urinary

• The urinary system is also known as the renal system and refers to the structures that produce and conduct urine to the point of excretion
• Functions of the Urinary System

  • Excretion
  • Regulation of blood volume and pressure
  • Regulation of concentration of solutes in the blood
  • Regulation of extracellular fluid pH
  • Regulation of red blood cell synthesis
  • Regulation of vitamin D synthesis
• Kidney Functions


  • Filter 200 liters of blood daily, allowing toxins, metabolic wastes, and excess ions to leave the body in urine
  • Regulate volume and chemical makeup of the blood
  • Maintain the proper balance between water and salts, and acids and bases
• Other Urinary System Organs

  • Urinary bladder - provides a temporary storage reservoir for urine (400-600ml)
  • Paired ureters - transport urine from the kidneys to the bladder
  • Urethra - transports urine from the bladder out of the body
• Layers of Tissue Supporting the Kidney

  • Renal capsule - layer of collagen fibers that encloses the kidney, prevents kidney infection
  • Adipose capsule - fatty mass that cushions/holds the kidney and helps attach it to the body wall
  • Renal fascia - outer layer of dense fibrous connective tissue that binds the kidney to abdominal wall
• Internal Anatomy (Frontal Section)

  • Cortex - the light colored, granular superficial region
  • Medulla - exhibits cone-shaped medullary (renal) pyramids separated by columns
  • Renal pelvis - flat funnel shaped tube lateral to the hilus within the renal sinus
• Internal Anatomy


  • Major calyces - large branches of the renal pelvis that empty urine into the pelvis
  • Minor calyces - smaller branches of the renal pelvis that collect urine draining from papillae
• Nephrons


  • Glomerulus - a number of capillaries associated with a renal tubule
  • Glomerular (Bowman's) capsule - blind, cup-shaped end of a renal tubule that completely surrounds the glomerulus
• Renal Corpuscle


  • The glomerulus and its Bowman's capsule
  • Glomerular endothelium - fenestrated epithelium that allows solute-rich, virtually protein-free filtrate to pass from the blood into the glomerular capsule
• Renal Tubule


  • Proximal convoluted tubule (PCT) - composed of cuboidal cells with numerous microvilli and mitochondria, reabsorbs water and solutes from filtrate and secretes substances into it
  • Loop of Henle - a hairpin-shaped loop of the renal tubule, with a proximal part similar to the PCT, a thin segment, and a thick segment
  • Distal convoluted tubule (DCT) - cuboidal cells without microvilli that function more in secretion than reabsorption
• Types of Nephrons


  • Cortical nephrons - 85% of nephrons, located in the cortex
  • Juxtamedullary nephrons - located at the cortex-medulla junction, have loops of Henle that deeply invade the medulla, have extensive thin segments, and are involved in the production of concentrated urine
• Capillary Beds of the Nephron

  • Every nephron has two capillary beds: the glomerulus, which is fed by an afferent arteriole and drained by an efferent arteriole, and the peritubular capillaries
• Filtration Membrane


  • Composed of three layers: fenestrated endothelium of the glomerular capillaries, visceral membrane of the glomerular capsule (podocytes), and basement membrane composed of fused basal laminae of the other layers
• Mechanisms of Urine Formation

  1. Glomerular filtration

  2. Tubular reabsorption

  3. Secretion
• Glomerular Filtration Rate (GFR)


  The total amount of filtrate formed per minute by the kidneys (Normal >60 mL/min)
• Factors governing filtration rate at the capillary bed are: total surface area available for filtration, filtration membrane permeability, and net filtration pressure
• Nephrolithiasis - Kidney stones
• Glomerulonephritis - Inflammation of the filtration membrane within renal corpuscle, causing increased membrane permeability and plasma proteins to enter the filtrate
• Net Filtration Pressure (NFP)

  Total pressure that promotes filtration = glomerular blood hydrostatic pressure - [capsular hydrostatic pressure + blood colloid osmotic pressure]
• If the GFR is too high

