Introduction to Urinary System Function PDF

Introduction to the Function of the Urinary System Dr. Catherine McDermott Learning Objectives This lecture aims to introduce and discuss the following: Renal Physiology  Glomerular filtration  Tubular reabsorption and secretion  Urine formation Cont...

Introduction to the Function of the Urinary System Dr. Catherine McDermott Learning Objectives This lecture aims to introduce and discuss the following: Renal Physiology  Glomerular filtration  Tubular reabsorption and secretion  Urine formation Control of bladder filling and micturition Lecture learning outcomes After actively participating in this weeks lecture and completing additional tasks as indicated, you will be able to: Describe the functions of the urinary system Describe the structure of kidneys Explain the process of glomerular filtration Explain the process of tubular reabsorption and tubular secretion Explain the role of sympathetic, parasympathetic and somatic nerves in control of micturition Urinary System Functions Excretion  Removal of waste products from the body Regulator  Volume and chemical composition of blood Water, salts, acids and bases  Blood pressure Production of hormones  Renin  Erythropoietin Metabolizing vitamin D to active form Gluconeogenesis The Nephron Structural and functional units of the kidney  Over 1 million per kidney  Glomerulus Renal Corpuscle  Glomerular/Bowman’s capsule  Renal tubule Proximal convoluted tubule (PCT) Loop of Henle (descending and ascending limb) Distal convoluted tubule (DCT) Collecting duct The Nephron  The Nephron Cortical nephron  85% of nephrons  Small portion of Loop of Henle projects into outer medulla Juxtamedullary nephron  15% of nephrons  Arise near cortex-medullary junction  Important in producing concentrated urine  Loops of Henle deeply invade medulla  Nephron Capillary Beds Every nephron has two capillary beds: Glomerulus  High pressure capillaries  Site of filtration Fluids and solutes forced into glomerular capsule  Fed and drained by arterioles Afferent and efferent arterioles Peritubular capillaries  Arise from efferent arteriole  Low pressure porous capillaries Readily reabsorb water and solutes from filtrate back into blood Vasa recta – long straight vessels serving juxtamedullary nephrons Juxtaglomerular Apparatus Region where distal portion of ascending limb lies against afferent arteriole  Juxtaglomerular / granular cells Enlarged smooth muscle cells Secretory granules containing renin Mechanoreceptor that sense BP in afferent arteriole  Macula densa Closely packed cells of ascending limb Chemoreceptors Changes in NaCl of filtrate Vasoconstriction Kidney Physiology: Urine Formation 1200ml blood pass through the glomeruli each minute  120-125 ml forced into renal tubules Concentrates filtrate formed by glomerular filtration  Prevent excess fluid loss Valuable materials reabsorbed and retained Filtrate Cell and protein free blood Loses most ions, nutrients and water in the collecting ducts What remains is called urine  Mostly metabolic wastes and unneeded substances  Urine Formation – 3 Processes 1. Glomerular Filtration  BP forces fluids and solutes across glomerular capillaries in glomerular capsule 2. Tubular Reabsorption  Removal of water and solutes from filtrate into peritubular capillary 3. Tubular Secretion  Movement of solutes from peritubular capillaries into tubular fluid 1. Glomerular Filtration Passive filtration process Small molecules move down pressure gradient across filtration membrane  Water, ions (e.g. Na+, Cl-, K+), small organic molecules (e.g. glucose, amino acids, nitrogenous waste)  Many of these reabsorbed Proteins and blood cells remain in the blood More efficient filter than other capillary beds  Large surface area  High pressure Glomerular Filtration Driven by blood pressure Net Filtration Pressure (NFP)  Glomerular hydrostatic pressure (BP)  Blood colloidal osmotic pressure  Capsular hydrostatic pressure Net filtration pressure  10 mmHg Glomerular Blood Pressure BP in glomerulus ~ 60 mmHg Afferent arterioles  Short, large diameter  Blood at high pressure Efferent arterioles  Smaller diameter  Maintain blood pressure in glomerulus by restricting outflow Glomerular Filtration Rate (GFR) The volume of filtrate formed each minute Directly proportional to NFP Adults – 120-125 ml/min Regulation of Glomerular Filtration  Renal autoregulation  Neural control Extrinsic Controls Sympathetic activity ↓ GFR by constricting renal arterioles Stress / emergency  Hormonal control Renin-angiotensin mechanism ↓ pressure leads to production of angiotensin II Constrict arterioles  2. Tubular Reabsorption Useful tubule contents returned to the blood Begins in proximal convoluted tubule Selective transepithelial process Healthy human kidney  All glucose and amino acids are completely reabsorbed Reabsorption of water and ions regulated and adjusted  Hormonal control Transport by passive diffusion, facilitated diffusion, active transport  Sodium Reabsorption Na+ ions most abundant in the filtrate Na+ reabsorption is almost always by active transport  Na+ enters tubule cells  Na+ is