URINARY

• Urinary system


  Consists of two kidneys that filter the blood, two ureters, a urinary bladder, and a urethra to convey waste substances to the outside

  
• Urinary system functions


  • Kidneys filter blood to keep it pure
  • Dispose of nitrogenous wastes from blood
  • Regulate the balance of water and electrolytes, acids and bases
  • Production of the hormones renin and erythropoietin

  
• Kidneys


  • Reddish brown, bean-shaped organ 12 centimeters long; enclosed in a tough, fibrous capsule
  • Positioned retroperitoneally on either side of the vertebral column between the twelfth thoracic and third lumbar vertebrae, with the left kidney slightly higher than the right

  
• Renal sinus

  Medial depression in the kidney leading to a hollow renal sinus into which blood vessels, nerves, lymphatic vessels, and the ureter enter

  
• Renal pelvis


  Subdivision of the renal sinus into major and minor calyces; small renal papillae project into each minor calyx

  
• Kidney structure


  • Two distinct regions: renal medulla and renal cortex
  • Renal medulla houses tubes leading to the papillae
  • Renal cortex contains the nephrons, the functional units of the kidney

  
• Renal blood vessels


  • Abdominal aorta gives rise to renal arteries leading to the kidneys
  • Renal arteries branch into successively smaller arteries: interlobar arteries, arcuate arteries, interlobular arteries, and afferent arterioles leading to the nephrons

  
• Kidney structures


  • Renal sinus
  • Hilum
  • Renal pelvis
  • Renal cortex
  • Renal medulla
  • Renal column

  
• Nephrons


  • A kidney contains one million nephrons, each of which consists of a renal corpuscle and a renal tubule

  
• Renal corpuscle


  The filtering portion of the nephron; made up of a ball of capillaries called the glomerulus and a glomerular capsule that receives the filtrate

  
• Glomerulus


  • A tuft of capillaries
  • Bowman's capsule: the container that surrounds the glomerulus, with two layers - visceral (inner) and parietal (outer)
  • Urinary space: the space between the two layers of Bowman's capsule where urine collects
• Podocytes


  Have a central cell body with primary processes; each primary process gives rise to secondary processes called pedicels (foot processes) that embrace the glomerular capillaries; foot processes from different podocytes interdigitate, with spaces between pedicels called filtration slits covered by a little diaphragm
• Renal filtration barrier

  • Fenestrated endothelial cells
  • Basal lamina
  • Filtration slits between foot processes (covered with a diaphragm)
• Glomerular basal lamina


  Made from fused basal laminae of endothelial cells and podocytes; lamina densa is a size barrier, lamina rara is a size and charge barrier
• Nephron structure


  • Renal tubule leads away from the glomerular capsule and first becomes a highly coiled proximal convoluted tubule, then leads to the nephron loop, and finally to the distal convoluted tubule
• Blood flow through nephron

  1. Afferent arterioles deliver blood to the glomeruli

  2. Efferent arterioles carry blood from the glomeruli to peritubular capillaries

  
• Anatomy of nephron

  • Glomerulus
  • Proximal tubule
  • Loop of Henle
  • Distal tubule
• Glomerulus


  The only place in the system where the blood is actually "filtered"; blood pressure is used to push plasma through capillary walls and into the Bowman's capsule
• Proximal tubule


  Nutrients (salts, vitamins, etc.) are moved out of the tubule through active transport; water follows the nutrients by osmosis
• Loop of Henle


  Tissue around the Loop of Henle is salty, from active transport and diffusion of sodium chloride; the salty conditions allow water to diffuse out of the loop
• Distal tubule


  Active transport is used to move more nutrients out of the concentrated urine; some ions, drugs, and toxins are actively pumped into the tubule
• Juxtaglomerular apparatus


  When sodium is high, macula densa cells trigger constriction of afferent arteriole, reducing blood flow to the glomerulus; when blood pressure falls, juxtaglomerular cells secrete renin, which increases blood pressure
• Collecting duct


  More water leaves the tube by osmosis, since the tube is surrounded by salty tissue; some urea leaves by diffusion, and may be cycled through the system
• Urine elimination


  Urine passes from the collecting ducts to the renal papillae, then to the minor and major calyces, and out the renal pelvis to the ureters, urinary bladder, and finally to the urethra, which conveys urine to the outside

  
• Ureters


  • Muscular tubes extending from the kidneys to the base of the urinary bladder; wall composed of three layers: mucous coat, muscular coat, and outer fibrous coat

  
• Urinary bladder


  A hollow, distensible, muscular organ lying in the pelvic cavity; internal floor includes the trigone, which is composed of the openings of the two ureters and the urethra; wall made up of four coats: inner mucous coat, submucous coat, muscular coat made up of detrusor muscle, and outer serous coat

  
• Urethra


  A tube that conveys urine from the urinary bladder to the outside; a muscular tube with urethral glands that secrete mucus into the urethral canal
• Female urethra


  • 3 to 4 cm long, external urethral orifice between vaginal orifice and clitoris, internal urethral sphincter detrusor muscle thickened, smooth muscle, involuntary control, external urethral sphincter skeletal muscle, voluntary control
• Male bladder and urethra

  • Urethra 18 cm long, internal urethral sphincter, external urethral sphincter, 3 regions: prostatic urethra (receives semen during orgasm), membranous urethra (passes through pelvic cavity), spongy urethra

  
• Voiding urine - micturition

  200 ml urine in bladder, stretch receptors send signal to sacral spinal cord, signals ascend to inhibitory synapses on sympathetic neurons and micturition center, signals descend to further inhibit sympathetic neurons and stimulate parasympathetic neurons, resulting in urinary bladder contraction and relaxation of internal urethral sphincter, external urethral sphincter relaxes
• Neural control of micturition
• Diseases and disorders of the urinary system

  • Acute renal failure
  • Chronic renal failure
  • Cystitis
  • Glomerulonephritis
  • Incontinence
  • Polycystic kidney disease
  • Pyelonephritis
  • Renal calculi