The Excretory System—Part 2

The Mammalian Kidney

• Functions:
  • Excretion
  • Maintaining water balance
• Structure:
  • Renal cortex (outer section)
  • Renal medulla (inner section)
• Functional unit: the nephron
  • Glomerulus: a tight ball of capillaries
  • Bowman's capsule: the blind end of the tubule.
    • Forms a cuplike swelling that surrounds the glomerulus.
  • The tubule is composed of three parts:
    • Proximal convoluted tubule (Latin proximus: nearest)
    • Loop of Henle
    • Distal convoluted tubule (Latin dis: apart)
• A number of nephrons empty into the collecting duct.

Figure 44.18, page 883, Campbell's Biology, 5th Edition

Circulatory System Meets Excretory System

• Circulatory system: from body to kidney
  • Blood arrives in the kidney from the renal artery.
  • As the artery becomes smaller it becomes an arteriole.
• Circulatory system: from kidney to nephron
  • The blood arrives in the nephron via an arteriole.
  • The arteriole becomes a network of capillaries formed into a tight ball called the glomerulus.
• From circulatory system to excretory system
  • Small ions, solutes, urea, and water are forced—under hydrostatic pressure—out of the circulatory system and into the excretory system.
  • Large molecules like proteins and large structures like cells are prevented from crossing the membrane.
  • The excretory system is a long tube that begins as a cup shaped swelling called Bowman's capsule, which surrounds the glomerulus (a part of the circulatory system).
    • The solution in the glomerulus is called blood since it is in the circulatory system.
    • The solution in Bowman's capsule is called filtrate since it is in the excretory system.
      • Because the membrane that does the filtering is nonselective with regard to small molecules, the filtrate includes solutes which are not waste products, such as glucose and vitamins.
      • These will need to be recovered from the filtrate.
  • Note that the excretory system of tubules is surrounded by capillaries called the peritubular capillaries (Greek peri: around).

The Long and Winding Road: the Filtrate's Journey to Becoming Urine

Overview: a Four Step Process

1. Filtration: movement of material from circulatory system to excretory system
2. Reabsorption: active and passive transport from the filtrate back into the blood
3. Secretion: active and passive transport from blood into the filtrate
4. Excretion: final elimination of waste filtrate which is now officially called urine

Figures 44.14 and 44.19, page 879 and 884, Campbell's Biology, 5th Edition

The Proximal Convoluted Tubule

• Reabsorption
  • Active transport of glucose, vitamins, nutrients, amino acids, etc. into interstitial fluid and eventually the peritubular capillaries.
  • Active transport of sodium (chloride follows passively).
  • Passive transport of water: as a result of the active transport of sodium chloride, water follows passively by osmosis.
• Secretion
  • Active transport of drugs and other toxins that have been processed by the liver.

Descending Limb of the Loop of Henle

• The osmolarity (concentration of solutes in solution) of the interstitial fluid surrounding the loop of Henle gets progressively greater going from the cortex to the inner medulla.
• The epithelial layer of the descending limb is permeable to water but not sodium chloride.
• Reabsorption
  • Passive transport of water out of the tubule due to osmosis.
  • Since the osmolarity is greater outside of the tubule, water moves down its concentration gradient.
  • As the water leaves the tubule, the filtrate becomes more concentrated.

Ascending Limb of the Loop of Henle

• The epithelial layer of the ascending limb is permeable to sodium chloride but not water.
  • Thin segment: passive transport of sodium chloride out of the tubule.
  • Thick segment: active transport of sodium chloride out of the tubule.
• The filtrate becomes more dilute, since sodium chloride is leaving and water can't.

Collecting Duct

• This is not part of the nephron, per se; instead, a number of nephrons dump their filtrate into the colleting duct.
• It goes back down into the inner medulla of the kidney.
• The epithelium is permeable to water but not sodium chloride.
  • Passive transport of water out of the collecting duct due to osmosis as the collecting duct travels through concentrated interstitial fluid.
• Result: concentrated urine with an osmolarity (1200 mosm∕L), much higher than blood (300 mosm∕L)

Figure 44.20, page 886, Campbell's Biology, 5th Edition

Antidiuretic Hormone (ADH)

• From Greek diuresis: to urinate
• Antidiuretic: preventing the output of urine
• Antidiuretic hormone
  • Produced in the hypothalamus
  • Released from the posterior pituitary
• Osmoreceptor cells in the hypothalamus continually monitor the osmolarity of the blood.
  • They stimulate the release of ADH when the blood osmolarity rises above a set point of 300 mosm∕L.
  • Sweating or diarrhea cause an increase in blood osmolarity.
  • ADH causes the epithelial cells lining the collecting ducts to become more permeable to water.
• Alcohol and coffee inhibit the release of ADH.
  • A hangover may partially be due to dehydration.

Figures 45.7(a) and 44.21(a), pages 901 and 888, Campbell's Biology, 5th Edition