  Needed substances cannot be reabsorbed quickly enough and are lost in the urine
• If the GFR is too low

  Everything is reabsorbed, including wastes that are normally disposed of
• Tubular Reabsorption


  • All organic nutrients are reabsorbed
  • Water and ion reabsorption is hormonally controlled (ADH)
  • Reabsorption may be an active (requiring ATP) or passive process
• Sodium Reabsorption: Primary Active Transport

  • Sodium reabsorption is almost always by active transport
  • Na+ enters the tubule cells at the luminal membrane and is actively transported out of the tubules by a Na+-K+ ATPase pump
• Reabsorption by PCT Cells

  • Active pumping of Na+ drives reabsorption of water by osmosis, aided by water-filled pores called aquaporins, cations and fat-soluble substances by diffusion, and organic nutrients and selected cations by secondary active transport
• Nonreabsorbed Substances


  • Substances are not reabsorbed if they lack carriers, are not lipid soluble, or are too large to pass through membrane pores
  • Urea, creatinine, and uric acid are the most important nonreabsorbed substances
• Creatinine is a waste product made by your muscles as part of regular, everyday activity
• Absorptive Capabilities of Renal Tubules and Collecting Ducts

  • Substances reabsorbed in PCT include: sodium, all nutrients, cations, anions, and water, urea and lipid-soluble solutes, and small proteins
  • Loop of Henle reabsorbs: H2O, Na+, Cl-, K+ in the descending limb
• Pumping of Na+


  1. Drives reabsorption of:

  2. Water by osmosis, aided by water-filled pores called aquaporins

  3. Cations and fat-soluble substances by diffusion

  4. Organic nutrients and selected cations by secondary active transport
• Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
• Reabsorption by PCT Cells

  1. Substances reabsorbed in PCT include:

  2. Sodium, all nutrients, cations, anions, and water

  3. Urea and lipid-soluble solutes

  4. Small proteins
• Loop of Henle reabsorbs

  1. H2O, Na+, Cl-, K+ in the descending limb

  2. Ca2+, Mg2+, and Na+ in the ascending limb
• DCT absorbs


  1. Ca2+, Na+, H+, K+, and water

  2. HCO3- and Cl-
• Collecting duct absorbs


  Water and urea
• Tubular Secretion


  1. Substances move from peritubular capillaries or tubule cells into filtrate

  2. Disposing of substances not already in the filtrate

  3. Eliminating undesirable substances such as urea and uric acid

  4. Controlling blood pH
• Anti-Diuretic Hormone (ADH)


  • Helps regulate the amount of water in the body
  • Released by the posterior pituitary gland when there is a decreased in blood volume or low blood pressure (occurs during dehydration or hemorrhage), which is detected by the baroreceptors (sensors of the heart and large blood vessels)
• Renin-Angiotensin-Aldosterone Mechanism


  1. When blood pressure suddenly decrease, or Na+ concentration in the filtrate is low, the kidneys will secrete renin

  2. Renin acts on angiotensin, a plasma protein produced by the liver, and converts it to angiotensin I

  3. Angiotensin I will be converted into Angiotensin II by the angiotensin-converting enzyme

  4. Angiotensin II will act on the adrenal cortex, causing to secrete aldosterone

  5. Aldosterone increases the rate of active transport (reabsorption) of Na+ in the distal convoluted tubules and collecting ducts

  6. Increased reabsorption of Na+ = increased blood volume
• Aldosterone and the Regulation of Na+ and Water in Extracellular Fluid
• Diuretics


  • Chemicals that enhance the urinary output
  • Any substance not reabsorbed
  • Substances that exceed the ability of the renal tubules to reabsorb it
  • Substances that inhibit Na+ reabsorption
• Osmotic diuretics


  • Alcohol - inhibits the release of ADH
  • Caffeine and most diuretic drugs - inhibit sodium ion reabsorption
  • Lasix and Diuril - inhibit Na+-associated symporters
• Figure 25.16 - Summary of Nephron Function