Table of contents
- 1. Introduction to Biology2h 42m
- 2. Chemistry3h 37m
- 3. Water1h 26m
- 4. Biomolecules2h 23m
- 5. Cell Components2h 26m
- 6. The Membrane2h 31m
- 7. Energy and Metabolism2h 0m
- 8. Respiration2h 40m
- 9. Photosynthesis2h 49m
- 10. Cell Signaling59m
- 11. Cell Division2h 47m
- 12. Meiosis2h 0m
- 13. Mendelian Genetics4h 44m
- Introduction to Mendel's Experiments7m
- Genotype vs. Phenotype17m
- Punnett Squares13m
- Mendel's Experiments26m
- Mendel's Laws18m
- Monohybrid Crosses19m
- Test Crosses14m
- Dihybrid Crosses20m
- Punnett Square Probability26m
- Incomplete Dominance vs. Codominance20m
- Epistasis7m
- Non-Mendelian Genetics12m
- Pedigrees6m
- Autosomal Inheritance21m
- Sex-Linked Inheritance43m
- X-Inactivation9m
- 14. DNA Synthesis2h 27m
- 15. Gene Expression3h 6m
- 16. Regulation of Expression3h 31m
- Introduction to Regulation of Gene Expression13m
- Prokaryotic Gene Regulation via Operons27m
- The Lac Operon21m
- Glucose's Impact on Lac Operon25m
- The Trp Operon20m
- Review of the Lac Operon & Trp Operon11m
- Introduction to Eukaryotic Gene Regulation9m
- Eukaryotic Chromatin Modifications16m
- Eukaryotic Transcriptional Control22m
- Eukaryotic Post-Transcriptional Regulation28m
- Eukaryotic Post-Translational Regulation13m
- 17. Viruses37m
- 18. Biotechnology2h 58m
- 19. Genomics17m
- 20. Development1h 5m
- 21. Evolution3h 1m
- 22. Evolution of Populations3h 52m
- 23. Speciation1h 37m
- 24. History of Life on Earth2h 6m
- 25. Phylogeny2h 31m
- 26. Prokaryotes4h 59m
- 27. Protists1h 12m
- 28. Plants1h 22m
- 29. Fungi36m
- 30. Overview of Animals34m
- 31. Invertebrates1h 2m
- 32. Vertebrates50m
- 33. Plant Anatomy1h 3m
- 34. Vascular Plant Transport1h 2m
- 35. Soil37m
- 36. Plant Reproduction47m
- 37. Plant Sensation and Response1h 9m
- 38. Animal Form and Function1h 19m
- 39. Digestive System1h 10m
- 40. Circulatory System1h 49m
- 41. Immune System1h 12m
- 42. Osmoregulation and Excretion50m
- 43. Endocrine System1h 4m
- 44. Animal Reproduction1h 2m
- 45. Nervous System1h 55m
- 46. Sensory Systems46m
- 47. Muscle Systems23m
- 48. Ecology3h 11m
- Introduction to Ecology20m
- Biogeography14m
- Earth's Climate Patterns50m
- Introduction to Terrestrial Biomes10m
- Terrestrial Biomes: Near Equator13m
- Terrestrial Biomes: Temperate Regions10m
- Terrestrial Biomes: Northern Regions15m
- Introduction to Aquatic Biomes27m
- Freshwater Aquatic Biomes14m
- Marine Aquatic Biomes13m
- 49. Animal Behavior28m
- 50. Population Ecology3h 41m
- Introduction to Population Ecology28m
- Population Sampling Methods23m
- Life History12m
- Population Demography17m
- Factors Limiting Population Growth14m
- Introduction to Population Growth Models22m
- Linear Population Growth6m
- Exponential Population Growth29m
- Logistic Population Growth32m
- r/K Selection10m
- The Human Population22m
- 51. Community Ecology2h 46m
- Introduction to Community Ecology2m
- Introduction to Community Interactions9m
- Community Interactions: Competition (-/-)38m
- Community Interactions: Exploitation (+/-)23m
- Community Interactions: Mutualism (+/+) & Commensalism (+/0)9m
- Community Structure35m
- Community Dynamics26m
- Geographic Impact on Communities21m
- 52. Ecosystems2h 36m
- 53. Conservation Biology24m
42. Osmoregulation and Excretion
Osmoregulation and Excretion
Problem 4
Textbook Question
The high osmolarity of the renal medulla is maintained by all of the following except
a. Active transport of salt from the upper region of the ascending limb.
b. The spatial arrangement of juxtamedullary nephrons.
c. Diffusion of urea from the collecting duct.
d. Diffusion of salt from the descending limb of the loop of Henle.

1
Understand the role of the renal medulla: The renal medulla is responsible for concentrating urine, and its high osmolarity is crucial for this function.
Identify the mechanisms that contribute to high osmolarity: These include active transport of ions, diffusion of urea, and the unique arrangement of nephron structures.
Examine option a: Active transport of salt occurs in the ascending limb of the loop of Henle, contributing to the osmolarity gradient.
Examine option b: Juxtamedullary nephrons have long loops of Henle that extend deep into the medulla, aiding in the concentration of urine.
Examine option d: Salt does not diffuse from the descending limb; instead, water is reabsorbed here, increasing the concentration of the filtrate. This option does not contribute to the high osmolarity of the medulla.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Osmolarity in the Renal Medulla
Osmolarity refers to the concentration of solutes in a solution. In the renal medulla, high osmolarity is crucial for the kidney's ability to concentrate urine. This is achieved through the countercurrent multiplier system, which involves the loop of Henle and the vasa recta, creating a gradient that allows for water reabsorption and urine concentration.
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Countercurrent Multiplier System
The countercurrent multiplier system is a mechanism in the kidneys that helps establish a concentration gradient in the renal medulla. It involves the loop of Henle, where the descending limb is permeable to water but not salt, and the ascending limb actively transports salt but is impermeable to water. This arrangement allows for the efficient reabsorption of water and solutes, maintaining high osmolarity.
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Role of Urea in Osmolarity
Urea plays a significant role in maintaining the osmolarity of the renal medulla. It diffuses from the collecting duct into the medullary interstitium, contributing to the osmotic gradient. This process helps in the reabsorption of water from the collecting ducts, further concentrating the urine. Urea recycling is essential for sustaining the high osmolarity necessary for kidney function.
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