4 The Cardiovascular System
Learn how blood, the heart, and blood vessels transport materials, circulate blood, and regulate flow to help maintain homeostasis.
The cardiovascular system at a glance
The cardiovascular system consists of , the heart, and vessels. Together, these components move oxygen, nutrients, hormones, and wastes around the body. Changes in heart activity and -vessel diameter help maintain stable conditions, or homeostasis, in tissues and organs.
connects cardiovascular function with other body systems: it carries oxygen from the lungs and absorbs nutrients from the digestive tract.
and its functions
is a fluid connective tissue composed of and . is mostly water and carries dissolved nutrients, hormones, ions, and waste products.
The have distinct roles:
Red cells, also called erythrocytes, contain hemoglobin, which carries most of the oxygen in and some carbon dioxide.
White cells, also called leukocytes, help defend the body against pathogens and other threats.
Platelets are cell fragments that help form clots and limit loss when a vessel is injured.
also distributes heat and helps maintain fluid balance and pH. Its transport, protective, and regulatory roles support the internal environment.
Heart chambers and valves
The heart has four chambers. The atria receive , and the ventricles pump it out. The septum separates the right and left sides. Four valves keep moving forward: the tricuspid and mitral valves lie between the atria and ventricles, while the pulmonary and aortic valves are at the exits of the ventricles.
The right side pumps to the lungs at relatively low pressure. The left side pumps throughout the body; its thicker muscular wall generates the higher pressure needed for .
The cardiac cycle and output
A heartbeat includes two phases. During diastole, the heart muscle relaxes and the chambers fill. During systole, the ventricles contract and eject .
The electrical signal that normally starts each heartbeat begins at the sinoatrial (SA) node. It spreads across the atria, pauses briefly at the atrioventricular (AV) node, and then travels through the atrioventricular bundle, bundle branches, and Purkinje fibers to coordinate ventricular contraction.
is the amount of pumped per minute. It depends on heart rate and :
For example, a heart rate of beats per minute and a of mL per beat produce a of mL per minute, or L/min.
vessels and circulation
travels through a continuous network: the heart sends through , arterioles, , venules, and veins, which return it to the heart.
carry away from the heart. Their relatively thick, elastic walls withstand higher pressure.
Arterioles adjust their diameter and are important in controlling resistance and directing into tissues.
have thin walls that allow gases, nutrients, water, and wastes to be exchanged between and surrounding tissues.
Venules and veins return to the heart. Veins operate at lower pressure, hold much of the body’s volume, and often have valves that help prevent backflow, especially in the limbs.
The heart drives two connected circuits. In , flows from the right ventricle to the lungs and returns to the left atrium. In , flows from the left ventricle to the body and returns to the right atrium.
The terms artery and vein describe the direction of flow, not its oxygen content. Pulmonary carry oxygen-poor , whereas pulmonary veins carry oxygen-rich .
Tracing through the heart
A red cell returning from the body enters the right atrium through the venae cavae. It passes through the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve into the pulmonary . In the lung , it releases carbon dioxide and takes up oxygen.
The cell then returns through the pulmonary veins to the left atrium. It passes through the mitral valve into the left ventricle, which pumps it through the aortic valve into the aorta and .
Regulating flow and pressure
flow depends on the heart’s pumping and the resistance of vessels. A useful approximate relationship is:
The nervous system can adjust circulation rapidly. in major sense stretch caused by pressure and send information to cardiovascular control centers in the brainstem. When pressure falls, the autonomic response typically increases sympathetic activity: heart rate and the strength of contraction rise, and many arterioles constrict. These changes help restore pressure. When pressure rises, responses generally shift in the opposite direction. The parasympathetic nervous system can slow the heart, chiefly by acting on the SA node.
Local tissue conditions also influence flow. Active tissues use more oxygen and produce more carbon dioxide and other metabolites; nearby arterioles can dilate, increasing delivery to those tissues. During exercise, flow can increase to working muscles as rises and local vessels adjust.
Over longer periods, the kidneys help regulate volume and therefore pressure by changing water and salt balance. Hormones, including those involved in the renin–angiotensin–aldosterone system, contribute to these adjustments. Rapid neural reflexes, local vessel responses, and longer-term kidney and hormonal control work together to match circulation to the body’s needs and help maintain homeostasis.