Urinary System

• The function of the urinary system:

  Helps maintain homeostasis by regulating water balance and by removing harmful substances from the blood.
• The blood is filtered by two kidneys, which produce urine, a fluid containing toxic substances and waste products. From each kidney, the urine flows through a tube, the ureter, to the urinary bladder, where it is stored until it is expelled from the body through another tube, the urethra.
• The urinary system consists of the kidneys, ureters, bladder, urethra.
• Kidney Location:

  • under back muscles,

  • behind parietal peritoneum (retroperitoneal)

  • just above the waistline;

  • The right kidney is usually a little lower and smaller than the left
• Three layers of connective tissue around the kidney:

  • Outer renal fascia holds the kidney to other structures.
  • Middle adipose capsule (renal fat pad) surrounds the kidney.
  • Inner renal capsule (fibrous capsule) surrounds the kidney
• Internal structure of the kidney

  1 Cortex- outer layer of kidney

  2 Medulla- inner portion of kidney

  3 Pyramids- triangular-shaped divisions of medulla

  4 Papillae- narrow, innermost ends of pyramids

  5 Columns - the cortical area between the pyramids.

  6 Minor calyx (8-18) - a funnel-shaped structure that surrounds the tip of the pyramid.

  7 Major calyx (2-3) - minor calyces join to form major calyces

  8 Pelvis- the funnel-shaped expansion of upper end of the ureter

  9 Calyces- divisions of the renal pelvis

  10 Hilum- depression where vessels enter/leave the kidney
• Nephrons are microscopic units of the kidneys responsible for filtering blood and processing urine.
• A nephron consists of the following parts:

  • Renal corpuscle
  • Renal tubule
• Renal corpuscle:

  • Bowman's capsule - the cup-shaped top of the nephron.
  • Glomerulus - a network of blood capillaries tucked into the Bowman's capsule.
• Renal tubule:

  • Proximal convoluted tubule - first segment of a renal tubule
  • Loop of Henle - extension of proximal tubule; consists of descending limb, loop, and ascending limb
  • Distal convoluted tubule - extension of ascending limb of Henle
  • Collecting tubule-straight extension of distal tubule
• Blood supply to nephron

  ➢ Afferent arteriole: enters Bowman’s capsule

  ➢ Glomerulus: coiled of capillaries in Bowman’s capsule

  ➢ Efferent arteriole: leaves Bowman’s capsule

  ➢ Peritubular capillaries: surrounds the nephron
• The functions of the kidneys:

  • Filtration
  • Secretion
  • Reabsorption
• Filtration:

  (chemicals leave blood and enter the nephron)

  ❖ small particles forced out from blood (glomerulus) into Bowman’s capsule: water, nutrients, urea, salts

  ❖ large particles remain in the blood (glomerulus): blood cells, proteins (albumins, prothrombin, fibrinogen, globulin)
• Secretion:

  The movement of substances into the urine in the distal and collecting tubules from blood in peritubular capillaries; hydrogen ions, potassium ions, and certain drugs are secreted by active transport; ammonia is secreted by diffusion.
• Reabsorption:

  Movement of substances out of renal tubules into the blood in peritubular capillaries; substances reabsorbed are water, nutrients, and various ions; water reabsorbed by osmosis from proximal tubules.
• Ureters

  Narrow long tubes with expanded upper end (renal pelvis) located inside the kidney and lined with mucous membrane. They have three layers: the inner mucosa, the middle muscularis, and the outer adventitia.
• Function of the ureters:

  Drain urine from renal pelvis to urinary bladder.
• Urinary bladder

  Elastic muscular organ, capable of great expansion. It is lined with mucous membranes arranged in rugae, like stomach mucosa. Circular smooth muscle fibers around the urethra form the internal urethral sphincter.
• The function of the bladder:

  1 Collection point for urine

  2 Stores urine before voiding

  3 Voiding
• Urethra:

  A narrow short tube from the urinary bladder to the exterior that is lined with mucous membrane.
•  In females, the urethra is about 3 to 4 cm (1.5 in) long and opens to the outside of the body between the vagina and the clitoris. In males, the urethra is about 15 to 20 cm (6 to 8 in) long and passes through the prostate gland, the urogenital diaphragm, and the penis.
• The functions of the urethra:

  • Serves as a passageway by which urine leaves the bladder to the exterior
  • Passageway by which male reproductive fluid leaves the body
• Micturition, or urination, is the process of releasing urine from the urinary bladder into the urethra.
• Regulatory sphincters:

  1 Internal urethral sphincter (involuntary)

  2 External urethral sphincter or compressor urethrae (voluntary)
• Emptying reflex:

  • Initiated by stretch reflex in the bladder wall
  • Bladder wall contracts
  • Internal sphincter relaxes
  • External sphincter relaxes and urination occurs
• Urinary retention:

  urine produced but not voided
• Urinary suppression:

  no urine produced but bladder is normal
• Incontinence:

  Urine is voided involuntarily

  May be caused by spinal injury, stroke, or weak sphincter muscle.
• How does the kidney regulate blood pressure?

  The renin-angiotensin-aldosterone system regulates the amount of extracellular fluid in the body. If blood pressure is low, renin is secreted by the juxtaglomerular cells of the kidneys to convert angiotensinogen to angiotensin I and then to angiotensin II. This narrows the blood vessels while aldosterone causes the kidneys to retain fluid and sodium. Thus, increasing volume and blood pressure.
• How do the kidneys regulate blood pressure?

  Also, a hormone called anti-diuretic hormone produced by the hypothalamus raises blood pressure by stimulating the kidneys to retain water. This raises blood pressure by increasing blood volume.