Chapter 25 The Urinary System Consists of two

Chapter 25 The Urinary System

Consists of two kidneys, two ureters, one urinary bladder and a urethra. Functions: 1. Regulates plasma concentrations of Na+, K+ , Cl-, Ca++ & Mg++, as well as glucose, AA & creatinine. 2. Endocrine function: regulates blood volume and blood pressure; produces erythropoietin and releases renin. Renin is produced by the Juxtaglomerular cells (JG) near afferent/efferent arterioles and regulates synthesis of Angiotensin-II. 3. Stabilizes blood p. H in concert with respiratory system and breathing (CO 2). 4. Eliminates organic waste products (e. g. ammonia as urea and uric acid, toxic substances and drugs). 5. Synthesizes calcitrol from Vitamin D and helps regulate Ca++ levels.

The Urinary System

The Kidneys Location: retroperitoneal in superior lateral abdominal cavity one on each side of body behind liver and stomach. Structure: Two bean shaped organs ~ 12 cm x 6 cm x 3 cm: GW ~ 150 gms. Surrounding Tissue Layers: – renal capsule – strong fibrous outer covering of kidney provides a barrier to infection from surrounding organs. – adipose capsule – middle layer of fat protects kidney from concussive blows and attaches kidney to posterior wall of abdomen. renal fascia – outermost layer of fibrous CT; surrounds kidney and adrenal glands; anchors them to surrounding structures.

Retroperitonel position

The kidneys Kidneys makeup ~ 0. 4% Body Weight but receive ~21% of Cardiac Output (5 l/min). Gross internal appearance of kidneys: Cortex (light pinkish in natural state) granular outer layer. Medulla (darker reddish brown) inner layer with cone shaped tissue masses (renal pyramids). Pelvis (inner core), conical shaped funnels that lead to the hilus and ureter. Often filled with adipose.

Blood flow to kidney Approximately 125 ml of blood is cleared by kidneys each minute =~ 140 -180 l/day. Vascular supply is via renal artery from aorta and return is via renal vein to inferior vena cava. Path is from aorta → renal artery → segmental arteries → interlobar arteries → arcuate arteries → interlobular arteries → afferent arteriole → glomerulus → efferent arterioles → peritubular capillaries or vasa recta → interlobular veins → arcuate veins → interlobar veins → renal veins → inferior vena cava.

Blood flow to kidney

Renal innervation The nerve supply to the kidneys is almost entirely sympathetically mediated. Nerves from the superior mesenteric ganglion enter at the renal hilus and regulate blood flow to the kidneys. If arterial blood flow drops the sympathetic nerves stimulate the release of renin which lead to synthesis of Angiotensin II (Ag-II), a very potent vasoconstrictor that will increase the blood pressure and consequently increase the blood flow.

The Nephron Consists of: Renal corpulse + renal tubule Afferent and efferent arterioles → glomerular capillaries which filter the blood plasma. Renal corpuscle = Glomerulus + Bowman’s capsule (glomerular capsule) - filters blood Renal tubule = Proximal and distal convoluted tubules plus loop of Henle. - Descending and ascending limbs of loop of Henle exhibit thick and thin segments.

The Nephron Renal corpuscle + Renal tubule

How urine is formed 1. Glomerular formation: blood plasma is filtered into glomerular space = glomerular filtrate. Lacks cells and proteins. Contains water + solutes (glucose, AA, FA, vitamins, nitrogenous watses, and electrolytes). 2. Tubular reabsorption of useful solutes (e. g. glucose, AA, electrolytes, etc. ) from filtrate → back into the blood. 3. Tubular Secretion of waste products (e. g. H+, drugs, toxic agents) from blood into filtrate for elimination from the body as urine. Water conservation: water is reabsorbed all along the nephron especially in collecting duct. (Helps maintain vascular volume and pressure).

