Concept 38.1 Circulatory Systems Can Be Open or Closed

• The metabolic needs of the cells of many small animals are met by direct exchange of materials with the external medium. Larger, active animals need a circulatory system that transports nutrients, respiratory gases, and metabolic wastes throughout the body.
   
• In open circulatory systems, found in mollusks and arthropods, extracellular fluid leaves vessels and percolates through tissues. Review Figure 38.1A
   
• In closed circulatory systems, found in annelids and vertebrates, the blood is contained in a system of vessels. Closed circulatory systems have several advantages, including faster transport and the ability to selectively direct blood to specific tissues, and the compartmentalization of functional elements such as red blood cells. Review Figure 38.1B
   
• The circulatory system of vertebrates consists of a heart, arteries, and arterioles that carry blood from the heart; capillaries, where materials are exchanged between blood and interstitial fluid; and venules and veins that carry blood back to the heart.
   

Concept 38.2 Circulatory Systems May Have Separate Pulmonary and Systemic Circuits

• The vertebrate heart evolved from two chambers in fishes; to three in lungfishes, amphibians, and most reptiles; to four in crocodilians, birds, and mammals. This evolutionary progression has led to an increasing separation of blood that flows to the gas exchange organs and blood that flows to the rest of the body. Review WEB ACTIVITY 38.1
   
• The simplest (two-chambered heart) has an atrium that receives blood from the body and a ventricle that pumps blood out of the heart. An aorta distributes blood to arteries.
   
• In crocodilians, birds, and mammals, blood circulates through two completely separate circuits: the pulmonary circuit, which transports oxygen-depleted blood to the lungs, and the systemic circuit, which transports oxygenated blood to the rest of the body.
   

Concept 38.3 A Beating Heart Propels the Blood

• The human heart has four chambers, with valves to prevent backflow. Blood flows from the right atrium and ventricle to the lungs, then to the left atrium and ventricle, and then to the rest of the body. Review Figure 38.2 and WEB ACTIVITY 38.2
   
• The cardiac cycle has two phases: systole, when the ventricles contract, and diastole, when the ventricles relax. The heart sounds (“lub-dup”) are made by the closing of the heart valves. Review Figure 38.3 and ANIMATED TUTORIAL 38.1
   
• The heartbeat is generated by pacemaker cells in the sinoatrial node that spontaneously depolarize, triggering an action potential that spreads to the rest of the heart through the atrioventricular node, bundle of His, and Purkinje fibers. Review Figure 38.5
   
• The autonomic nervous system controls heart rate by changing the rate of depolarization of the pacemaker cells. Norepinephrine from sympathetic nerves increases heart rate, and acetylcholine from parasympathetic nerves decreases it. Review Figure 38.4
   

Concept 38.4 Blood Consists of Cells Suspended in Plasma

• Blood consists of a liquid plasma and cellular components (erythrocytes, or red blood cells, platelets, and leukocytes, or white blood cells). All of the cellular components are produced in the bone marrow. Review Figure 38.7
   
• Erythrocytes transport oxygen. Their production in the bone marrow is stimulated by erythropoietin, which is produced in response to hypoxia (low oxygen levels) in the tissues.
   
• Platelets, along with circulating proteins, are involved in blood clotting, which results in a meshwork of fibrin threads that help seal vessels. Review Figure 38.8
   

Concept 38.5 Blood Circulates through Arteries, Capillaries, and Veins

• Arteries and arterioles contain elastic fibers that enable them to withstand high pressure, and smooth muscle that can change their diameters. Review Figure 38.9 and WEB ACTIVITY 38.3
   
• Capillaries have thin, permeable walls that allow exchange of materials with the interstitial fluid. Capillaries in most tissues other than the brain have pores that allow many large molecules to pass through. Review Figures 38.9 and 38.10
   
• Starling's forces suggest that blood volume is maintained in the capillary beds by an exchange of fluids driven by both blood pressure and osmotic pressure. Review Figure 38.11
   
• Veins have a high capacity for storing blood. Return of blood to the heart is aided by gravity, skeletal muscles, and breathing. Review Figure 38.12
   
• The lymphatic system returns interstitial fluid to the blood. Review Figure 38.13
   

Concept 38.6 Circulation Is Regulated by Autoregulation, Nerves, and Hormones

• Deviations in blood pressure can be corrected by altering total peripheral resistance (TPR), heart rate (HR), or stroke volume (SV). Review INTERACTIVE TUTORIAL 38.1
   
• Autoregulation causes arterioles to dilate in response to low oxygen, high carbon dioxide, and certain metabolic waste products. This increases blood flow to the area. Review Figure 38.14
   
• Blood pressure and heart rate are also regulated by the autonomic nervous system, which responds to information about both blood pressure and blood composition that is integrated by regulatory centers in the medulla. Review Figure 38.15
   
• Many hormones affect blood pressure, including epinephrine, angiotensin, and ADH. Review Figure 38.16