The Circulatory System
35.1 Circulatory Systems Deliver Nutrients and Remove Wastes
1. A circulatory system consists of fluid, a network of vessels, and a pump. The fluid delivers nutrients and oxygen to tissues and removes metabolic wastes.
2. In simple animals, open body cavities give interior structures direct contact with fluids from the environment so that nutrients and oxygen can diffuse into cells and wastes can diffuse out. In more complex animals, a heart pumps the fluid to the body cells.
3. In an open circulatory system, the fluid bathes tissues directly in open spaces before returning to the heart.
4. In a closed circulatory system, the heart pumps the fluid through a system of vessels to cells and back.
5. Respiratory pigments increase the oxygen-carrying capacity of blood or hemolymph.
6. Tunicates have blood and a rudimentary heart, and a blood flow that goes back and forth. Lancelets lack a heart, but introduce unidirectional circulation.
7. Vertebrates have closed circulatory systems that increase in complexity in fishes, amphibians, reptiles, birds, and mammals, reflecting the challenges of life on land. A double heart and double circulation that separates oxygenated from deoxygenated blood enables land vertebrates to be active.
35.2 The Human Circulatory System
8. The heart is the muscular pump that drives the human circulatory system. The heart has two atria that receive blood and two ventricles that propel blood in the body. Heart valves separate the right atrium and right ventricle, the left atrium and left ventricle, and the site where the pulmonary circulation and systemic circulation pathways leave the heart. Heart valves ensure one-way blood flow.
9. The pacemaker or sinoatrial (SA) node, a collection of specialized cardiac muscle cells in the right atrium, sets the heart rate. From there, heartbeat spreads to the atrioventricular (AV) node and then along Purkinje fibers through the ventricles.
10. The circulatory system leads to and from the lungs and to and from the rest of the body. Blood leaves the heart through the aorta and travels in increasingly narrower arteries and arterioles to the capillaries, where nutrient and waste exchange occur. Blood flows from the capillaries to venules and then to veins, and it reenters the heart through the venae cavae. Arteries have thicker, more elastic walls than veins.
11. The pumping of the heart and constriction of blood vessels produces blood pressure. Systole is the pressure exerted on blood vessel walls when the ventricles contract. The low point, diastole, occurs when the ventricles relax.
12. Human blood is a mixture of cells and cell fragments (collectively called formed elements), proteins, and molecules that are or suspended in plasma. The formed elements include red blood cells, white blood cells, and platelets. Red blood cells carry oxygen. White blood cells protect against infection. Platelets break and collect near a wound, releasing chemicals that trigger blood clotting. Blood cells originate in bone marrow.
13. The circulatory system is controlled to help maintain homeostasis. When the volume of blood entering the heart changes, ventricular contraction changes in response to adjust cardiac output. The autonomic nervous system, under the influence of the brain’s vasomotor center, speeds or slows heart rate and dilates or constricts blood vessels in a way that adjusts blood pressure.
35.3 The Lymphatic System Maintains Circulation and Protects Against Infection
14. The lymphatic system is a network of vessels that collect fluid from the body’s tissues, purify it, and return it to the blood. Lymph nodes filter lymph, and the spleen, thymus, and possibly the tonsils manufacture lymphocytes. The blood and lymphatic systems continually supply tissues with nutrients and oxygen and remove or destroy wastes.