HISTOLOGÍA: APARATO URINARIO

Biology Lecture: Urinary System Overview

In this lecture, the speaker delves into the intricacies of the urinary system, focusing on the kidneys' role in maintaining homeostasis and their endocrine functions.

Fundamentals of the Urinary System

• The urinary system comprises two kidneys, two ureters, a bladder, and a urethra.

• Kidneys play a crucial role in maintaining bodily homeostasis by regulating water, electrolytes, metabolites, and eliminating metabolic waste products.

• Apart from their excretory function, kidneys are also considered endocrine organs. They synthesize erythropoietin to regulate red blood cell production and renin to control blood pressure.

Structure of the Kidneys

• Kidneys are large reddish organs shaped like beans located on either side of the vertebral column in the abdominal cavity.

• The renal hilum allows entry and exit of renal vessels and nerves. It gives rise to the renal artery, renal vein, renal nerve, and pelvis that leads to the ureter.

• The renal capsule is a connective tissue covering surrounding each kidney. It consists of an outer layer of fibroblasts and collagen fibers and an inner layer of myofibroblasts.

Renal Cortex and Medulla

• The cortex is the outer reddish-brown region containing renal corpuscles, convoluted tubules, straight tubules, connecting tubules.

• Beneath the cortex lies the medulla which appears paler. It forms conical structures called pyramids that help in urine concentration.

Structure of the Kidney

The discussion focuses on the anatomical structure of the kidney, detailing the arrangement of various components such as pyramids, papilla, and renal columns.

Anatomical Structure of the Kidney

• The conical structure of the kidney is oriented towards the cortex, with its base in continuity with the cortex. The apex, known as papilla, points towards a minor calyx where multiple minor calyces unite to form major calyces.

• The papilla's end forms a cribriform area perforated by openings from collecting ducts that converge at this point before emptying into the minor calyx. This area with multiple openings is termed as cribriform area.

• In between the pyramids lies cortical tissue known as renal columns. These columns separate adjacent pyramids and are integral to the kidney's overall structure.

Nephron and Renal Filtration

• Each pyramid is divided into an outer medulla closer to the cortex, further subdivided into external and internal regions, and an inner medulla extending from medullary collecting duct to the apex (papilla). The nephron serves as both structural and functional units within kidneys.

• A nephron's initial segment is called a renal corpuscle responsible for glomerular ultrafiltration comprising two main components: glomerulus - a cluster of capillaries irrigated by an afferent arteriole and drained by an efferent arteriole; Bowman's capsule encloses this glomerulus forming part of renal corpuscle.

Renal Filtration Apparatus

Aquaporin 1 and Glomerular Basement Membrane

In this section, the discussion revolves around Aquaporin 1 and the Glomerular Basement Membrane, highlighting their composition and significance in renal function.

Aquaporin 1 Function

• Aquaporins facilitate rapid water movement through the endothelium.

Composition of Glomerular Basement Membrane

• The glomerular basement membrane is primarily composed of collagen type 4.

• Mutations in the gene encoding Alpha five chain of collagen type four lead to Alport syndrome, a hereditary glomerulonephritis.

Podocytes and Filtration Barrier

This segment delves into podocytes, their role in regulating glomerular fluid flow, and the components of the filtration barrier.

Podocyte Structure

• Podocytes regulate glomerular fluid flow.

• Podocytes emit evaginations around glomerular capillaries, forming secondary processes known as pedicels.

Filtration Barrier Components

• The filtration barrier includes slit diaphragms covered by thin diaphragms containing crucial molecules like nephrin for kidney filtration.

Membrana Basal Glomerular Functions

This part emphasizes the critical role of the glomerular basement membrane as a physical barrier and selective ion filter.

Functions of Glomerular Basement Membrane

• The glomerular basement membrane acts as a physical barrier and an ion-selective filter with specific layers like lamina rara externa and interna.

• Despite its protein restriction capacity, significant protein amounts cross daily but are reabsorbed at the proximal convoluted tubule level.

Glomerular Basement Membrane and Juxtaglomerular Apparatus

This section discusses the structure of the glomerular basement membrane, mesangial cells, and the juxtaglomerular apparatus in the kidney.

Glomerular Basement Membrane and Mesangial Cells

• The glomerular basement membrane is shared among multiple capillaries within the glomerulus. Mesangial cells are found within this shared membrane space.

• Mesangial cells have functions such as phagocytosis, endocytosis, defense against pathogens in the renal corpuscle, support of structural integrity, secretion of various substances like interleukin 1 and prostaglandin 2, and modulation of glomerular distension.