Steps in urine formation

Kidney (Renal) nephrons The Nephron is structural and functional unit of kidney Cortical nephrons – ~ 2. 1 million/ kidney; The glomerulus is higher in the cortex with short loop of Henle dipping into the medulla. – Account for 85% of kidney nephrons – Dilute the urine but do not concentrate it Juxtamedullary nephrons – ~ 0. 4 million/ kidney. The glomerulus is near cortical/medullary border in cortex and loop of Henle is very long descending deep into the medulla. – Account for 15% of kidney nephrons – Functions in concentrating of urine

Renal nephrons Cortical nephron Juxtamedullary Medullary border

Cortical and juxtamedullary nephrons Renal corpuscles for both nephrons are all in the cortex. Loop of Henle dips into the medulla for both nephrons. Cortical nephrons loops are shorter and dip slightly into the medulla. Juxtamedullary nephron loops dip deeper into the medulla.

SEM of Glomeruli

TEM of a glomerular capillary shows podocyte with pedicels and filtration slits and pores.

Nephron histology Bowman’s capsule- inner (visceral) layer of podocytes wrapped around capillaries and outer (parietal) layer of simple squamous epithelium Proximal convoluted tubules (PCT)- consists of simple cuboidal epithelium with microvilli (brushed border). Distal convoulted tubule (DCT)- consists of simple cuboidal epithelium and no microvilli; (Affected by aldosterone and PTH). Loop of Henle – “U” shaped with the thick segment of cuboidal epithelium and thin segment of simple squamous epithelium. Collecting ducts are not part of nephron but empty into papillary duct and then minor calyx. They are lined with simple cuboidal epithelium and 2 types of cells (intercalated and principal cells). Effected by 2 hormones ANP & ADH

Glomerular capsule with parietal and visceral layers; afferent and efferent arterioles, JGA etc.

Cortical nephron -short loop of Henle with peritubular capillaries Functions to form urine.

Juxtamedullary nephron -Glomerulus is In cortical layer. Vasa recta goes deep into medulla. -Function is to concentrate urine and conserve water.

Juxtaglomerular apparatus Occurs at vascular pole of renal capsule. Consists of three cell types: 1. Macula densa group of sensory epithelial cells in the distal convoluted tubule, next to afferent arteriole MD monitors flow and urine composition. Communicates with the JG cells. 2. Juxtaglomerular (JG) cells = enlarged sm. m. cells on afferent and efferent arterioles. When stimulated they constrict to reduce flow through the glomerulus. JG cells also secrete renin into blood → AG-II. 3. Mesangial cells located between afferent and efferent arterioles and may modulate communication between the two arterioles. See next slide for clarification.

Glomerulus with Juxtaglomerular apparatus (Saladin 25. 9) Symp. Nerve JG cells Macula densa Afferent arteriole DCT

Histologic slide of a Glomerular capsule with parietal and visceral layers; afferent and efferent arterioles, Macula densa, JG cells, etc.

Nephron fctns. 1. Filtration 2. Reabsorption 3. Secretion

Summary of urine flow Afferent arteriole enters Glomerulus → glomerular filtrate → Proximal convoluted tubule → Descending loop of Henle → Ascending loop of Henle → Distal convoluted tubule → Collecting duct → → → Papillary duct → Minor calyx → Major calyx → Renal pelvis → Ureter → Urinary bladder → → Urethra → Outside.

Summary of urine flow

Ureters Muscular tube lined with transitional epithelium. Wall consists of two layers of smooth muscle (inner longitudinal and outer circular layer). Urine moves by peristalsis “milking action” from renal pelvis to bladder. Enter bladder from posterior and below. Small flap of mucosa acts as a valve. ~ 25 cm long; 1. 7 cm diam.

Urinary Bladder Muscular storage bag lined with transitional epithelium, with lamina propria underneath and detrusor muscle in wall. Detrusor muscle is 3 separate layers of smooth muscle. Rugae present when empty Vol. can hold up to 800 ml. when full. Two ureters and urethra form “trigone” at base of bladder.

Urethra Histology: First part of tract is transitional, → pseudostratified columnar → stratified squamous epithel. at external orifice. Males 3 areas: 1. Prostatic urethra 2. Membranous urethra 3. Spongy urethra in penis. ♂ urethra serves for urine excretion and sperm ejaculation. ♀ urethra for urine excretion only.

Bladder and urethra of female

Urethra Urine release is controlled by an internal and external urethral sphincter. Internal sphincter is a thickened extension of the detrusor muscle and is involuntarily controlled. External sphincter is composed of skeletal muscle and is under voluntary control. It is located where the urethra penetrates the pelvic floor.