Final answer:
Antidiuretic hormone (ADH) is the hormone that causes urine volume to be as low as about 0.5 L/day and appear darker yellow. ADH's action on the kidneys results in more water being reabsorbed. The color and volume of urine are indicators of hydration status and health conditions.
Step-by-step explanation:
The hormone responsible for reducing urine volume and causing it to appear darker yellow in color is antidiuretic hormone (ADH). When ADH is released, it prompts the kidneys to reabsorb more water, leading to a decrease in urine volume. Urine can thus be as little as about 0.5 L/day under the influence of ADH, especially during dehydration or when the body needs to conserve water. This reduced urine output, termed oliguria, often appears noticeably darker due to the increased concentration of waste products like urobilin or urochrome.
Conversely, when there is an abundance of water in the bloodstream, the pituitary gland lowers the release of ADH, which results in the production of a larger volume of urine that is usually pale yellow or colorless. Conditions such as diabetes insipidus, where there is a deficiency of ADH, or diabetes mellitus, which affects the osmotic balance in the kidneys, can lead to polyuria – an excessive production of urine.
The characteristics of urine, including volume and color, are good indicators of a person's hydration status and can also reflect various health conditions. It's important to stay aware of your body's hydration needs, especially during activities such as exercise, which can affect urine concentration.