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Cardiovascular System

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton, Jess Bell, Stacy Schiurring, Rucha Gadgil, Harshit Anand, Kim Jackson, Uchechukwu Chukwuemeka and Ahmed M Diab  

Introduction

The cardiovascular system, also known as the circulatory system, acts as a transportation system to supply the body with oxygenated blood and nutrients and remove waste products, such as carbon dioxide and metabolites from the body's cells. In essence, it is a sophisticated system of vessels (arteries, capillaries, and veins) powered by a strong muscular pump (the heart).[1]

Many mechanisms regulate the cardiovascular system, including changes in blood volume, hormonal factors, changes in electrolytes and osmolarity, various medications, the kidneys, etc. Both the parasympathetic and sympathetic divisions of the autonomic nervous system also play important roles in regulating the cardiovascular system.[2]

This article explores the cardiovascular system, covering the major anatomical structures and components, heart physiology and function and circulatory pathways. It also includes links to information on commonly encountered cardiovascular diagnoses.

Cardiovascular System Components and Function

Blood

There are three layers found in a centrifugal blood sample: the top layer contains blood plasma, the thin middle layer (known as the buffy coat) contains white blood cells and platelets, and the bottom layer contains red blood cells.[3]

Blood is a specialised connective tissue that carries nutrients, oxygen, hormones, and water to the cells and carbon dioxide and waste products away from the cells.[4] Blood is involved in regulating body temperature and pH and has a role in immunological functions and inflammatory responses.[5] It is also responsible for blood clot formation to prevent excess blood loss.

Blood is made up of four main components: (1) plasma, (2) red blood cells, (3) white blood cells, and (4) platelets. Plasma makes up approximately 55-60% of a person's total blood volume, red blood cells make up approximately 40-45%, and white blood cells and platelets make up less than 1%.[6]

Plasma is the liquid component of blood. It is a mixture of water, sugar, fat, protein, and salts. The primary function of plasma is to transport blood cells throughout the body along with nutrients, waste products, antibodies, clotting proteins, chemical messengers (eg. hormones), and proteins that help maintain the body's fluid balance.[7]

Red blood cells (RBCs, also known as erythrocytes) are biconcave, disc-shaped cells that are specialised for oxygen transport throughout the body. Red blood cells lack a nucleus and organelles. This maximises space for haemoglobin, the iron-containing protein that binds oxygen. Each red blood cell contains approximately 270 million haemoglobin molecules and can carry over a billion oxygen molecules. Red blood cells pick up oxygen in the lungs where high oxygen levels promote haemoglobin saturation, then release it in tissues where lower oxygen and higher carbon dioxide concentrations facilitate unloading (Bohr effect, please see additional resources for more information). They also help transport carbon dioxide back to the lungs for elimination.[8]

White blood cells (WBCs, also known as leukocytes) are immune system cells that defend the body against infections, foreign substances, and abnormal cells. There are several specialised types of white blood cells, each with distinct functions. Neutrophils are the most abundant white blood cell. They act as first responders to bacterial infections, engulfing pathogens through phagocytosis and releasing antimicrobial substances. Lymphocytes (includes B cells that produce antibodies and T cells) coordinate immune responses by killing infected cells or regulating immunity. Monocytes circulate in blood and become tissue macrophages that consume pathogens and debris while presenting antigens to other immune cells. Eosinophils combat parasites and mediate allergic reactions, while basophils release histamine during inflammation. White blood cells can migrate from blood vessels into tissues and circulate through both blood and lymphatic systems to provide comprehensive immune surveillance and protection.[9]

Platelets (also known as thrombocytes), are essential for blood clotting and haemostasis. These disc-shaped structures circulate for 8-10 days and contain granules filled with clotting factors, growth factors, and vasoactive substances. When blood vessels are injured, platelets rapidly adhere to exposed collagen through surface receptors. They then undergo activation and shape changes. This triggers platelet aggregation, resulting in an initial haemostatic plug that seals the vessel breach. Platelets also promote fibrin formation, stable clot development, aid wound healing, and participate in inflammatory responses.[10]

If you would like to learn more about blood and its components, please watch the following optional video:

[11]

If you would like to learn about blood-related diagnostics and medical diagnoses, please see the following optional reading pages:

Heart

The heart is a strong, muscular organ that beats regularly to pump oxygenated blood to the systemic system and deoxygenated blood to the pulmonary system.[12] It is located in the thoracic cavity, behind the sternum.[12] The heart and its great vessels are suspended in a fibrous fluid-filled sac called the pericardium, which stabilises the heart, supports cardiac contractions, and separates it from other structures in the thorax.[13]

