What is the significance of peptides like vasopressin, oxytocin, and glutathione in biological systems? Discuss their functions and importance.

Peptides play a crucial role in biological systems by acting as signaling molecules, hormones, and antioxidants. Among them, vasopressin, oxytocin, and glutathione are significant due to their distinct biochemical functions that regulate physiological processes.  

### Vasopressin (Antidiuretic Hormone, ADH)  
Vasopressin is a nonapeptide hormone secreted by the hypothalamus and stored in the posterior pituitary gland. It plays a key role in water homeostasis and blood pressure regulation.  

1. Water Retention: Vasopressin regulates kidney function by increasing the permeability of the collecting ducts to water in the nephrons, reducing water excretion. This is mediated by the activation of V2 receptors in renal tubules, which stimulate aquaporin-2 (AQP2) channels.  
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