Lecture 29: The Renal System Flashcards Preview

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Give an overview of the urinary system's organization and anatomy

* The urinary system consists of:
- Two kidneys and ureters
- The urinary bladder and urethra
* In this lecture, we will briefly review the gross anatomy of the kidney and then focus on the histology and function of the urinary system with emphasis on the renal tubules.
* Each kidney has somewhat of an oval shape (kind of like a kidney bean) with a convex lateral border and a slightly concave or indented medial border through which pass the renal artery and vein and the ureter.
* A sagittal section of the kidney reveals the following regions; refer to Figure 14-1.
- An outer capsule surrounding the entire kidney except for the hilum.
- An outer cortical area beneath the capsule which has a cuplike form with the open side of the cup shape facing the hilum.
- A juxtamedullary cortex deep to the outer cortical area.
- A medullary area made up of triangular-shaped renal pyramids and divided into:
-- An outer medullary area deep to the juxtamedullary cortex.
-- An inner medullary area deep to the outer medullary area.
- The renal pelvis into which empty the renal pyramids.
* Each kidney is fed by a large renal artery branching directly off the abdominal aorta.

- See Slide 4


Describe the nephron

* The nephron is the functional unit of the kidney.
- There are approximately one million of these units per kidney.
* Each nephron consists of:
- A renal corpuscle composed of:
-- A cup-shaped capsule referred to as Bowman’s capsule
-- A glomerulus: This is a capillary knot situated between two arterioles that lies within the indentation of the Bowman’s capsule.
- A renal tubule that is a continuation of the Bowman’s capsule.
* Nephrons empty into collecting ducts which, in turn, empty into the renal pelvis.


Describe the two types of nephrons

* Cortical nephrons:
- The renal corpuscles of these nephrons are located in the cortex.
- Their glomeruli empty into peritubular capillary networks that surround the proximal and distal convoluted tubules of their own nephrons and those of surrounding nephrons.
- They have short loops of Henle that do not extend deeply into the medulla.
* Juxtamedullary nephrons:
- The renal corpuscles of these nephrons are located in the cortex next to the cortical-medullary junction (hence the name).
- Their glomeruli empty into vasa recta consisting of long, straight arterioles and venules that run parallel to the loops of Henle and collecting ducts.
- They have long loops of Henle that extend deeply into the medulla

- See slide 7


Describe the vascular supply to the kidney

* Branches of the renal artery within the kidney are organized into the following sequence (Refer to Figures 14-1 through 14-3):
- Interlobar arteries →
- Arcuate arteries →
- Interlobular arteries →
- Afferent arterioles → Glomeruli → Efferent arterioles →
- Vasa recta or peritubular capillaries →
- Interlobular veins → • Arcuate veins →
- Interlobar veins →
- Renal vein

- See Slide 9-10


Describe the glomerulus

* The glomerulus is a capillary bed inserted between two arterioles:
- Afferent arteriole
- Efferent arteriole
* Note that a typical capillary bed is inserted between an arteriole and a venule.
* Refer to Figure 14-2.
* The efferent arteriole feeds into either:
- A capillary loop, the vasa recta
- A capillary network, the peritubular capillaries

* Afferent arteriole:
- Juxtaglomerular cells:
-- Circular smooth muscle cells at vascular pole
-- Secrete renin (hypertensive factor)
* Glomerular formation of capillaries
* Efferent arteriole


Describe bowman's capsule in the renal corpuscle

* This is a double-layered, cup-shaped dilation of the nephron; it consists of an inner and an outer layer:
- Parietal layer:
-- This is an outer layer of simple squamous epithelium.
-- The squamous epithelium is continuous with the simple cuboidal epithelium of the proximal convoluted tubule.
- Visceral layer:
-- This is the inner layer of podocytes in contact with the endothelium of the glomerular capillaries.
* Bowman’s space is located between the parietal and visceral layers and is continuous with the lumen of the proximal convoluted tubule.


