AHE II 4.4 - URINARY SYSTEM (PART 1)

• Course objective: Recall the body's mechanisms for producing and expelling filtrate.
• Identify gross and microscopic structures of the urinary system.
• Trace the pathway of filtrate.

Lecture Learning Objectives

• Describe kidney and urinary tract functions.
• Explain the gross anatomy of the kidneys including location, surrounding tissues, and external/internal structures.
• Trace the blood supply to and within the kidney.
• Describe the microscopic anatomy of nephrons, focusing on location and function of structures, and their role in urine formation
• Trace the flow of filtrate and urine from renal corpuscles to outside the body.
• Explain the role of the juxtaglomerular apparatus, including its cells and hormones.
• Recall the autonomic innervation of the kidney.
• List the components of the urinary tract and their gross anatomy, innervation, blood supply, and roles in urination.
• Compare the male and female urethra.
• Describe the process of micturition, including the innervation and steps of the micturition reflex.

Urinary System Introduction

• All cells produce waste products that enter the bloodstream.
• The urinary system filters waste from the bloodstream.
• The organs are the kidney, ureters, urinary bladder, and urethra.

Kidney Functions

• Kidneys filter waste from the blood and convert the filtrate into urine.
• Kidneys regulate blood volume under hormonal direction, affecting blood pressure.
• Kidneys regulate erythrocyte production by measuring oxygen levels during filtration, and erythropoietin to increase RBC production when oxygen is low.
• Kidneys regulate inorganic ion balance (sodium, potassium, phosphate) and acid-base balance.

Urinary Tract Functions

• The urinary tract includes the ureters, bladder, and urethra.
• Ureters transport urine from the kidneys to the bladder for storage.
• The urinary bladder temporarily stores urine.
• Micturition (urination) is the expulsion of urine via the urethra which transports urine out of the body.

Kidney Location

• The kidneys are symmetrical, bean-shaped organs.
• Kidneys are retroperitoneal, with their posterior aspect contacting the posterior abdominal wall.
• The superior poles are near vertebral level T12, while the inferior poles are at L3.
• The right kidney is slightly inferior due to the liver.

Kidney Surrounding Tissue

• The kidney is surrounded and supported by several layers, from superficial to deep:
• Paranephric fat: Adipose tissue providing cushion and insulation.
• Renal fascia: Dense irregular connective tissue that encapsulates the adrenal gland and anchors the kidney to the posterior abdominal wall and peritoneum.
• Perinephric fat (adipose capsule): Adipose tissue that cushions and insulates.
• Fibrous capsule (renal capsule): Dense irregular connective tissue that covers the outer surface of the kidney, maintaining its shape and protecting it from trauma and infection.

Kidney External Anatomy

• Renal hilum: The concave medial border where vessels, nerves, and the ureter connect to the kidney.
• Renal sinus: Continuous with the hilum, housing renal arteries, veins, lymph vessels, nerves, renal pelvis, calyces, and adipose tissue.

Kidney Internal Anatomy

• Renal cortex: Outer region.
• Renal columns: Inward projections of the renal cortex into the renal medulla
• Renal medulla: Inner region.
• Renal pyramids: Bounded laterally by renal columns; each kidney contains 8-15 pyramids.
• Corticomedullary junction: Where the cortex and medulla meet, at the base of each renal pyramid.
• Renal lobe: Consists of a renal pyramid and the surrounding renal cortex.
• Filtrate flows through an increasing series of spaces:
• Renal papilla: The apex of each renal pyramid, opening into a minor calyx.
• Minor calyx: Funnel shaped space that merges with the major calyx
• Major calyx: The space where several minor calyces meet.
• Renal pelvis: Large, funnel-shaped space where major calyces drain, collecting urine and connecting directly to the ureter.

Kidney Blood Supply

• 20-25% of resting cardiac output flows through the kidneys.
• The right and left renal arteries arise from the lateral aspects of the abdominal aorta at the level of L1 or L2.
• Renal arteries branch into 2-5 segmental arteries prior to entering the kidney at the renal hilum.
• Branching vessels are often named for the region they traverse; veins and arteries often share the same name.
• Segmental arteries branch from the renal artery into the renal sinus.
• Interlobar arteries branch from segmental arteries within the renal sinus; they travel between kidney lobes in renal columns.
• Arcuate arteries arch around the base of the pyramids within the corticomedullary junction.
• Interlobular arteries branch from arcuate arteries into the cortex.
• Interlobular arteries extend afferent arterioles.

