Urinary System

• Urinary system


  Consists of two kidneys, two ureters, one urinary bladder, and one urethra
• Functions of the urinary system

  • Remove wastes, toxins, water through process of excretion
  • Maintain homeostasis of many factors (blood pH, electrolytes, and blood pressure)
  • Participate in production of erythrocytes (synthesise erythropoietin)
  • Transport, store, and eliminate urine from the body
• Kidneys


  Help maintain homeostasis throughout the body by regulating ion levels in the blood, regulating blood volume and blood pressure, regulating blood pH, producing hormones, and excreting wastes
• Location of the kidneys


  • Lie on either side (lateral) of the vertebral column between the peritoneum and the back wall of the abdominal cavity
• Structure of the kidneys

  • Enclosed in a renal capsule, surrounded by adipose tissue
  • Three main areas: renal cortex, renal medulla, renal pelvis
• Renal cortex


  • Contains the glomerulus, Bowman's capsule, proximal convoluted tubule, and distal convoluted tubule of the nephrons
• Renal medulla


  • Contains the loop of Henle and collecting duct of the nephrons; renal pyramids, renal columns, major and minor calyces
• Renal pelvis


  • Where the ureter joins the kidney
• Blood supply of the kidneys

  • Blood enters through the renal artery and leaves through the renal vein
  • The functional units of the kidney are called the nephrons, numbering about a million in each kidney
• Functions of the nephron

  • Filtration, reabsorption, and secretion
• Structure of the nephron

  • Consists of a renal corpuscle where blood plasma is filtered, and a renal tubule into which the filtered fluid passes
  • The two parts of the renal corpuscle are the glomerulus and glomerular (Bowman's) capsule
  • The three main sections of the renal tubule are the proximal convoluted tubule, the nephron loop, and distal convoluted tubule
• Structure of the glomerulus


  • A loop of capillaries twisted into a ball shape, surrounded by the Bowman's capsule
  • The filtration barrier consists of endothelial cells of glomerular capillaries, glomerular basement membrane, and epithelial cells of Bowman's Capsule (podocytes)
• Podocytes


  • Specialised epithelial cells of Bowman's capsule which form the visceral layer of the capsule
  • Foot-like processes project from these podocytes and interdigitate to form filtration slits
• Juxtaglomerular apparatus


  • Lies close to the glomerulus and has three components: macula densa, juxtaglomerular cells, and extraglomerular mesangial cells
  • Macula densa cells detect low sodium concentration and stimulate renin secretion from juxtaglomerular cells, activating the RAAS pathway
  • Juxtaglomerular cells produce renin
  • Extraglomerular mesangial cells play a role in renal autoregulation of blood flow and regulation of systemic blood pressure
• Blood supply of the kidneys

  • Renal artery → Segmental artery → Interlobar artery → Arcuate artery → Interlobular (cortical radiate) artery → Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries
  • Interlobular vein → Arcuate vein → Interlobar vein → Renal vein
• Functions of the nephron and collecting ducts

  1. Glomerular filtration

  2. Tubular reabsorption

  3. Tubular secretion
• Glomerular filtration


  • Blood pressure forces water and most solutes in blood plasma across the wall of glomerular capillaries
  • Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys
• Tubular reabsorption and secretion

  1. Tubular reabsorption occurs in the PCT part of the renal tubule, where almost all nutrients are reabsorbed

  2. Filtrate enters the descending limb of the loop of Henle, and exits the ascending limb

  3. By the time the filtrate reaches the DCT, most of the urine and solutes have been reabsorbed

  4. Excretion of wastes occurs due to lack of reabsorption combined with tubular secretion

  5. Kidneys also maintain an acid-base balance by secreting excess H+ ions
• Bowman's capsule: ultrafiltration takes place
• Glomerulus: capillaries in which the pressure forces all solutes in the blood plasma to be forced through the capillary walls. This includes ions, amino acids, glucose, urea, water. Proteins and erythrocytes do not pass through as the
• PCT