The wall of the heart has three layers. From innermost to outermost, these layers are the: endocardium, myocardium, and epicardium.[14]

Structure of the Heart

The heart contains four chambers (right atrium, right ventricle, left atrium, left ventricle) that are divided by a muscular septum into a right and left pump. Each pump has an atrium and a ventricle. The four valves of the heart ensure a constant flow of blood in the right direction through the heart's chambers. The two atrioventricular valves direct blood from the atria to the ventricles and the semilunar valves direct blood flow from the ventricles to the outgoing arteries.[15][16]

The right atrium receives deoxygenated blood from the body (via the inferior and superior vena cavae) and the heart itself and sends it through the right atrioventricular valve (also known as the tricuspid valve) into the right ventricle. The right ventricle then pumps this blood through the pulmonary valve into the lungs via the pulmonary trunk. After gas exchange occurs in the lungs, the left atrium receives oxygenated blood via the pulmonary veins and sends it through the left atrioventricular valve (also known as the bicuspid or mitral valve) into the left ventricle. The left ventricle then pumps this blood through the aortic valve to the entire body via the aorta.

Circulatory Systems

The cardiovascular system consists of two main circulatory systems: (1) pulmonary and (2) systemic circulation systems. The pulmonary circulation, powered by the right ventricle, holds around 10% of the total blood volume at any given time. It is responsible for the oxygenation of blood via the lungs. It is a low-pressure system, with lower intravascular pressure compared to the systemic circulation.[1][2] The systemic circulation, powered by the left ventricle, contains approximately 85% of the total blood volume at any given time. It provides oxygenated blood and nutrients to the body. It involves high intravascular pressure. The remaining 5% of the blood volume is within the heart chambers themselves.[2][15][17]

Sinoatrial Node

The heart has its own pacemaker, called the sinoatrial (SA) node. The electrical signals generated here get carried through the left atrium, via the Bachmann’s bundle, and trigger atrial contraction. The signal then travels to the atrioventricular (AV) node; to the atrioventricular bundle (also known as the bundle of His) and finally through Purkinje fibres, which trigger ventricular contraction. Because our heart rate must be able to adapt to our body's changing physiological demands, it is also regulated by the endocrine and autonomic systems—for example, parasympathetic nervous system action decreases heart rate while sympathetic nervous system action increases heart rate. Without these extrinsic regulatory influences, the sinoatrial node would establish a resting heart rate of around 100 beats per minute.[18]

Vascular Supply to the Heart

The heart has its own blood supply.[19] 80% of blood to the heart comes from the left coronary artery, which divides into the left anterior descending artery and the circumflex coronary artery. The right coronary artery supplies the remaining 20% of blood to the heart.[16]

Cardiac Cycle

The cardiac cycle has two main phases: diastole (relaxation) and systole (contraction). During diastole, the heart's ventricles fill with blood as the atrioventricular valves open and semilunar valves remain closed. This phase includes isovolumetric relaxation (where all valves are closed and ventricular pressure drops), followed by ventricular filling and atrial contraction. Systole begins with isovolumetric contraction (where ventricular pressure rises with all valves closed until it exceeds arterial pressure). The semilunar valves then open during ventricular ejection, pumping blood into the pulmonary artery and aorta while the atrioventricular valves remain closed to prevent blood backflow. The cardiac cycle completes as ventricular pressure falls below arterial pressure, causing the semilunar valves to close. The atrioventricular valves stay closed until ventricular pressure drops below atrial pressure to begin the next diastolic phase. This coordinated sequence ensures efficient blood circulation throughout the body.[20]

Heart sounds are created by the opening and closing of the cardiac valves as blood moves through the heart chambers during the cardiac cycle. Audible sounds are created by the vibrations of these structures to produce the characteristic "lub-dub" sounds heard during auscultation.[21] Heart auscultation involves the use of a stethoscope to listen to the sounds produced by the heart.

The cardiac cycle produces characteristic heart sounds through valve closure during pressure changes. The first heart sound, "lub" (S1), occurs at the beginning of systole when the atrioventricular valves close as ventricular pressure rises above atrial pressure during isovolumetric contraction. The second heart sound, "dub" (S2), occurs at the end of systole when the semilunar valves close as ventricular pressure falls below arterial pressure during isovolumetric relaxation. S2 is typically higher-pitched than S1 due to higher arterial pressures. Additional sounds include S3, which can occur during early diastolic filling, and S4, which occurs during atrial contraction just before S1. The timing of these sounds directly corresponds to cardiac cycle phases, with S1 marking the onset of systole and S2 marking its end, making heart sounds valuable clinical tools for assessing cardiac function and valve operation.[21]

If you would like to learn about heart-related diagnostics and medical diagnoses, please see the following optional reading pages:

Blood Vessels

Blood is transported throughout the body by an extensive network of arteries, capillaries, and veins.

