The cardiovascular system delivers oxygen and nutrients while removing metabolic wastes through a muscular pump and a network of vessels. The heart wall is composed of three layers: the outer epicardium, a serous layer covering the surface; the thick myocardium of cardiac muscle responsible for contraction; and the inner endocardium, a smooth lining continuous with the heart valves. The whole heart is enclosed in a tough outer sac, the pericardium. Internally, four chambers work in sequence. The right atrium and right ventricle receive deoxygenated blood and pump it to the lungs, while the left atrium and left ventricle receive oxygenated blood from the lungs and pump it to the body. Four valves enforce one-way flow: the tricuspid and mitral atrioventricular valves on the right and left respectively, and the pulmonary and aortic semilunar valves at the outlets to the pulmonary artery and aorta.
Electrical coordination of the cardiac cycle is handled by the conducting system, beginning with the sinoatrial node as the heart's natural pacemaker, continuing through the atrioventricular node, the bundle of His, the bundle branches, and ending at the Purkinje fibers, which trigger ventricular contraction. Blood flows through three categories of vessels adapted to their roles. Arteries carry blood away from the heart and have thick, elastic walls that withstand high pressure. Veins return blood to the heart under lower pressure and therefore have thinner walls along with valves that prevent backflow. Capillaries are the smallest exchange vessels with walls only one cell thick, allowing diffusion between blood and tissues. Arterioles and venules, the smaller relatives of arteries and veins, connect the capillary beds to the larger vessels.
The respiratory system brings in oxygen and removes carbon dioxide through a branching airway that transitions from conducting to respiratory zones. The upper respiratory tract, including the nose, pharynx, and larynx, warms, humidifies, and filters incoming air. The lower tract continues through the trachea, which branches into the primary bronchi entering the lungs, then into smaller bronchi and bronchioles, and finally into alveoli where gas exchange occurs. Each lung is suspended in the thoracic cavity within a double pleural membrane of visceral and parietal pleura separated by a thin pleural cavity that reduces friction during breathing. With three lobes on the right and two on the left to accommodate the heart, the lungs together contain approximately 300 million alveoli enveloped by pulmonary capillaries, providing enormous surface area for gas diffusion. The diaphragm, a dome-shaped skeletal muscle separating the thoracic and abdominal cavities and innervated by the phrenic nerve, is the primary muscle of inspiration: when it contracts and flattens, thoracic volume increases and air flows in.