Juxtaglomerular Apparatus

• The juxtaglomerular apparatus consists of three components: the terminal portion of the distal convoluted tubule in continuity with afferent and efferent arterioles, extraglomerular mesangial cells (lacis cells), and juxtaglomerular cells derived from smooth muscle cells of the afferent arteriole.

• Smooth muscle cells modify into granule-containing secretory cells known as juxtaglomerular cells. Additionally, at this site, there are macula densa cells that regulate renal blood flow.

Renin-Angiotensin-Aldosterone System

This part delves into the renin-angiotensin-aldosterone system's role in maintaining sodium homeostasis and renal hemodynamics.

Renin-Angiotensin System Functionality

• The juxtaglomerular cells synthesize and release renin. Renin catalyzes angiotensinogen hydrolysis to produce angiotensin I.

• Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme in pulmonary capillaries. Angiotensin II acts on adrenal glands to stimulate aldosterone synthesis/release and causes vasoconstriction in arterioles.

Aldosterone Effects

• Aldosterone acts on distal nephron segments to enhance sodium reabsorption along with water reabsorption via an osmotic gradient mechanism while promoting potassium secretion.

• By increasing blood volume through sodium retention, aldosterone indirectly influences venous return, preload, stroke volume enhancement leading to increased cardiac output hence elevating blood pressure for long-term regulation.

Renal Tubular Function Overview

In this section, the speaker discusses the renal tubular function by detailing the different segments of the nephron and their significance in reabsorption processes.

Tubule Contorneado Proximal

• The proximal convoluted tubule is crucial for reabsorption, responsible for reclaiming a significant portion of filtered substances.

• It features a brush border with microvilli covered by glycocalyx, tight junctions, folds, basal infoldings, and basal striations.

• Key proteins in this segment include the sodium-potassium ATPase pump for sodium reabsorption and aquaporin 1 for water transport.

• Apart from sodium and water reabsorption, it also retrieves nearly 100% of glucose through sodium-glucose cotransporters like SGLT2.

• Additionally, it recovers around 98% of amino acids via ion exchange transporters.

Tubule Recto Proximal

• Following the proximal convoluted tubule is the less developed proximal straight tubule with high-affinity glucose cotransporters associated with sodium to facilitate glucose absorption into capillaries.

• This segment ensures any remaining glucose not reabsorbed in the proximal convoluted tubule is reclaimed.

Thin Descending Limb & Thin Ascending Limb

• The thin descending limb is highly permeable to water due to aquaporins, leading to substantial water reabsorption but minimal sodium and urea uptake.

• Conversely, the thin ascending limb is very permeable to sodium and chloride through cotransporters but impermeable to water. This results in ion retention and a shift from hyperosmotic urine to hypoosmotic urine.

• Cells in this segment produce uromodulin influencing sodium and chloride reabsorption.

Role of Uromodulin in Renal Function

This part delves into uromodulin's significance beyond aiding in sodium and chloride reabsorption within renal tubules.

Uroanalysis Significance

• Uromodulin production by epithelial cells impacts not only ion reabsorption but also plays a role in uroanalysis or general urine examination.

Detailed Overview of Renal Tubules and Blood Supply

In this section, the focus is on the structure and function of renal tubules, including the production of uromodulin in the Asa de Henle and the transport of ions like chloride, sodium, and potassium in different segments of the tubules.

Renal Tubule Structure and Function

• Uromodulin produced in the Asa de Henle forms a matrix that encloses components in a cylindrical shape.

• The uromodulin acts as a protein "tamord" that encapsulates pathogens, such as in urinary tract infections. Uromoglobulin plays a crucial role clinically in urine analysis.

• The distal straight tubule transports chloride, sodium, and potassium ions from the tubular lumen to the interstitium through electro-neutral transporters. Some potassium ions re-enter to create a positive charge for reabsorption of calcium and magnesium.

• The distal convoluted tubule primarily facilitates calcium reabsorption regulated by thyroid hormone. It also plays roles in sodium reabsorption, potassium secretion, bicarbonate ion reabsorption under acidic conditions, chloride reabsorption, and ammonium secretion.

• The connecting tubule serves as a transition region between the distal convoluted tubule and collecting ducts. It contributes significantly to potassium secretion.

Continuation: Renal Tubules Functionality

This section delves into further details about specific segments of renal tubules such as cortical collecting ducts with flat to cuboidal cells and medullary collecting ducts with cuboidal to columnar cells.

