When it's hot or when the organism starts sweating a lot, this causes an amount of water to be lost from the body. Water is important for the organism to survive and it's absence wi lead to death, therefore Osmoreceptors cells in the hypothalamus in the brain, sends signals to increase the rate of water re-absorption by the kidney. This causes the urine to be of a low volume and more concentrated. This allows enough water to be used by the body as a lot has been lost during sweating.