Why is the first urine of day most concentrated?

The first urine of the day is typically the most concentrated because it has been stored in the bladder for a longer period of time, allowing for more water to be reabsorbed by the body. This process of water reabsorption is controlled by the hormone vasopressin (also known as antidiuretic hormone), which is released by the pituitary gland in response to decreased blood volume or increased blood osmolality. When the body senses that it is in need of water, vasopressin is released, causing the kidneys to reabsorb more water from the urine, resulting in a more concentrated urine.

Factors such as fluid intake, dietary salt intake, perspiration, and medications can also influence the concentration of urine. Dehydration, high salt intake, and certain medications can lead to more concentrated urine, while adequate hydration, a low-sodium diet, and diuretic medications can result in more dilute urine.

Typically, the first urine of the day is darker and more yellow in color compared to subsequent urinations throughout the day, indicating its higher concentration. As the day progresses and more fluids are consumed, the urine becomes less concentrated and paler in color.