venous capillary drainage and are returned to the systemic circulation).

thick ascending limb - That more distal portion of the ascending limb of the loop of the nephron (of Henle) which is lined by a simple cuboidal epithelium; it is impermeable to water at all times and insensitive to the action of antidiuretic hormone = ADH = vasopressin; however, it plays a major role in electrolyte pumping by the sodium-potassium ATPase pumps (sodium ions are extracted from the filtrate and replaced in the filtrate by potassium ions; the sodium ions, and chloride ions which follow the sodium ions passively, enter the interstitial fluid of the medulla and from there into the venous capillary drainage and are returned to the systemic circulation).

distal convoluted tubule - The second or last coiled segment of the renal tubule which receives the plasma filtrate from the loop of the nephron; it is lined by a simple cuboidal epithelium lacking a brush border; it is the lesser site for tubular reabsorption (<20% of the filtrate volume) of water (obligatory water resorption), nutrients, electrolytes, small proteins, e.g., albumin, etc.; it also plays a major role in tubular secretion of certain electrolytes, particularly H⁺ ions,  and nitrogenous wastes as well as removing "toxins," (plant alkaloids and various drugs) from the plasma and actively transporting them into the urine; its actions are regulated locally by autoregulatory mechanisms and it is responsive to a variety of hormones including antidiuretic hormone = ADH = vasopressin, angiotensin II, aldosterone, aand atrial natriuretic peptide; it empties urine into the drainage system of collection ducts.

Distal Convoluted Tubule

collecting ducts - Those renal tubules which form a drainage system to collect urine from the distal portions of nephrons and transport that urine to the papillary ducts at the medullary tips of the renal pyramids where the urine passes into the minor calyxes; they are lined by a simple cuboidal to simple columnar epithelium; they participate in the active transport of sodium chloride and urea from the urine to the medullary interstitial space which maintains the osmotic gradient between the cortex and the medulla of the kidney; they are permeable to water only under the influence of antidiuretic hormone = ADH = vasopressin.

Collecting Ducts

            renal corpuscle → {afferent arteriole → glomerular capillaries → efferent arteriole} → proximal convoluted tubule  → loop of the nephron {descending limb of the loop of the nephron + ascending limb of the loop of the nephron}→ distal convoluted tubule → collecting duct system → papillary ducts

4. the four processes the nephron carries out to form urine and describe where these processes occur in the nephron.

renal corpuscle	plasma filtration
proximal convoluted tubule	tubular reabsorption (~80%; DCT and loop also play a role)
loop of the nephron	adjust water concentration in the urine (in cooperation with the collecting ducts; facultative water reabsorption regulated by ADH)
distal convoluted tubule	tubular secretion (PCT also plays a role)