The Renal Tubule

The renal (uriniferous8) tubule is a duct that leads away from the glomerular capsule and ends at the tip of a medullary pyramid. It is about 3 cm long and divided into four major regions: the proximal convoluted tubule, nephron loop, distal convoluted tubule, and collecting duct (see fig. 23.5). Only the first three of these are parts of an individual nephron; the collecting duct receives fluid from many nephrons. Each region of the renal tubule has unique physiological properties and roles in the production of urine.

The Proximal Convoluted Tubule The proximal convoluted tubule (PCT) arises from the glomerular capsule. It is the longest and most coiled of the four regions and thus dominates histological sections of renal cortex. The PCT has a simple cuboidal epithelium with prominent microvilli (a brush border), which attests to the great deal of absorption that occurs here. The microvilli give the epithelium a distinctively shaggy look in tissue sections.

The Nephron Loop After coiling extensively near the renal corpuscle, the PCT straightens out and forms a long U-shaped nephron loop (loop of Henle9). The first portion

9Friedrich G.J. Henle (1809-85), German anatomist of the loop, the descending limb, passes from the cortex into the medulla. At its deep end it turns 180° and forms an ascending limb that returns to the cortex. The nephron loop is divided into thick and thin segments. The thick segments have a simple cuboidal epithelium. They form the initial part of the descending limb and part or all of the ascending limb. The cells here are heavily engaged in active transport of salts, so they have very high metabolic activity and are loaded with mitochondria. The thin segment has a simple squamous epithelium. It forms the lower part of the descending limb, and in some nephrons, it rounds the bend and continues partway up the ascending limb. The cells here have low metabolic activity but are very permeable to water.

The Distal Convoluted Tubule When the nephron loop returns to the cortex, it coils again and forms the distal convoluted tubule (DCT). This is shorter and less convoluted than the PCT, so fewer sections of it are seen in his-tological sections. It has a cuboidal epithelium with smooth-surfaced cells nearly devoid of microvilli. The DCT is the end of the nephron.

The Collecting Duct The DCTs of several nephrons drain into a straight tubule called the collecting duct, which passes down into the medulla. Near the papilla, several collecting ducts merge to form a larger papillary duct; about 30 of these drain from each papilla into its minor calyx. The collecting and papillary ducts are lined with simple cuboidal epithelium.

The flow of fluid from the point where the glomeru-lar filtrate is formed to the point where urine leaves the

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Peritubular Capillaries Function

Peritubular capillaries

Interlobular artery

Interlobular vein

Renal corpuscle

Glomerular— capsule

-Proximal convoluted tubule

Glomerulus -

Juxtaglomerular apparatus

Efferent arteriole Afferent arteriole

Nephron loop

Descending-limb

Thick-

segment Thin -

segment

Ascending -limb

Collecting duct

Peritubular capillaries

-Proximal convoluted tubule

Efferent arteriole Afferent arteriole

Renal corpuscle

Glomerular— capsule

Glomerulus -

Juxtaglomerular apparatus

Nephron loop

Descending-limb

Thick-

segment Thin -

segment

Ascending -limb

Nephron Loop

Collecting duct

Juxtamedullary nephron

Cortical nephron

Figure 23.5 Structure of the Nephron. (a) Location of the nephrons in one wedge-shaped lobe of the kidney. (b) Structure of two nephrons. For clarity, vasa recta are shown only on the left and peritubular capillaries only on the right. Note that juxtamedullary nephrons are closer to the corticomedullary junction and have longer nephron loops than cortical nephrons. Vasa recta come only from the nephrons closest to the medulla. (c) The true proportions of the nephron loops relative to the convoluted tubules.

Juxtamedullary nephron

Cortical nephron

Cortex

Outer medulla

Inner medulla

Figure 23.5 Structure of the Nephron. (a) Location of the nephrons in one wedge-shaped lobe of the kidney. (b) Structure of two nephrons. For clarity, vasa recta are shown only on the left and peritubular capillaries only on the right. Note that juxtamedullary nephrons are closer to the corticomedullary junction and have longer nephron loops than cortical nephrons. Vasa recta come only from the nephrons closest to the medulla. (c) The true proportions of the nephron loops relative to the convoluted tubules.

Outer medulla

Inner medulla

Saladin: Anatomy & I 23. The Urinary System I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 23 The Urinary System 885

Afferent arteriole

Blood-flow

Efferent arteriole

Afferent arteriole

Blood-flow

Efferent arteriole

Saladin Pressure Afferent Arterioles

Proximal convoluted tubule

Blood flow

Glomerular capsule Parietal layer Capsular space

Podocytes of visceral layer Glomerulus

Proximal convoluted tubule

Blood flow

Distal convoluted tubule

Macula densa

Glomerular capsule Parietal layer Visceral layer Capsular space

Glomerular capillaries Proximal convoluted tubule

Peritubular capillaries (b)

Distal convoluted tubule

Macula densa

Glomerular capillaries Proximal convoluted tubule

Peritubular capillaries (b)

Peritubular Capillaries

Figure 23.6 The Renal Corpuscle. (a) Anatomy of the corpuscle; (b) light micrograph.

Figure 23.6 The Renal Corpuscle. (a) Anatomy of the corpuscle; (b) light micrograph.

body is: glomerular capsule ^ proximal convoluted tubule ^ nephron loop ^ distal convoluted tubule ^ collecting duct ^ papillary duct ^ minor calyx ^ major calyx ^ renal pelvis ^ ureter ^ urinary bladder ^ urethra.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • prisca
    Which region of renal tubule has abundunant microvilli?
    5 months ago

Post a comment