Functional Anatomy of the Kidney PDF

24.3 Functional Anatomy of the Kidney The functional anatomy of the kidney encompasses structures that include nephrons, collecting tubules, collecting ducts, and their associated structures. 24.3a Nephron LEARNING OBJECTIVES 8. Identify and describe a renal corpuscle and its components. 9. Identify...

24.3 Functional Anatomy of the Kidney The functional anatomy of the kidney encompasses structures that include nephrons, collecting tubules, collecting ducts, and their associated structures. 24.3a Nephron LEARNING OBJECTIVES 8. Identify and describe a renal corpuscle and its components. 9. Identify and describe the location and structure of the three components of a renal tubule. 10. Name and compare the two types of nephrons and the functional differences between them. The nephron (nef′ron; nephros = kidney) is the microscopic, functional filtration unit of the kidney. Each nephron consists of two major structures: a renal corpuscle and a renal tubule ( figure 24.4). All of the renal corpuscles and almost all of the renal tubules reside in the cortex (the exception being the nephron loop, which also extends into the medulla). Figure 24.4 Nephron Structure. The nephron is composed of a renal corpuscle and renal tubule. (a) The nephron is shown in its anatomic orientation relative to its position in the cortex and medulla of the kidney. (b) In this diagrammatic representation, the nephron has been straightened out, with each component color-coded for clarity. Note that neither the collecting tubule nor the collecting duct is part of a nephron. APR Module 13: Urinary: Animations: Kidney Microscopic Anatomy Page 953 Renal Corpuscle The renal corpuscle (kōr′pus-l; corpus = body, cle = tiny) is an enlarged, bulbous (round) portion of a nephron housed within the renal cortex. It is composed of two structures: the glomerulus and the glomerular capsule. The glomerulus (glō-mer′yū-lŭs; glomus = ball of yarn, ulus = small) is a thick tangle of capillary loops, which are also called glomerular capillaries. Blood enters the glomerulus by an afferent (af′er-ent; aferrens = to bring to) arteriole and exits by an efferent (ef′er-ent; efferens = to bring out) arteriole. The glomerular (glō-mer′yū-lăr) capsule (Bowman capsule) is formed by two layers: an internal, permeable visceral layer that directly overlies the glomerulus (described in section 24.5b) and an external, impermeable parietal layer composed of simple squamous epithelium. Between these two layers is a capsular space that receives the filtrate (described in section 24.5c), which is then modified to form urine. The renal corpuscle has two opposing poles: a vascular pole and a tubular pole. The vascular pole is where both the afferent and efferent arterioles attach to the glomerulus, and the tubular pole is where the renal tubule originates. Renal Tubule The renal tubule makes up the remaining part of a nephron and is composed of a simple (single layer) epithelium resting on a basement membrane. It consists of three continuous sections: the proximal convoluted tubule, the nephron loop, and the distal convoluted tubule. The convoluted tubules reside in the cortex, whereas the nephron loop typically extends from the cortex into the medulla. The proximal convoluted tubule (PCT) is the first region of the renal tubule. It originates at the tubular pole of the renal corpuscle and is composed of a simple cuboidal epithelium with tall, apical microvilli that markedly increase its surface area and thus its reabsorption capacity. When viewed with a light microscope, the apical surface of the proximal convoluted tubule looks fuzzy due to the brush border formed by these tall microvilli (see section 4.6c). Page 954 The nephron loop (loop of Henle) originates at a sharp bend in the proximal convoluted tubule. Each nephron loop has two limbs: a descending limb and an ascending limb that are continuous at a “hairpin turn” within the medulla. The descending limb extends from the proximal convoluted tubule to the tip of the nephron loop, whereas the ascending limb of the nephron loop extends from the tip of the nephron loop to the distal convoluted tubule. Portions of both limbs are classified as either thick or thin according to the epithelia composing them. The thick segments of each limb are composed of a simple cuboidal epithelium, whereas the thin segments of each limb are composed of a simple squamous epithelium. (Note that the lumen size is the same for the two segments.) The distal convoluted tubule (DCT) originates in the renal cortex at the end of the nephron loop’s thick ascending limb and extends to a collecting tubule. Like the proximal convoluted tubule, the distal convoluted tubule is composed of a simple cuboidal epithelium. However, the distal convoluted tubule epithelial cells are smaller and have only sparse, short, apical microvilli. Thus, the apical surface of the distal convoluted tubule appears clear and not fuzzy when viewed with a light microscope. Two Types of Nephrons The renal corpuscle and both the proximal and distal convoluted tubules are housed within the cortex; the nephron loops extend from the cortex medially toward or into the medulla, as previously described. The relative position of the renal corpuscle in the cortex and the length of the nephron loop are used to classify nephrons into two categories: cortical nephrons and juxtamedullary nephrons ( figure 24.5). Figure 24.5 Two Types of Nephrons. Cortical nephrons are located almost completely in the cortex and have short nephron loops that barely penetrate into the medulla. Juxtamedullary nephrons, which lie close to the corticomedullary junction, have relatively long nephron loops that extend deep into the medulla. Cortical nephrons are oriented with their renal corpuscles near the peripheral edge of the cortex and have a relatively short nephron loop that barely penetrates the medulla. Thus, the bulk of a cortical nephron resides within the cortex. Approximately 85% of nephrons are cortical nephrons. The remaining 15% of nephrons are called juxtamedullary (jŭks′tă-med′yŭ-lār-ē; juxta = near) nephrons. Their renal corpuscles lie adjacent to the corticomedullary junction, and they have relatively long nephron loops that extend deep into the medulla. Juxtamedullary nephrons are important in establishing a salt concentration gradient within the interstitial space that lies outside the nephron loop, the collecting tubules, and the collecting ducts—thus allowing for the regulation of urine concentration by antidiuretic hormone (ADH) (see sections 17.7b and 24.6f). WHAT DID YOU LEARN? 