Key Arteries and Vessels

• Interlobular arteries extend afferent arterioles in the cortex.
• Afferent arterioles enter renal corpuscles, forming glomerulus capillaries where filtration occurs.
• Blood leaves the renal corpuscle through efferent arterioles.
• Peritubular capillaries are in the cortex, associated with proximal and distal convoluted tubules.
• Vasa recta are in the medulla; straight vessels associated with the nephron loop.

Microscopic Anatomy

• Nephron: The filtration unit of the kidney; approximately 2.5 million nephrons per kidney.
• Renal corpuscle: Glomerulus + Glomerular capsule.
• Renal tubules: PCT, nephron loop, and DCT

Nephron Types

• Cortical nephrons (85%): Primarily located in the cortex with short nephron loops that barely penetrate the medulla.
• Juxtamedullary nephrons (15%): Renal corpuscle close to the corticomedullary junction and have long nephron loops that extend deep into the medulla; blood supply via vasa recta.

Urine Formation

• Nephrons form urine through three interrelated processes:
• Filtration: Occurs in the renal corpuscle.
• Tubular reabsorption: Occurs in the renal tubules.
• Tubular secretion: Occurs in the renal tubules.

Renal Corpuscles

• Located in the renal cortex.
• Glomerulus: Ball-shaped capillary bed, supplied by the afferent and drained by the efferent arteriole.
• Glomerular (Bowman) capsule: Cup-shaped structure surrounding the glomerulus, has a parietal layer as the outer wall, and visceral layer, directly on the glomerulus.

Filtration and Podocytes

• Visceral layer: Comprised of podocytes (specialized cells) with a primary role in filtration.
• Pedicels: Long processes of podocytes that wrap and support glomerular capillaries.
• Filtration slits: Thin spaces between pedicels that allow materials from blood plasma to pass into the capsular space.
• Glomerular capillaries are fenestrated, facilitating filtration.
• Filtration membrane: Fenestrated capillaries + basement membrane + filtration slits of podocytes.
• Blood flows into the glomerulus via the afferent arteriole.
• Water and solutes from the blood plasma (filtrate) move into the capsular space of the renal corpuscle.

Filtrate Movement

• The afferent arteriole is larger than the efferent arteriole, creating higher pressure.
• The glomerulus/glomerular capillaries are "leaky."
• The filtrate contains too much water and vital nutrients and ions.
• Additional modification of filtrate occurs in renal tubules.

Tubular Reabsorption and Secretion

• Tubular reabsorption reclaims water and solutes that are filtered out of blood.
• Tubular secretion further modifies contents of the filtrate

Proximal Convoluted Tubule (PCT)

• Starts at the tubular pole of the renal corpuscle.
• Primary Function: Reabsorb most nutrients, ions, vitamins, plasma proteins, 60-65% water; returned to peritubular capillaries.
• Lined with tall microvilli to increase reabsorption.

Nephron Loop (Loop of Henle)

• Sharp bend in PCT.
• Primary Function: Assists in the reabsorption of sodium and chloride ions, which are returned to the blood via the vasa recta.
• Comprised of a descending and ascending limb and is defined on thickness created by epithelial type.
• Classified as either thick (cuboidal epi.) or thin (simple squamous epi.)
• Terminates at DCT.

Distal Convoluted Tubule (DCT)

• Begins in the cortex.
• Main role is to secrete ions (ex. potassium, hydrogen) into tubular fluid.
• Also reabsorbs water under the influence of ADH and aldosterone.
• Smaller cells with fewer microvilli than PCT.
• Drains into collecting tubules.
• Contacts the afferent arteriole at the vascular pole which is the macula densa of the juxtaglomerular apparatus.

Juxtaglomerular Apparatus Definition

• Is made up of macula densa cells and juxtaglomerular cells which act together to control blood pressure.
• Macula densa: Group of modified epithelial cells in the DCT that contact juxtaglomerular cells.
• Juxtaglomerular cells: Modified smooth muscle cells of the afferent arteriole near the entrance of the renal corpuscle.
• Macula Densa cells detect changes in tubular fluid which signals changes in blood volume or solute concentration.
• If changes are detected, juxtaglomerular cells are stimulated to release renin.
• Renin activates the renin-angiotensin pathway, which leads to aldosterone production in the adrenal cortex to increase blood ion concentration and blood volume.

