Table of contents

1. Introduction to Anatomy & Physiology5h 43m
2. Cell Chemistry & Cell Components12h 36m
3. Energy & Cell Processes10h 6m
4. Tissues & Histology10h 3m
5. Integumentary System2h 20m
6. Bones & Skeletal Tissue2h 16m
7. The Skeletal System2h 35m
8. Joints2h 17m
9. Muscle Tissue2h 33m
10. Muscles1h 11m
11. Nervous Tissue and Nervous System1h 35m
12. The Central Nervous System1h 6m
- Introduction to the Central Nervous System
  18m
- The Cerebrum
  48m
13. The Peripheral Nervous System1h 26m
14. The Autonomic Nervous System1h 38m
15. The Special Senses2h 41m
16. The Endocrine System2h 48m
17. The Blood3h 22m
18. The Heart3h 42m
19. The Blood Vessels3h 35m
20. The Lymphatic System3h 16m
21. The Immune System14h 37m
22. The Respiratory System3h 20m
23. The Digestive System2h 5m
24. Metabolism and Nutrition4h 0m
25. The Urinary System2h 39m
26. Fluid and Electrolyte Balance, Acid Base Balance37m
27. The Reproductive System2h 5m
28. Human Development1h 21m
29. Heredity3h 32m

25. The Urinary System

The Nephron

Anatomy & Physiology

25. The Urinary System

The Nephron

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1:35 minutes

Problem 21

Textbook Question

Explain why the filtrate becomes hypotonic as it flows through the ascending limb of the nephron loop. Also explain why the filtrate at the bend of the nephron loop (and the interstitial fluid of the deep portions of the medulla) is hypertonic.

Verified step by step guidance

Understand the structure and function of the nephron loop (loop of Henle). The nephron loop has two main segments: the descending limb and the ascending limb. The descending limb is permeable to water but not to solutes, while the ascending limb is impermeable to water but actively transports solutes (such as Na+, K+, and Cl-) out of the filtrate.

Explain the process in the descending limb. As the filtrate flows down the descending limb, water is reabsorbed into the surrounding interstitial fluid due to the osmotic gradient in the medulla. This causes the filtrate to become increasingly concentrated (hypertonic) as it reaches the bend of the nephron loop.

Describe the conditions at the bend of the nephron loop. At the bend, the filtrate is highly concentrated (hypertonic) because water has been removed in the descending limb, and solutes remain in the filtrate. The interstitial fluid in the deep medulla is also hypertonic due to the countercurrent multiplier mechanism, which establishes a gradient of increasing osmolarity deeper into the medulla.

Explain the process in the ascending limb. As the filtrate flows up the ascending limb, active transport pumps move solutes (Na+, K+, Cl-) out of the filtrate into the interstitial fluid. Since the ascending limb is impermeable to water, water cannot follow the solutes, causing the filtrate to become progressively less concentrated (hypotonic).

Summarize the relationship between the nephron loop and the medullary osmotic gradient. The countercurrent multiplier mechanism ensures that the interstitial fluid in the medulla remains hypertonic, which is crucial for water reabsorption in the collecting ducts. The filtrate becomes hypotonic in the ascending limb due to solute reabsorption without water movement, while the filtrate and interstitial fluid at the bend of the loop are hypertonic due to water reabsorption in the descending limb.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Nephron Loop Function

The nephron loop, or loop of Henle, plays a crucial role in concentrating urine and regulating water and electrolyte balance. It consists of a descending limb that is permeable to water and an ascending limb that is impermeable to water but actively transports sodium and chloride ions out into the interstitial fluid, creating a concentration gradient.

Osmosis and Filtrate Concentration

Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. As the filtrate ascends the nephron loop, the active transport of ions out of the filtrate reduces its solute concentration, making it hypotonic compared to the surrounding interstitial fluid, which retains higher solute levels.

Hypertonicity of Medullary Interstitial Fluid

The interstitial fluid in the deep portions of the medulla becomes hypertonic due to the accumulation of solutes, primarily sodium and urea, as a result of the countercurrent multiplication mechanism. This hypertonic environment is essential for the kidney's ability to concentrate urine and retain water, as it facilitates the reabsorption of water from the collecting ducts.

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