Describe the renal corpuscle

* The renal corpuscle is involved in producing a filtrate of blood.
* The rest of the nephron, including the convoluted portions and the loop of Henle, as well as the collecting tubules, is involved in modifying the filtrate.
* The formation of the urine involves three process:
- Filtration (occurring in the renal corpuscle)
- Reabsorption (occurring in the renal tubule)
- Secretion (occurring in the renal tubule)
* Whatever is filtered and secreted but not reabsorbed forms the excretory product of the kidneys – urine.

- See Slide 17


Describe the mesangium

* The mesangium is part of the glomerulus.
- The mesangiumis formed by mesangial cells embedded in a mesangial matrix.
- Intraglomerular mesangial cells are located between nearby capillaries in the glomerulus and cover endothelium not covered by podocytes.
- Extraglomerular mesangial cells are located between the afferent and efferent arterioles at the vascular pole and are associated with the juxtaglomerular apparatus.
* Characteristics and functions of mesangial cells:
- Contractile: utilize cytoskeletal contractile proteins to modify blood flow through glomerular capillaries.
- Phagocytic: take up worn out glomerular basal lamina for recycling and also phagocytose immunoglobulins trapped in the basal lamina.
- Proliferative
- Synthesize matrix and collagen
- Secrete prostaglandins and endothelins
- Respond to angiotensin II
- Provide mechanical support and regulate blood flow

- See slide 20-21 (Keep an eye on the macular densa, and mesangliar structures)


Describe the juxtaglomerular apparatus

* Components of the juxtaglomerular apparatus:
- Macula densa
- Extraglomerular mesangial cells
- Juxtaglomerular cells
* Macula densa:
- Formed by elongated, densely packed cells in the wall of the convoluted tubule. Polarity of these cells is reversed.
- Responds to changes in sodium and chloride concentration in the urine or hypotensin.
- Signals renin release from juxtaglomerular cells
- Involved in regulation of fluid-electrolyte balance and blood pressure regulation.
* Juxtaglomerular cells:
- Modified smooth muscle cells associated with macula densa and afferent arteriole.
- Secrete renin into the blood when stimulated by the macula densa.
- Innervated by sympathetic nerve endings which increase renin release.
- Covered in next lecture (See slide 24)


Describe the uriniferous renal tubule

* The simple squamous epithelium of Bowman’s capsule becomes continuous with the simple cuboidal epithelium of the proximal convoluted tubule. The proximal convoluted tubule is the first part of the renal tubule. Refer to Figure 14-4.
* The parts of the renal tubule are:
- The proximal convoluted tubule
- The descending limb of Henle (with thick and thin portions)
- The loop of Henle
- The ascending limb of Henle (with thick and thin portions)
- The distal convoluted tubule
* The distal convoluted tubule is continuous with the collecting duct.
- See slide 26


Describe the components of the renal filtration barrier

* Fenestrated endothelium of glomerular capillaries
* Fused basal laminae of endothelial cells and podocytes
* Filtration slits:
- Fenestrated endothelium of capillaries
- Basal lamina:
-- Contains type IV collagen, fibronectin, laminin, heparin sulfate
- Filtration slits created by adjacent pedicels of podocytes:
-- Covered by filtration slit diaphragm
-- Pedicels are attached to basal lamina via α3β1 integrins.


Describe the selectivity of the renal filtration barrier

* Water, glucose, and most ions pass through filter into Bowman’s space.
* Proteins are typically excluded from passing across the filter.
* Molecules with a large negative charge are excluded.