Arteries and veins have three layers of tissue. From innermost to outermost, these layers are the tunica intima, tunica media and tunica externa (or adventitia). The capillaries only contain the tunica intima.[22]

Arteries

Arteries always carry blood away from the heart. In the systemic circuit, they carry oxygenated blood away from the heart to the body via the aorta; in the pulmonary circuit, they carry deoxygenated blood away from the heart to the lungs via the pulmonary arteries.[23]

Names written in red denote arteries, and names written in blue denote veins.

The aorta originates at the left ventricle. All systemic arteries are either direct or indirect branches of the aorta. The aorta divides into four regions: (1) ascending aorta, (2) aortic arch, (3) thoracic aorta, and (4) abdominal aorta.[24]

The coronary arteries branch from the ascending aorta and supply the heart with blood. The head, neck and arms are supplied by the brachiocephalic artery, the left common carotid artery, and the left subclavian artery; these arteries branch from the aortic arch. The thoracic and abdominal aorta divide into many visceral and parietal branches, which supply the viscera and the walls of the thoracic and abdominal cavities. The abdominal aorta eventually divides into the common iliac arteries, which supply the pelvis, gluteal regions, and lower limbs.[16]

There are elastic arteries, muscular arteries, and arterioles. Elastic arteries are the largest arteries. They are closest to the heart (e.g. aorta, pulmonary arteries). They have large amounts of elastin in their tunica media, which enables them to have a "relatively constant pressure gradient despite the constant pumping action of the heart."[25] Muscular arteries have more smooth muscle cells in their tunica media than elastic arteries.[25] Arteries gradually transition into arterioles. Arterioles feed blood into capillary networks and are involved in regulating the rate of blood flow, blood pressure and vascular resistance.[16][26]

Capillaries

Capillaries are thin-walled vessels, consisting of just the tunica intima. This structure enables the exchange of gases (oxygen and carbon dioxide), nutrients and waste between the capillaries and the tissue interstitium.[25][27] The exchange of fluids at the capillaries depends on hydrostatic and osmotic pressure.[28]

Veins

Veins always carry blood towards the heart. In systemic circulation, they carry deoxygenated blood back towards the heart via the inferior and superior vena cavae; in pulmonary circulation, they carry oxygenated blood towards the heart via pulmonary veins.

Capillaries feed into venules, which eventually transition into larger veins. Veins have three layers like arteries, but they are thin-walled and less elastic as the blood being transported is at a lower pressure. Veins often need to transport blood against the pull of gravity, therefore they contain a system of valves that prevent the backflow of blood. They also rely on the contraction of skeletal muscles to aid in the transport of blood.[2] These thin-walled vessels are sensitive to compression when subjected to prolonged static pressure. Capillary bed compression can lead to tissue ischaemia and pressure wound formation.

Optional related reading: the lymphatic system plays a role in draining excess interstitial fluid, immunity, and transporting dietary lipids. It is technically not part of the cardiovascular system, but it is involved in fluid circulation throughout the body and works in tandem with the cardiovascular system.

If you would like to learn about vascular-related diagnostics and medical diagnoses, please see the following optional reading pages:

Additional Resources

Optional Videos

  • The following optional 23-minute video provides an overview of the cardiovascular system.

[29]

  • The following optional 18-minute video explains the connection between the cardiac cycle and heart sounds.

[30]

  • The following optional 5:30 minute video explains the Bohr effect

[31]

Clinical Resources

The Cardiovascular System 3D models (Innerbody)

