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

Renal Physiology: Overview

Anatomy & Physiology

25. The Urinary System

Renal Physiology: Overview

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Problem 16

Textbook Question

When the renal threshold for a substance exceeds its tubular maximum:
(a) More of the substance will be filtered
(b) More of the substance will be reabsorbed
(c) More of the substance will be secreted
(d) The amount of the substance that exceeds the tubular maximum will be found in the urine
(e) Both a and d occur

Verified step by step guidance

Understand the concept of renal threshold and tubular maximum: The renal threshold is the plasma concentration of a substance at which it begins to appear in the urine because its reabsorption capacity in the renal tubules is exceeded. The tubular maximum (Tm) is the maximum rate at which the renal tubules can reabsorb a substance.

Recognize that when the renal threshold is exceeded, the substance's concentration in the blood surpasses the tubular maximum, meaning the renal tubules cannot reabsorb all of the filtered substance.

Analyze the options provided: (a) More of the substance will be filtered. This is incorrect because filtration depends on the plasma concentration and glomerular filtration rate, not the renal threshold or tubular maximum.

Evaluate option (d): The amount of the substance that exceeds the tubular maximum will be found in the urine. This is correct because the excess substance that cannot be reabsorbed due to the tubular maximum will be excreted in the urine.

Conclude that the correct answer is (d), as it directly relates to the physiological process occurring when the renal threshold exceeds the tubular maximum.

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Key Concepts

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

Renal Threshold

The renal threshold refers to the concentration level of a substance in the blood at which the kidneys begin to excrete it into the urine. When the blood concentration of a substance exceeds this threshold, the kidneys cannot reabsorb all of it, leading to its presence in urine. Understanding this concept is crucial for analyzing how substances are handled by the kidneys.

Tubular Maximum

The tubular maximum (Tm) is the maximum rate at which a substance can be reabsorbed from the renal tubules back into the bloodstream. Each substance has a specific Tm, and once this limit is reached, any additional amount will not be reabsorbed and will instead be excreted in urine. This concept is essential for understanding the limits of renal reabsorption.

Filtration and Excretion

Filtration in the kidneys involves the movement of substances from the blood into the renal tubules, while excretion refers to the elimination of these substances in urine. When the renal threshold is exceeded, it indicates that the kidneys are filtering more of the substance than can be reabsorbed, resulting in increased excretion. This relationship is key to answering the question regarding the fate of substances in renal physiology.

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