4 Cardiovascular System Physiology
Learn how blood, the heart, and blood vessels work together to circulate materials, regulate flow and pressure, and maintain tissue perfusion.
and its functions
The cardiovascular system consists of , the heart, and vessels. It supports homeostasis by delivering oxygen, nutrients, and hormones to tissues; carrying carbon dioxide and other wastes away; distributing heat; and helping defend the body and limit loss. flow also helps maintain stable conditions around cells.
is a fluid connective tissue composed of and formed elements. is mostly water and contains dissolved electrolytes, nutrients, hormones, wastes, and proteins. Albumin helps maintain the osmotic pressure that keeps fluid in the bloodstream; other proteins include antibodies and clotting factors.
The formed elements have different roles:
Red cells, or erythrocytes, use hemoglobin to carry most of the ’s oxygen and some carbon dioxide.
White cells, or leukocytes, defend against pathogens and other threats.
Platelets are cell fragments that help form clots when a vessel is damaged.
Together, ’s transport, defense, and clotting functions support the stable internal environment needed by tissues.
The heart and its pumping cycle
The heart is a muscular pump with four chambers. The right atrium receives returning from the body, and the right ventricle pumps it toward the lungs. The left atrium receives from the lungs, and the left ventricle pumps to the body. The left ventricle has a thicker muscular wall because it must generate enough pressure to drive through the systemic circulation.
Valves keep moving in one direction by opening and closing in response to pressure differences, which helps prevent backflow. passes from the atria to the ventricles through the tricuspid and mitral, or bicuspid, valves. It leaves the ventricles through the pulmonary and aortic semilunar valves.
The alternates between relaxation, called diastole, when the chambers fill, and contraction, called systole, when the ventricles eject . Autorhythmic cells initiate each heartbeat. The sinoatrial node normally sets the pace; the signal spreads across the atria, then the atrioventricular node briefly delays it before it travels through the ventricles. This sequence coordinates atrial contraction before ventricular contraction.
The route of flow
moves through the heart and lungs before returning to the body. The sequence is:
from the body enters the venae cavae, then passes through the right atrium, tricuspid valve, and right ventricle.
It passes through the pulmonary valve and pulmonary arteries to lung capillaries. There, carbon dioxide leaves the and oxygen enters.
returns from the lungs through the pulmonary veins to the left atrium, then passes through the mitral valve to the left ventricle.
It leaves through the aortic valve and aorta to reach body tissues.
Arteries are defined by carrying away from the heart, while veins carry toward the heart. These terms describe direction, not oxygen content: pulmonary arteries carry oxygen-poor , and pulmonary veins carry oxygen-rich .
and its determinants
is the volume of pumped by one ventricle per minute. It is calculated as:
is the volume ejected with each beat. For example, a heart rate of beats per minute and a of mL per beat give a of mL per minute, or about L/min. At rest, the output of the right and left ventricles is approximately equal over time; otherwise, would accumulate in the pulmonary or systemic circulation.
depends especially on three factors:
: How much the ventricle fills before contraction. Greater filling generally stretches cardiac muscle and increases contraction force through the Frank–Starling mechanism, within physiological limits.
Contractility: The strength of contraction at a given filling level. Sympathetic stimulation and circulating epinephrine can increase it.
: The pressure or resistance the ventricle must overcome to eject . A higher can make ejection more difficult.
Heart rate and can change with activity and physiological demand. During exercise, sympathetic stimulation raises heart rate and contractility, while increased venous return can raise .
vessels, circuits, and pressure
travels through a branching network in this order: arteries, , capillaries, venules, and veins. Each type of vessel contributes differently to circulation.
Arteries carry away from the heart. Their elastic, muscular walls tolerate higher pressure and help maintain flow between beats.
change diameter through smooth-muscle contraction or relaxation. They are major resistance vessels and help direct into different tissues.
Capillaries have thin walls that permit exchange of gases, nutrients, and wastes between and surrounding tissues.
Venules and veins return to the heart at lower pressure. Veins can hold substantial volumes of . Valves in many limb veins help prevent backward flow, and contraction of nearby skeletal muscles helps move venous toward the heart.
The carries from the right side of the heart to the lungs and back to the left side. The carries from the left side of the heart to the body and back to the right side. These circuits are connected in series, so passes through both during each full trip around the body.
flows down pressure gradients, from higher to lower pressure. opposes flow and depends strongly on vessel radius; a small change in arteriole diameter can substantially change resistance and tissue flow. In broad terms, mean arterial pressure is related to and systemic :
This relationship shows that pressure can change when either the heart’s output or the vessels’ resistance changes.
Regulation and homeostasis
Cardiovascular regulation helps keep pressure and tissue perfusion adequate while adjusting flow to changing needs. Neural, hormonal, kidney-related, and local mechanisms interact.
Rapid neural control
in major arteries sense stretch associated with pressure. If pressure falls, signals to cardiovascular centers in the brain can increase sympathetic activity and reduce parasympathetic activity. Heart rate, contractility, and vascular tone then rise, helping restore pressure. When pressure rises, the reflex response generally shifts in the opposite direction.
Hormonal and kidney-related control
Epinephrine and norepinephrine can increase cardiac activity and alter vessel tone. Over longer periods, the kidneys help regulate volume by adjusting salt and water excretion. Hormonal systems, including the renin–angiotensin–aldosterone system and antidiuretic hormone, can conserve salt or water and influence vessel tone. Atrial natriuretic peptide promotes sodium and water loss when volume is elevated.
Local tissue control
Active tissues release chemical signals that can dilate nearby and increase local flow. This helps match delivery to metabolic demand, such as directing more to working skeletal muscle during exercise and to digestive organs after a meal.
These controls operate together. During exercise, for example, increases while local changes in vessel diameter redistribute flow toward tissues with greater demand. The kidneys and hormonal mechanisms help stabilize volume and pressure over longer periods.