The Respiratory System
The respiratory system delivers oxygen into the blood, and excretes carbon dioxide waste. Oxygen enters the lungs and carbon dioxide exits the lungs. Inhalation occurs because of the diaphragm enlarging, causing oxygen to rush in, and elastic rebound causing the carbon dioxide to be forced out. Oxygen enters the blood at the alveoli in the lungs; it enters naturally because of passive diffusion, there is less oxygen in the blood then the lungs. Carbon dioxide exits the blood the same way.
The Circulatory System
The circulatory system delivers oxygen and nutrients from the blood into the tissues, and picks up carbon dioxide and waste for excretion. The circulatory system is made of the heart and blood vessels. The blood vessels narrow into capillaries to deliver the oxygen by passive diffusion into the body's cells. The blood circulates through two systems, the pulmonary circuit and the systemic circuit.
The heart has the left and right side, and has four chambers. The heart is specialized for pumping blood. Deoxygenated blood enters the right atrium, and enters the right ventricle, before heading to the lungs in the pulmonary artery. It returns from the lungs oxygenated, to enter the left atrium, then left ventricle, to be pumped by the aorta to the rest of the body.
The Lymphatic System
The lymphatic system is a network of vessels which pick up fluids and fat which has leaked from the blood, and filters it for disease.