Urinary System I Bio 178 A+P

• Functions of the Urinary System

  • Regulates blood plasma concentrations of ions
  • Adjusts and regulates blood volume and blood pressure
  • Regulation of blood pH
  • Conserve (and even make) important nutrients
  • Eliminate waste, including toxic substances and drugs
  • Synthesis of hormones (erythropoietin, stimulates RBC production), and assist in production of Vitamin D
• Structures of the Urinary System

  • Kidneys
  • Ureter
  • Bladder
  • Urethra
• Kidneys


  • Paired, processes blood plasma
• Ureter


  • Transports urine to the urinary bladder
• Bladder


  • Stores urine, humans don't modify urine in bladder
• Urethra


  • Transport of urine during micturition/urination
• External Anatomy of Kidneys

  • Paired (in most people)
  • Outer fibrous capsule protects the kidneys
  • Surrounded by perinephric adipose tissue capsule
  • Medial hilum where renal artery enters and renal vein and ureter leaves
  • Inner layer of capsule folds into the hilum and creates renal sinus (cavity)
• Sectional Anatomy of Kidneys

  • Renal cortex - superficial layer
  • Renal medulla - darker in color (more blood vessels), divided into several renal pyramids that point toward the renal sinus
  • Renal columns separate adjacent renal pyramids
  • Urine produced in each pyramid empties into minor calyx, and several of these merge into a major calyx
  • Major calyces combine to form the renal pelvis, which merges with the ureter
  • Renal arteries/renal veins pass through the renal sinus, which often contains adipose tissue to protect and maintain the position of structures
• Types of Nephrons


  • Cortical nephrons - 85% of nephrons found in cortex with a portion of their loop of Henle dipping into medulla
  • Juxtamedullary nephrons - 15% of nephrons dip down deep into pyramids of the medulla
• Renal Corpuscle


  • Fused, where filtration occurs
  • Consists of afferent/efferent arterioles, glomerulus (capillary network), and Bowman's capsule (expanded region of renal tubule, simple squamous epithelium)
• Parts of Renal Tubule

  • Proximal convoluted tubule (PCT) - simple cuboidal
  • Loop of Henle (descending/ascending) - simple cuboidal/simple squamous
  • Distal convoluted tubule (DCT) - simple cuboidal
  • Collecting duct - drains to papillary duct and thence to minor calyx
• Capillaries in Nephron


  • Two capillary beds in series (portal system)
  • Glomerular capillaries
  • Peritubular capillaries
  • Vasa recta - extension of peritubular capillaries, surrounds long loop of Henle in juxtamedullary nephrons
• Blood Supply to Kidneys

  • Arterial: renal art. -> segmental arteries -> Interlobar arteries -> Arcuate arteries -> Radiate arteries -> afferent arteriole -> glomerulus
  • Portal system: glomerulus -> efferent arteriole -> peritubular capillaries and vasa recta (juxtamedullary nephrons only)
  • Venous: peritubular capillaries -> radiate veins -> arcuate veins -> interlobar veins -> renal vein -> inferior vena cava
• Ureters


  • Paired, muscular tubes that collect urine from renal pelvis and transport it to the urinary bladder
  • The ureteral opening in the bladder is slit-like to prevent backflow when the bladder contracts
  • Mucosa of the ureters is transitional epithelium
  • Lined by smooth muscles, peristalsis helps propel urine
• Urinary Bladder


  • A hollow, muscular organ that functions to temporarily store urine
  • Position differs slightly between males and females
  • Ureteral openings are located near the base, at the corners of the trigone
  • Opening to the urethra (the neck of the bladder) contains a muscular internal urethral sphincter to regulate urine flow
• Histology of Bladder


  • Mucosa of transitional epithelium
  • Submucosa containing blood vessels/nerves/lymph vessels
  • Muscularis layer (detrusor muscle) with three layers of single unit smooth muscle
• Urethra


  • Extends from the neck of the bladder to the exterior of the body
  • Passes through urogenital diaphragm, where the external urethral sphincter (skeletal muscle) is located
  • Mucosa begins as transitional epithelium near the neck of the bladder, changing to simple squamous for most of its length
  • Differs in length and function between males and females
• Micturition Reflex


  1. Peristaltic waves move urine to the bladder

  2. Increased urine in the bladder initiates the micturition reflex involving both the spinal cord and brain

  3. Spinal reflex: Stretch receptors send signals to the sacral spinal cord, PNS fibers signal detrusor to contract

  4. CNS pathway: Interneuron sends signal to thalamus and cerebral cortex, can initiate voluntary relaxation of external urethral sphincter
• Filtration


  Occurs in the renal capsule only, produces a filtrate that resembles plasma but without most of the plasma proteins
• Selectivity of Filtration


  • Fenestrated capillary epithelium - pores let water, ions and small molecules through
  • Basal lamina - acts like a mesh or sieve, excludes most proteins
  • Bowman's capsule epithelium - podocytes create filtration slits
• Only a portion of the blood plasma actually leaves the circulation and enters the tubule (about 1/5th of the total plasma volume), this is called the filtration fraction</b>
• Filtration fraction can be regulated by changing the filtration barriers and by changing the filtration pressures
• Filtration


  1. Glomerular hydrostatic pressure

  2. Colloid oncotic pressure

  3. Hydrostatic pressure in Bowman's capsule

  4. Net filtration pressure
• Glomerular hydrostatic pressure (Pglom)


  Glomerular capillary pressure, normally around 50 mmHg, provides driving pressure to move fluid from capillary to tubule
• Colloid oncotic pressure (glom)


  Higher concentration of proteins in plasma than in filtrate, keeps fluid in capillary (pulls fluid back, negative pressure), approximately 25 mmHg
• Hydrostatic pressure in Bowman's capsule (PCapsule)

  The fluid in Bowman's capsule and the tubule exerts a hydrostatic pressure as well, approximately 15 mmHg that resists fluid flow out of the glomerular capillaries
• Net filtration pressure


  Pglom - glom - Pcapsule + capsule = 10 mmHg
• Careful regulation of mean arterial pressure and associated cardiovascular mechanisms, along with renal mechanisms, exist to ensure that the net filtration pressure is about 10 mmHg
• Glomerular Filtration Rate (GFR)


  The amount of fluid entering the nephron per unit time, a measure of the effectiveness of kidney filtration
• Factors influencing GFR


  • Leakiness of the filtration apparatus (filtration coefficient)
  • Net filtration pressure
• GFR remains almost constant at 180ml/day as long as mean arterial pressure remains between 80-180mmHg
• Local regulation of GFR

  1. Myogenic response

  2. Tubuloglomerular feedback
• Myogenic response

  The intrinsic ability of the vascular smooth muscle of the afferent arteriole to respond to pressure changes (i.e., stretch)
• Tubuloglomerular feedback


  A local signaling mechanism in which the fluid flow through the tubule feeds back to regulate GFR in Bowman's capsule
• Macula densa


  • A group of cells in the ascending tubule
• Juxtaglomerular (JG) cells


  • A group of specialized smooth muscle cells in the adjacent wall of the afferent arteriole
• Systemic Control of Glomerular Filtration Rate

  1. Neural control

  2. Endocrine/hormone effects
• Neural control


  GFR can be modified through actions of sympathetic neurons that innervate both the afferent and efferent arterioles (causing vasoconstriction)
• Endocrine/hormone effects


  Important hormonal pathways that influence GFR, most importantly the renin-angiotensin-aldosterone pathway
• Every day 180 liters of renal filtrate is formed, yet we only produce ~1.5 liters of urine