  Part of the renal tubule where almost all nutrients are reabsorbed, either by passive or active transport
• Sodium (Na+)


  The most abundant ion, most of which is reabsorbed by active transport
• Loop of Henle


  • A U-shaped tube
  • Filtrate enters the descending limb and exits the ascending limb
  • Descending limb is water-permeable, water travels to interstitial space
  • Ascending limb is permeable to sodium and chloride ions
• Tubular Reabsorption and Secretion

  1. By the time filtrate reaches the DCT, most of the urine and solutes have been reabsorbed

  2. If body requires additional water, all of it can be reabsorbed at this point

  3. Excretion of wastes occurs due to lack of reabsorption combined with tubular secretion

  4. Products like metabolic wastes, urea, uric acid, and certain drugs, are excreted by tubular secretion

  5. Most of the tubular secretion happens in the DCT, but some occurs in the early part of the collecting duct

  6. Kidneys also maintain an acid-base balance by secreting excess H+ ions
• Bowman's capsule


  • Ultrafiltration takes place
• Glomerulus


  • Capillaries where pressure forces all solutes in blood plasma to be forced through, including ions, amino acids, glucose, urea, water
  • Proteins and erythrocytes do not pass through as they are too large
• Proximal Convoluted Tubule


  • First coiled region of the tubule, where products needed in the blood (ions, glucose, amino acids etc) are reabsorbed into the blood
• Loop of Henle


  • A long loop of tubule which spans the cortex and medulla, used to concentrate the urine
  • A salty environment is created in the medulla in order to cause water to osmose of water out of the nephron, and salts
• Distal Convoluted Tubule


  • Second coiled region of the tubule, where osmosis and diffusion of solutes occurs
  • Antidiuretic Hormone (ADH) affects the permeability
• Collecting Duct


  • Urine travels through the collecting duct down to the pelvis
• Angiotensin II


  • Enhances reabsorption of Na+ and Cl-
  • Stimulates the adrenal cortex to release aldosterone, which stimulates the collecting ducts to reabsorb more Na+ and Cl- and secrete more K+
• Atrial natriuretic peptide (ANP)

  Inhibits reabsorption of Na+ (and Cl- and water) by the renal tubules, which reduces blood volume
• Antidiuretic hormone (ADH)


  The major hormone that regulates water reabsorption, via negative feedback
• Renin-Angiotensin-Aldosterone System


  ACE-angiotensin-converting enzyme
• Low Blood Calcium Level

  • Parathyroid Hormone release by Parathyroid Gland
  • Ca reabsorption stimulated, P reabsorption inhibited
• Transportation, Storage, and Elimination of Urine

  1. Urine produced by the nephrons drains into the minor calyces, which join to become major calyces that unite to form the renal pelvis

  2. The two ureters transport urine from the renal pelvis of the right and left kidneys to the urinary bladder

  3. From the urinary bladder urine is then discharged from the body through the urethra
• The Ureter


  • Function: Responsible for transporting urine from the kidney to the bladder via peristaltic movements
  • Wall consists of 3 layers: transitional epithelium on the inside (mucosa), smooth muscle in the middle, and an outer layer of areolar connective tissue
• The Urinary Bladder


  • Function: To store urine prior to micturition
  • Mucosa contains transitional epithelium and rugae
  • Muscular layer consists of three layers of smooth muscle called the detrusor muscle
  • Outer coat is a fibrous covering
• The Urethra


  • In females, it lies directly behind the pubic symphysis and is embedded in the front wall of the vagina, between the clitoris and vaginal opening
  • In males, the urethra passes vertically through the prostate, the deep perineal muscles, and finally the penis
• Micturition


  The micturition reflex discharges urine from the urinary bladder by means of contraction of the detrusor muscle and relaxation of the internal urethral sphincter muscle, and by inhibition of the external urethral sphincter