- See slide 28


Describe the characteristics of the proximal convoluted tube

* Extends from the urinary pole of the renal corpuscle to the beginning of the loop of Henle
* Characteristics:
- Simple cuboidal epithelium made up of truncated pyramidal cells
- Apical (facing lumen of tubule) tight junctions and brush border
- Basolateral infoldings and interdigitations
- Basal striations with abundant mitochondria (for active transport across basal membrane)
- Large, pale-staining nuclei
- Eosinophilic cytoplasm


Describe the functions of the proximal convoluted tube

* Reabsorbs most of the filtrate, including water.
* Removes essentially all the glucose and amino acids from the filtrate.
* Reabsorbs most of the sodium and chloride ions.
* Characterized by a complex set of transporters, co-transporters, channels, and exchangers.

- See Slide 32


Describe the characteristics of the Loop of Henle

* Extends from the proximal to distal convoluted tubules.
- Made up of descending limb and ascending limb.
* Each limb has a thin and a thick portion:
- Thick segments of the loop of Henle are composed of simple low cuboidal epithelium.
- Thin segments of the loop of Henle are composed of simple squamous epithelium.
- Cells of thick ascending limb have no brush border and more basal vertical striations.
- The ascending limb is impermeable to water.


Describe the functions of the Loop of Henle

* Descending thin limb is permeable to water and sodium and chloride ions.
* Ascending thin limb is impermeable to water and actively pumps chloride ion out of the tubule, allowing sodium ions to follow.
* The major function of the loop of Henle is to establish the counter-current exchange system which creates the osmotic conditions necessary to pull water out of the distal convoluted tubule and collecting duct in the presence of ADH.

- See Slide 46


Describe the distal convoluted tube

* Lined with simple cuboidal epithelium with smaller cells than those of the proximal convoluted tubule.
* The lumen is larger than that of the proximal convoluted tubule and there is no brush border.
* The cytoplasm is less eosinophilic than that of the proximal convoluted tubule.
* The distal convoluted tubule is impermeable to water except in the presence of ADH.
- Antidiuretic hormone (AKA: vasopressin)
- Derived from the posterior pituitary
- Functions to reduce water loss

- See Slide 48


Describe the Collecting Duct

* The collecting duct has a different embryological origin than that of the nephron.
* Its wall consists of simple cuboidal epithelium.
* Two major specialized cell types are associated with the collecting duct:
- Principal cells: Reabsorb sodium ions and water and secrete potassium via ATPase pump
- Intercalated cells: Secrete either hydrogen ion or bicarbonate ion

- See Slide 50


Describe the excretory passages

* Renal pelvis, ureter, and urinary bladder:
- Layers:
-- Mucosa
-- Submucosa(not clearly demarcated)
-- Muscularis
-- Adventitia (FECT external to muscularis; serosa over superior surface of bladder
- Mucosa:
-- Transitional epithelium (urothelium), basement membrane, lamina propria of FECT; some loose lymphoid tissue and a few smooth muscle cells
- Muscularis:
-- Inner longitudinal and outer circular smooth muscle coat in ureter
-- Lower third of ureter has a third external longitudinal coat.
-- Layers of smooth muscle in urinary bladder form detrusor muscle.

- See Slide 63


Describe the male urethra

* Prostatic urethra:
- Section of urethra passing through prostate gland
- Mucosa lined with transitional epithelium
- Highly vascularized with veins with abundant elastic tissue
* Membranous urethra:
- Tall pseudostratified columnar cells but variable
- Extends through urogenital diaphragm and receives striated muscle cells forming the external sphincter of the bladder
* Cavernous urethra:
- Lined with pseuduostratified epithelium with patches of stratified squamous epithelium
* Glands:
- Lucanae of Morgagni are invaginations of mucous membrane containing single or groups of intraepithelial mucous cells.
- Glands of Littre are branched tubular glands opening into the lacunae of Morgagni


Describe the female urethra

* Mucosa is lined primarily with stratified or pseudostratified columnar epithelium with intraepithelial nests of mucous glands.
* Lamina propria is highly vascularized with veins with an abundance of elastic tissue.
* Submucosais not discernible.
* Muscularis has an inner longitudinal and an outer circular layer of smooth muscle cells but is not well defined.
* Adventitia is not discernible.