6 Diagram and label the components of a nephron similar to the one pictured in 7 figure 24.4b. Describe both the location and anatomic structure of each of the components of a nephron. 8 Compare and contrast cortical and juxtamedullary nephrons. Page 955 24.3b Collecting Tubules and Collecting Ducts LEARNING OBJECTIVES 13. Explain the relationship between collecting tubules and collecting ducts. 14. Compare and contrast the functions of the two types of specialized epithelial cells found within collecting tubules and ducts. Several nephrons drain into each collecting tubule (or junctional tubule). Each kidney contains thousands of collecting tubules, and a series of collecting tubules empty into larger collecting ducts. Both collecting tubules and collecting ducts project through the renal medulla toward the renal papilla. Numerous collecting ducts then empty into a papillary duct located within the renal papilla. Papillary ducts are the most distal portion of the collecting ducts and empty into a minor calyx (see figure 24.9). (Note: The nephrons that ultimately drain into a single collecting duct compose a lobule, which is a microscopic structure within the cortex.) The epithelial cells of the collecting tubules are cuboidal-shaped, but the cells become very tall columnar cells in the collecting ducts near the renal papilla. The striations seen within the renal pyramid are formed collectively by the straight ascending and descending tubules of nephron loops and the collecting ducts (see figures 24.3 and 24.4). The collecting tubules and collecting ducts contain two types of specialized epithelial cells called principal cells and intercalated cells ( figure 24.4b). Principal cells have cellular receptors to bind both aldosterone (released from the adrenal cortex) and antidiuretic hormone (released from the posterior pituitary). Intercalated cells (types A and B) are specialized epithelial cells that help regulate urine pH and blood pH. Both types of cells are described in section 24.6d, but for now keep in mind that type A cells eliminate acid (H+) and type B cells eliminate base ( ). Histology sections of the kidney are provided in figure 24.6. Figure 24.6 Histology of Renal Cortex and Medulla. (a) A photomicrograph of a section through the renal cortex, which houses renal corpuscles and most of the tubule of a nephron. Proximal convoluted tubules stain more darkly and appear fuzzier than distal convoluted tubules. (b) A photomicrograph of a transverse section through the renal medulla compares nephron loops and collecting ducts. (a, b) ©McGraw-Hill Education/Al Telser APR Module 13: Urinary: Histology: Renal Cortex: Renal Corpuscle WHAT DID YOU LEARN? 9 Differentiate between the functions of principal cells and the intercalated cells within the kidney. 24.3c Juxtaglomerular Apparatus LEARNING OBJECTIVES 13. Identify the location and describe the structure of the juxtaglomerular apparatus. 14. Explain the functions of both granular cells and cells of the macula densa. The nephron is “stretched out” in figure 24.4b to more clearly view its components. A representation of the normal orientation of a nephron would show the distal convoluted tubule of the nephron contacting the afferent arteriole of the same nephron ( figure 24.7). This accurate physical arrangement allows us to understand the anatomic features of a specialized region of the nephron called the juxtaglomerular (JG) (jŭks′tă-glō-mer′ū-lăr) apparatus, an important structure that helps regulate filtrate formation and systemic blood pressure. Figure 24.7 Juxtaglomerular Apparatus. With the nephron in the normal orientation, there is physical contact between the afferent arteriole and the adjacent distal convoluted tubule (DCT) forming the juxtaglomerular apparatus. The juxtaglomerular apparatus is composed of granular cells of the afferent arteriole and macula densa cells of the distal convoluted tubule. The juxtaglomerular apparatus monitors blood pressure and releases renin into the blood in response to either low blood pressure or stimulation by the sympathetic division of the ANS. (PCT = proximal convoluted tubule) APR Module 13: Urinary: Dissection: Lower Urinary: Sagittal view: Urinary Bladder Page 956 The primary components of the JG apparatus include both granular cells and macula densa cells. Granular cells (or juxtaglomerular cells) are modified smooth muscle cells of the afferent arteriole located near its entrance into the renal corpuscle. Granular cells have two functions: (a) They contract when stimulated either by stretch or by the sympathetic division of the autonomic nervous system, and (b) they synthesize, store, and release the enzyme renin (re′nin). Renin is required in the production of angiotensin I, which is then converted by angiotensin-converting enzyme (ACE) to angiotensin II (see section 20.6b and figure 20.15). The macula densa (mak′ū-lă = spot, den′să = dense) is a group of modified epithelial cells in the wall of the distal convoluted tubule where it contacts the granular cells. The cells of the macula densa are located only in the tubule wall adjacent to the granular cells of the afferent arteriole, and they are narrower and taller than other distal convoluted tubule epithelial cells. The macula densa cells detect changes in the sodium chloride (NaCl) concentration of tubular fluid within the lumen of the distal convoluted tubule. Macula densa cells signal granular cells in the afferent arteriole to release renin through paracrine stimulation (see section 17.3b). Extraglomerular mesangial cells ( figure 24.7), located just outside the glomerulus within the gap between the afferent arteriole and efferent arteriole, are also components of the JG apparatus. These cells communicate with the other cells of the JG apparatus via gap junctions and the release of paracrine hormones. Their specific function is not well understood. (Note: The mesangial cells [or intraglomerular mesangial cells] associated with the glomerulus that are shown in figure 24.7 are described in section 24.5b.) WHAT DID YOU LEARN? 10 What are the two primary cellular components of the juxtaglomerular apparatus, how is each stimulated, and what substance is released in response to stimulation?