Altered Filtrate

• After leaving the distal convoluted tubule (DCT) it becomes tubular fluid (also referred to as filtrate) passes through the collecting ducts and the fluid can not be altered. Finally entering the minor calyces.
• Filtrate becomes urine after leaving the collecting duct and entering minor calyces through the papillary duct which can no longer be modified.
• Minor calyces> major calyces > renal pelvis > ureter

Kidney Innervation

• Enters at the renal hilum with renal artery and vein.
• The kidneys have a renal plexus of nerves with sympathetic innervation from T10-T12 which causes vasoconstriction and can cause referred pain in T10-T12 dermatomes.
• Parasympathetic innervation has no known effect.

Urinary Tract Organs

• Ureters: Retroperitoneal tubes conducting urine from kidneys to urinary bladder.
• Connects directly to renal pelvis and exits at renal hilum.
• Urinary bladder: Expandable muscular container, stores urine.
• Ureteral openings are located in posterolateral walls, insert at oblique angle.
• Neck of bladder: Located inferiorly and connects to urethra.
• Urethra: Fibromuscular tube that connects the urinary bladder neck to the exterior of the body.

Ureter Layers & Composition

• Three tunics composing the walls:
• Mucosa: Transitional epithelium and lamina propria allow stretching.
• The folds in the mucosa allow urine to fill certain sections.
• Muscularis: Is made up of Inner longitudinal and outer circular layers that cause peristaltic waves to propel urine to move through the lumen.
• Adventitia helps anchor ureter to posterior abdominal wall.

Ureter Blood Supply & Innervation

• Blood supply: Closest major artery, which are the renal, aorta, common iliac, and internal iliac arteries.
• Autonomic nervous system with Parasympathetic and Sympathetic innervation- Referred pain from T11 and L2 dermatomes.

Urinary Bladder Location

• Retroperitoneal, located posterior to the pubic symphysis.
• Anterior in regards to Females: anteroinferior to uterus.
• Anterior in regards to Males: anterior to rectum and superior to prostate gland.
• Median umbilical ligament: Extends towards umbilicus.
• Trigone: Posteroinferior triangular area in urinary bladder wall and does not move as the bladder fills.
• Embryonically: All formed from distal ureters.

Urinary Bladder Wall

• Four tunics form the urinary bladder wall
• Mucosa: Transitional epithelium and highly vascularized lamina propria, allows greater distention
• Submucosa: Dense irregular CT to help support urinary bladder wall.
• Muscularis: Three layers of muscle collectively known as the detrusor muscle: inner longitudinal, middle circular, and outer longitudinal layers; creates the internal urethral sphincter.
• Adventitia: Formed from areolar connective tissue.
• Blood is supplied by the internal iliac aa.

Urethra Makeup

• Lined with mucus secreting urethral glands.
• Bundles of smooth muscle fibers and mucosa help propel urine to outside of the body.
• There are Internal urethral sphincters (involuntary) found at the neck of the bladder and are made of smooth muscle.
• External urethral sphincter: Aids when expelling urine, located below the internal sphincter, with voluntary movement that is made of skeletal muscle.

Female Urethra Differences

• Transports urine from the bladder to the exterior of the body.
• Between 3-5 cm long.
• External urethral orifice at the female perineum.

Male Urethra Differences

• Both reproductive and urinary functions that transports urine and semen through.
• 18-20 cm long.
• Three divisions:
• Prostatic urethra: Extends through prostate gland; is transitional.
• Membranous urethra: Shortest; extends through pelvic diaphragm.
• Spongy urethra: Longest, passes through the penis, ending at the external urtheral orifice.

Micturition/Urination Control

• Expulsion of urine.
• Controlled by parasympathetic and sympathetic axons.
• Is turned on by Parasympathetic movement; pelvic splanchnic nerve helps urine go though and stimulate contraction and allows relaxation of the internal.
• Is turn off through Sympathetic activity; which contracts/holds internal sphincter and inhibits muscle activity.

Micturition Reflex

• Bladder distension activates mechanoreceptors, signaling the micturition reflex center.
• Parasympathetic impulses both relax the internal urethral sphincter and contract the detrusor muscle.
• Voluntary relaxation of the external urethral sphincter allows expulsion of urine; abdominal and expiratory muscles help.
• As bladder empties, detrusor muscle relaxes, and the micturition reflex center is inactivated.