Dogs are usually in heat for 21 days. Female dogs have a cyclic sexuality that is independent of season unlike most animals. As for the male dogs, they do not have any cycles and are always ready for intimacy like most males of any species including human beings. Female dogs have a cycle of six months on average meaning they can be in heat twice a year. The heat cycle has three distinct stages. They include: Proestrus. During this stage, one may notice some bleeding from the vulva of the dog. The bleeding can either be heavy or light.
Important hormonal proteins include the hormones insulin and parathyroid hormone. These two protein hormones have important physiological roles in the body. Insulin has the role of facilitating the entry of glucose molecules into the liver cells.
Other medically useful human peptides that have been made widely available because of genetic engineering are human growth hormone, which is used to treat persons with congenital dwarfism and tissue-type plasminogen activator (t-PA), which is a promising new treatment for persons who suffer a heart attack.