Urine is formed from

Options

A. Tubular absorption
B. Glomerular filtration
C. Tubular secretion
D. Both A and B

Answer

### Step-by-Step Solution

1. **Understanding Urine Formation**: Urine formation is a complex process that involves multiple steps, primarily occurring in the kidneys.

2. **Step 1: Ultrafiltration**:
- The first step in urine formation is ultrafiltration, which occurs in the glomerulus of the nephron.
- Blood enters the glomerulus, where it is filtered through a specialized filtration unit made up of three layers:
- **Epithelial Cells**: The outermost layer that forms the boundary.
- **Basement Membrane**: A thin layer that provides structural support and acts as a filtration barrier.
- **Endothelial Membrane**: The innermost layer that lines the blood vessels.
- **Visceral Membrane (Podocytes)**: This layer contains specialized cells called podocytes that have foot-like projections, which help in the filtration process.
- During ultrafiltration, particles smaller than 4 angstroms can pass through, while those larger than 8 angstroms are retained in the blood.

3. **Step 2: Tubular Reabsorption**:
- After ultrafiltration, the filtrate moves into the renal tubules where tubular reabsorption occurs.
- Approximately 70% of the filtered substances, including water, ions, and nutrients, are reabsorbed back into the bloodstream.
- Glucose is reabsorbed almost completely (about 100%) in the proximal convoluted tubule (PCT).

4. **Step 3: Tubular Secretion**:
- Tubular secretion is the process where additional substances are secreted from the blood into the tubular fluid.
- This includes waste products and excess ions such as hydrogen ions, potassium ions, and ammonia.
- This step is crucial as it helps in the regulation of electrolytes and the elimination of waste products.

5. **Conclusion**:
- The formation of urine involves both glomerular filtration and tubular secretion.
- Therefore, urine is formed from the processes of ultrafiltration in the glomerulus and tubular secretion in the renal tubules.

### Final Answer
Urine is formed from glomerular filtration and tubular secretion.

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