Choose the correct one regarding urinary excretion

Options

A. Urinary excretion : Glomerular filtration - Tubular reabsorption `+` Tubular secretion
B. Urinary excretion : Tubular reabsorption `+` Glomerular filtration `-` Tubular secretion
C. Urinary excretion : Tubular secretion `+` Tubular reabsorption
D. Urinary excretion : Tubular secretion `-` Glomerular filtration

Answer

**Step-by-Step Solution:**

1. **Understanding Urinary Excretion**: The process of urinary excretion involves the formation of urine, which occurs through three main steps: glomerular filtration, tubular reabsorption, and tubular secretion.

2. **Step 1 - Glomerular Filtration**: This process takes place in the Bowman's capsule, where blood is filtered from the glomerulus (a tuft of capillaries). The filtrate formed here enters the lumen of the Bowman's capsule.

3. **Step 2 - Tubular Reabsorption**: After filtration, approximately 80-90% of the filtrate is reabsorbed back into the blood. A significant portion (70-80%) of this reabsorption occurs in the proximal convoluted tubule (PCT). This means that the substances reabsorbed are returned to the body and are not excreted.

4. **Step 3 - Tubular Secretion**: In this step, certain substances such as ammonia, hydrogen ions (H+), and potassium ions (K+) are secreted from the blood back into the filtrate at both the PCT and the distal convoluted tubule (DCT). These substances will ultimately be excreted in the urine.

5. **Conclusion**: The substances that are excreted in urine are those that are filtered and those that are secreted. The reabsorbed substances are not part of the urine. Therefore, the correct statement regarding urinary excretion is that it involves glomerular filtration minus tubular reabsorption plus tubular secretion.

6. **Final Answer**: Based on the understanding of the process, option number one is the correct answer regarding urinary excretion.

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