Ch14 urinary system

Functions of the Urinary System

• Excretion: removal of waste material from blood plasma and disposal of waste in urine
• Elimination: removal of waste from other organ systems
• Water balance: kidney tubules regulate water reabsorption and urine concentration
• Regulation of pH: volume, and composition of body fluids
• Production of erythropoietin for hematopoiesis and renin for blood pressure regulation

Anatomy of the Urinary System

• Kidneys: pair of bean-shaped organs located retroperitoneally, responsible for blood filtering and urine formation
• Renal capsule: layer of fibrous connective tissue covering the kidneys
• Renal cortex: outer region of the kidneys where most nephrons are located
• Renal medulla: inner region of the kidneys where some nephrons are located, and urine is collected to be excreted outward
• Renal calyx: duct-like sections of renal medulla for collecting urine from nephrons and directing urine into renal pelvis
• Renal pyramid: connective tissues in the renal medulla binding various structures together
• Renal pelvis: central urine collecting area of renal medulla
• Hilum: concave notch of kidneys where renal artery, renal vein, ureter, nerves, and lymphatic vessels converge
• Ureter: tubule that transports urine (mainly by peristalsis) from the kidney to the urinary bladder
• Urinary bladder: spherical storage organ that contains up to 400 ml of urine

Functions of Nephron Components

• Renal capsule: filteration of H2O and dissolved substances from the plasma
• Glomerulus: filtration of H2O and dissolved substances from the plasma
• Glomerular capsule: receives the glomerular filtrate
• Proximal convoluted tubule:
  • Reabsorption of glucose, amino acids, creatine, lactic acid, citric acid, uric acid, and ascorbic acid by active transport
  • Reabsorption of proteins by pinocytosis
  • Reabsorption of H2O by osmosis
  • Reabsorption of Cl- and other negatively charged ions by electrochemical attraction
  • Active secretion of substances such as penicillin and hydrogen ions
• Descending limb of nephron loop: reabsorption of H2O by osmosis
• Ascending limb of nephron loop: reabsorption of Na, K, and Cl- by active transport
• Distal convoluted tubule:
  • Reabsorption of Na by active transport
  • Reabsorption of H2O by osmosis
  • Active secretion of hydrogen ions
  • Secretion of K both actively and by electrochemical attraction
• Collecting duct: reabsorption of H2O by osmosis

Juxtaglomerular Apparatus

• Located at the point of contact between the distal convoluted tubule and the afferent and efferent arterioles
• JG cells detect changes in blood pressure in the afferent arterioles and secrete renin
• Macula densa cells respond to changes in the solute concentration of the filtrate in the tubule

Glomerular Filtration

• Urine formation begins when waste and water and dissolved materials are filtered out of the glomerular capillary
• Urinary excretion = glomerular filtration + Tubular secretion - Tubular reabsorption
• Glomerular capillaries are much more permeable than capillaries in other tissues
• Filtration pressure = forces favoring filtration (Glomerular capillary hydrostatic pressure & capsular osmotic pressure) - forces opposing filtration (capsular hydrostatic pressure & Glomerular capillary osmotic pressure)

Regulation of GFR

• Neural regulation: sympathetic nerves can cause the constriction or relaxation of the afferent arteriole, resulting in a change of GFR
• Renal autoregulation: the juxtaglomerular apparatus secretes vasoconstriction substances to either afferent arteriole, in response to GFR changes and NaCl levels
• ADH causes the distal convoluted tubules and collecting ducts to become more permeable and increase H2O reabsorption by osmosis

Mechanism of forming dilute & concentrated urine

• Urine formation and adjustment of blood composition involves three major processes: Glomerular filtration, Tubular reabsorption, and Secretion
• Tubular reabsorption: the kidney must have mechanisms for reabsorption of the many solutes (Na+, K+, glucose, chloride -) and H2O that it filters each day
• Substances are selectively reabsorbed from the glomerular filtrate
• The peritubular capillary is adapted for reabsorption, carrying low-pressure blood and being very permeable
• Most reabsorption (70%) occurs in the proximal tubule
• Different modes of transport reabsorb various substances in particular segments of the renal tubule

Countercurrent Mechanism

• The fluid entering the loop of Henle has an osmolarity of 100 mosm/l
• The descending limb is very permeable to H2O, and Na+ and Cl- ions are reabsorbed
• The ascending limb is impermeable to H2O but actively transports Cl- out of the tubular fluid into interstitial fluid, with Na+ ion following passively
• Thus, a small horizontal gradient of 200 is established between the ascending and descending limbs