Final answer:
Ghrelin is known as the 'hunger hormone' and is responsible for stimulating appetite and increasing hunger.
Step-by-step explanation:
The hormone that increases hunger is ghrelin. Ghrelin is a hormone produced mainly by the stomach, with small amounts released by the small intestine, pancreas, and brain. It is known as the 'hunger hormone' because it stimulates appetite, increases food intake, and promotes fat storage. Hormones like leptin and insulin have opposite effects; leptin decreases hunger and is produced by adipose tissue, and insulin is secreted by the islets of Langerhans in the pancreas when blood glucose levels rise, assisting in glucose metabolism rather than hunger modulation.