Specific Segments of Renal Tubules

• The cortical collecting duct is vital for regulating calcium absorption under thyroid hormone control.

• Cortical collecting ducts are lined with flat to cuboidal cells while medullary collecting ducts have cuboidal to columnar cells. Both types contain clear cells or principal cells responsible for water permeability via aquaporin 2 channels regulated by antidiuretic hormone (vasopressin).

Understanding Hormonal Regulation in Renal System

This part focuses on hormonal actions within renal structures like aldosterone's impact on sodium-potassium balance through gene expression modulation.

Hormonal Regulation Mechanisms

• Aldosterone binds to mineralocorticoid receptors within renal epithelial cell cytoplasm to enhance gene expression related to sodium channel proteins for sodium reabsorption and ATPase proteins for potassium-sodium exchange.

• Aldosterone's actions lead to increased blood volume and blood pressure due to enhanced sodium retention which subsequently promotes water retention. Aldosterone primarily affects principal cells within collecting ducts.

Insights into Interstitial Tissue Surrounding Nephrons

This segment explores interstitial tissue composition around nephrons comprising fibroblast-like cells synthesizing collagen & glucosaminoglycans along with macrophages aiding defense mechanisms.

Interstitial Tissue Composition

• Interstitial tissue surrounding nephrons consists of fibroblast-like cells producing collagen & glucosaminoglycans alongside macrophages involved in defense mechanisms.

Irrigation and Innervation of the Kidney

This section discusses the vascular supply and innervation of the kidney, detailing the intricate network of blood vessels and nerve structures involved in renal function.

Renal Vascular Supply

• The cortical capillaries drain into peritubular capillaries, which then connect to interlobular veins.

• The medullary vascular network drains directly into arcuate veins, leading to interlobular veins and eventually the renal vein.

• Proximal renal surface capillaries drain into stellate veins, connecting to interlobular veins and ultimately the renal vein.

Renal Innervation

• The renal plexus is primarily sympathetic, regulating smooth muscle contraction for urine production.

• Urine from nephrons flows sequentially through minor calyces, major calyces, and drains into the renal pelvis before entering ureters.

Structure of Urinary Tract Epithelium

This segment explores the composition and function of epithelial layers lining the urinary tract, focusing on their impermeability properties.

Urothelium Composition

• The urinary tract is lined with impermeable transitional epithelium called urothelium.

• Urothelium consists of superficial dome cells that change shape based on bladder fullness.

• Uroplakins in urothelium confer impermeability to small molecules like water and urea.

Underlying Layers

• Beneath urothelium lies a dense collagen layer known as lamina propria.

• Unlike mucosa-rich digestive systems, urinary tracts lack muscularis mucosae but contain smooth muscle layers.

Muscle Layers in Urinary Tract

This part delves into the unique arrangement of muscle layers within different segments of the urinary system compared to other organ systems.

Muscle Arrangement

• Urinary tract muscles have an inner longitudinal layer and outer circular layer configuration.

• The ureters exhibit three muscle layers: inner longitudinal, middle circular, and outer longitudinal (distal end only).

Innervation of Bladder

Discusses how sympathetic and parasympathetic fibers regulate bladder function through specific receptors for contraction or relaxation.

Bladder Inervvation

• s Sympathetic fibers originate from inferior hypogastric plexus releasing norepinephrine at beta 3 receptors for relaxation.

Detailed Overview of Urinary System Anatomy

In this section, the anatomy of the urinary system is discussed, focusing on the differences between male and female urethras.

Urethra Anatomy in Males

• The male urethra measures approximately 20 centimeters in length and consists of three portions:

• The prostatic urethra, measuring 3 to 4 centimeters, extends from the bladder neck through the prostate gland and is lined with urothelium.

• The membranous urethra, about one centimeter long, extends to the bulb of the penis where it forms the external sphincter in the perineal space.

• The spongy or penile urethra measures 15 centimeters and opens at the body surface near the glans penis. It transitions from pseudostratified columnar epithelium to stratified squamous epithelium distally.

Urethra Anatomy in Females

• In females, the urethra is significantly shorter, measuring between three to five centimeters. Similar to males, it transitions from urothelium to stratified squamous epithelium before termination.

• Secretory ducts from bulbourethral (Cowper's) glands and periurethral glands (Skene's glands) empty into the female urethra.

Conclusion

• The discussion concludes by highlighting that while there are differences in male and female urethral anatomy, both exhibit similar histological transitions towards their terminations.

• Reference is made to using Ross' Histology as a bibliographic source for further study on this topic.