References

  1. ↑ 1.0 1.1 Witzleb E. Functions of the vascular system. InHuman physiology. Berlin: Heidelberg, Springer;1989. p480-542.
  2. ↑ 2.0 2.1 2.2 2.3 Chaudhry R, Miao JH, Rehman A. Physiology, Cardiovascular. [Updated 2022 Oct 16]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Available from:https://www.ncbi.nlm.nih.gov/books/NBK493197/
  3. ↑ Akadeum LifeSciences. What Is a Buffy Coat?. Available from: https://www.akadeum.com/blog/what-is-buffy-coat/ (accessed 07/July/2025).
  4. ↑ Kamrani P, Marston G, Arbor TC, et al. Anatomy, Connective Tissue. [Updated 2023 Mar 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538534/
  5. ↑ Cherian VT. Physiological functions of blood. In: Liu H, Kaye AD, Jahr JS. Editors. Blood substitutes and oxygen biotherapeutics. Springer, Cham, 2022.
  6. ↑ Sharma R, Sharma S. Physiology, Blood Volume. [Updated 2023 April 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Available from:https://www.ncbi.nlm.nih.gov/books/NBK526077/
  7. ↑ Mathew J, Sankar P, Varacallo MA. Physiology, Blood Plasma. [Updated 2023 Apr 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK531504/
  8. ↑ Barbalato L, Pillarisetty LS. Histology, Red Blood Cell. [Updated 2022 Nov 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539702/
  9. ↑ Tigner A, Ibrahim S, Murray I. Histology, White Blood Cell. [Updated 2022 Nov 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK563148/
  10. ↑ Fountain J, Lappin S. Physiology, Platelet. [Updated 202# July 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470328/
  11. ↑ Dr Matt & Dr Mike. Blood Components (Hematocrit). Available from: http://www.youtube.com/watch?v=fE1GfwKsvjc [last accessed 13/5/2025]
  12. ↑ 12.0 12.1 Rehman I, Rehman A. Anatomy, Thorax, Heart. [Updated 2023 Aug 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470256/
  13. ↑ Rehman I, Nassereddin A, Rehman A. Anatomy, Thorax, Pericardium. [Updated 2023 July 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482256/
  14. ↑ Arackal A, Alsayouri K. Histology, Heart. [Updated 2023 Jan 2]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK545143/
  15. ↑ 15.0 15.1 Morris JL, Nilsson S. The circulatory system. InComparative physiology and evolution of the autonomic nervous system 2021 Sep 28 (pp. 193-246). Routledge.
  16. ↑ 16.0 16.1 16.2 16.3 Xuan D. Overview of the Cardiovascular and Respiratory Systems Course. Physiopedia Plus, 2025.
  17. ↑ Satish M, Tadi P. Physiology, Vascular. InStatPearls [Internet] 2019. StatPearls Publishing. Available from:https://www.statpearls.com/kb/viewarticle/31012 (accessed 28 August 2020)
  18. ↑ Gordan R, Gwathmey JK, Xie LH. Autonomic and endocrine control of cardiovascular function. World J Cardiol. 2015 Apr 26;7(4):204-14.
  19. ↑ Goodwill AG, Dick GM, Kiel AM, Tune JD. Regulation of coronary blood flow. Compr Physiol. 2017 Mar 16;7(2):321-382.
  20. ↑ Pollock J, Makaryus AN. Physiology, Cardiac Cycle. [Updated 2022 Oct 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459327/
  21. ↑ 21.0 21.1 Dornbush S, Turnquest A. Physiology, Heart Sounds. [Updated 2023 July 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Available from: https://www.ncbi.nlm.nih.gov/books/NBK541010/
  22. ↑ Gao Y. Architecture of the blood vessels. In: Biology of vascular smooth muscle. Springer: Singapore, 2022.
  23. ↑ Segers P, Verdonck P. Principles of vascular physiology. 2002. In: Lanzer, P., Topol, E.J. (eds) Pan Vascular Medicine. Springer, Berlin, Heidelberg.
  24. ↑ Nursing Times. Vascular system 1: anatomy and physiology. Available from: https://www.nursingtimes.net/cardiovascular/vascular-system-1-anatomy-and-physiology-26-03-2018/ (last accessed 13 May 2025).
  25. ↑ 25.0 25.1 25.2 Tucker WD, Arora Y, Mahajan K. Anatomy, Blood Vessels. [Updated 2023 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470401/
  26. ↑ Rahman M, Siddik AB. Anatomy, Arterioles. [Updated 2023 Jan 13]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK555921/
  27. ↑ Godwin L, Tariq MA, Crane JS. Histology, Capillary. [Updated 2023 Apr 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK546578/
  28. ↑ Manning GS, Kay AR. The physical basis of osmosis. J Gen Physiol 2 October. 2023;155(10):e202313332.
  29. ↑ Susannaheinze.Cardiovascular System | Summary. Available from: http://www.youtube.com/watch?v=JDWeq0xg9nA[last accessed 31/8/2020]
  30. ↑ YouTube. Heart Sounds and Auscultation | Ninja Nerd Nursing. The Evolution of Dance. Available from: https://www.youtube.com/watch?v=OlnkTJTAUvQ [last accessed 04/July/2025]
  31. ↑ YouTube. The Bohr Effect | For A-level & Up Revision. The Evolution of Dance. Available from: https://www.youtube.com/watch?v=n0MMzv0NITw[last accessed 15/July/2025]
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