The circulatory system is responsible for transporting substances like oxygen, nutrients, hormones around the body, and removing carbon dioxide and wastes to be eliminated. It has three main components: blood, blood vessels, and the heart. Blood carries oxygen from the lungs to tissues, nutrients from the intestines to cells, and waste products to the kidneys. It also regulates temperature, transports hormones, and protects the body from pathogens. The heart has four chambers and valves that pump blood through two circuits - pulmonary circulation to the lungs and systemic circulation to the rest of the body. An aneurysm is a bulge or weakening in an artery wall often caused by high blood pressure or other factors.