actively transported across the basolateral membrane by Na+ / K+-ATPase pump  Na+ and water rapidly taken up by adjacent peritubular capillaries Active pumping of Na+ drives reabsorption of:  Water by osmosis  Cations and fat-soluble substances  Organic nutrients and selected cations by secondary active transport (symport or antiport) Transport Maximum (TM) Almost every substance that is reabsorbed using a transport protein has a TM Reflects number of transport proteins in renal tubule High TM value  Plenty of carriers  Glucose, amino acids Transporters saturated  Excess is excreted in urine  Hyperglycemia in uncontrolled diabetes mellitus Reabsorption in PCT Reabsorption in Loop of Henle Permeability of tubule epithelium changes Water reabsorption not coupled to solute reabsorption  Can leave descending but not ascending limb as aquaporins are scarce or absent  Opposite true for solutes Reabsorption in DCT and Collecting Duct Reabsorption from this point dependent on body’s specific needs  Regulated by hormones Aldosterone ADH ANP Parathyroid hormone (PTH) Reabsorption: Hormonal Regulators Anti-diuretic hormone (ADH)  Hypothalmic neurons called osmoreceptors monitor solute concentrations in the blood  Becomes too concentrated ADH released by posterior pituitary Targets collecting ducts via cAMP system Water reabsorbed from filtrate  Other stimuli include pain, low BP and drugs such as nicotine, barbiturates and morphine  Alcohol inhibits ADH release Reabsorption: Hormonal Regulators Aldosterone  Fine tunes reabsorption of remaining Na+  Adrenal cortex releases aldosterone Decreased blood volume or BP Low extracellular Na+ High extracellular K+  DCT and collecting duct Synthesize and retain more luminal Na + and K+ channels Basolateral Na+ / K+-ATPase pumps Na+ retained in body Increase blood volume  Reabsorption: Hormonal Regulators Atrial natriuretic peptide  Hormone secreted by specialized cardiac muscle cells in the atria Increased BP  Inhibits the renin-angiotensin system and release of aldosterone  Inhibits Na+ and water reabsorption  ↓ blood volume and BP 3. Tubular Secretion Substances move from capillaries into renal tubule Essential for:  Removing substances not already filtered E.g. drugs and metabolites bound to plasma proteins  Eliminating waste products reabsorbed by passive processes Urea and uric acid  Removing excess K+  Controlling blood pH by secretion of H+ or HCO3- PCT is the main site of secretion  Except for K+  Secretion in DCT and collecting duct FUNCTION & REGULATION OF THE URINARY BLADDER Bladder Hollow, distensible muscular organ that acts as a temporary reservoir for urine The Urothelium: not just a passive barrier Inner epithelial layer: continuous with the ureters above and the urethra below Originally thought to solely act as a barrier to water and solutes between the lumen and underlying bladder muscle Urothelium plays active role in sensory detection and coordinating responses to chemical and mechanical stimuli  Close proximity to bladder nerves  Presence of receptors and ion channels, similar to those of afferent nerves  Releases mediators which stimulate tissue response Urothelial Signalling Mediators ATP Acetylcholine PGE2 Nitric Oxide Urothelial derived inhibitory factor (UDIF) – Prof Russ Chess-Williams Detrusor Muscle Contraction of bladder muscle responsible for the expulsion of urine During filling, detrusor cells relax and elongate to accommodate for the increase in bladder contents without increasing intravesical pressure Innervation of the Bladder Bladder Efferent nerves:  Sympathetic stimulation: causes detrusor relaxation during filling and contraction of the bladder neck/IUS Detrusor Hypogastric nerve muscle  Parasympathetic stimulation: causes detrusor contraction and thus micturition Pelvic nerve External urethral sphincter  Somatic innervation: voluntary control ACh Nicotinic receptor Pudendal nerve The Physiology of Storage and Micturition The phases of bladder storage and micturition are coordinated by the afferent and efferent nerves Storage:  As urine accumulates the bladder expands  Walls stretch and thin and rugae disappear  Detrusor muscle relaxed. Allows the bladder to store urine without a significant increase in pressure  IUS and EUS closed to maintain continence  400-500ml represents a normal micturition volume Pontine storage centre  ↓ parasympathetic and ↑ sympathetic activity to bladder Neural Control: Storage Noradrenaline causes bladder relaxation Noradrenaline causes internal sphincter contraction Micturition / Urination The act of emptying the bladder (voiding/micturition) 1. Detrusor muscle must contract 2. Internal urethral sphincter must relax/open 3. External urethral sphincter must relaxopen Afferent stretch receptors in bladder relay awareness of bladder fullness to the pons Pontine micturition centre  ↑ parasympathetic and ↓ sympathetic stimulation of the lower urinary tract Neural Control: Voiding Acetylcholine causes contractions of the bladder, ATP co-released Nitric Oxide relaxes the urethra References Human Anatomy and Physiology, 9th Edition by Marieb and Hoehen, Chapter 25, p. 955-977