Cardiovascular System

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton, Harshit Anand, Uchechukwu Chukwuemeka and Kim Jackson  

Introduction[edit | edit source]

Circulatory system.png

The vessels of the cardiovascular system are the heart, arteries, capillaries, and veins.

This is a transport system, within which the blood is propelled by the heart in a closed circuit through vessels.

This continual circulation of fluid throughout the body serves:

  • As a means of delivery and removal of substances;
  • It provides all the living cells of the organism with the materials required for their normal functions (e.g., O2 and nutrients)
  • It carries away the products of cell metabolism (CO2 and other metabolites).
  • These substances do not enter and leave the bloodstream directly; their passage is indirect, by way of the interstitial (extracellular) fluid[1].

The regulation of the cardiovascular system occurs via an innumerable number of stimuli, including changing blood volume, hormones, electrolytes, osmolarity, medications, adrenal glands, kidneys, and much more. The parasympathetic and sympathetic nervous systems also play a key role in the regulation of the cardiovascular system[2].

Sections[edit | edit source]

The human circulatory system consists of two main sections arranged one after the other (in

series) :

  • Systemic circulation: Left ventricle as the pump (85% of the circulation, remaining 5% is in the heart) with a high intravascular pressure[3]
  • Pulmonary circulation: Right ventricle as the pump[1] (10% of the circulation); a low-pressure system with low intravascular pressure

The pulmonary circulation allows for oxygenation of the blood, and the systemic circulation provides for oxygenated blood and nutrients to reach the rest of the body[2].

Organs involved[edit | edit source]


Heart[edit | edit source]

The organ that pumps the blood through the vessels. It pumps blood directly into arteries, more specifically the aorta or the pulmonary artery.

  • The heart pumps oxygenated blood out of the left ventricle and into the aorta to begin systemic circulation.
  • After the blood has supplied cells throughout the body with oxygen and nutrients, it returns deoxygenated blood to the right atrium of the heart.
  • The deoxygenated blood flows down from the right atrium to the right ventricle.
  • The heart then pumps it out of the right ventricle and into the pulmonary arteries to begin pulmonary circulation.
  • The blood moves to the lungs, exchanges carbon dioxide for oxygen, and returns to the left atrium.
  • The oxygenated blood shoots from the left atrium to the left ventricle below, to begin systemic circulation again.
  • Each half of the heart has a receiving chamber called Atrium and a pumping chamber called Ventricle.
  • Cardiac output in humans is generally 5-6 L/min in an at-rest to more than 35 L/min in elite athletes during exercise.[4]
  • The heart weighs about 300gm.
  • Two thin-walled atria are separated from each other by an interatrial septum.
  • Two thick-walled ventricles are separated from each other by an interventricular septum.

LV = Left Ventricle

RV = Right Ventricle

LA = Left Atrium

RA = Right Atrium

LV ➜ RA ➜ RV ➜ lungs ➜ LA ➜ LV

LV ➜ Aortic valve ➜ Aorta ➜ Sup. & Inf. Vena Cava ➜ RA ➜ Tricuspid valve ➜ RV ➜ Pulmonary valve ➜ Pulmonary artery ➜ lungs ➜Pulmonary vein ➜ LA ➜ Mitral valve ➜ LV.

Types of Valves[edit | edit source]

Atria-Ventricular Valve (AV Valve)

  1. Tricuspid valve
  2. Mitral valve

Semilunar valve

  1. Pulmonary valve
  2. Aortic valve

Arteries[edit | edit source]

Artery and Vein.jpg

Arteries supply the body with oxygenated blood – with the exception of the pulmonary arteries from the heart; these carry deoxygenated blood to the lungs, and the umbilical artery, which carries deoxygenated blood from the foetus to the placenta. Blood travels from the arteries to the arterioles and on to the capillaries, where gaseous exchange takes place.

The largest artery is the aorta, which extends from the left ventricle down the left side of the body. It divides into four major regions, the ascending aorta, aortic arch, thoracic aorta, and abdominal aorta.

Arteries can be divided into elastic arteries, muscular arteries, and arterioles

  1. Elastic arteries are the largest (1-2.5cm in diameter) and comprise large amounts of elastin as well as smooth muscle. They have a large lumen with a low resistance to blood flow and can expand and recoil to accommodate changes in blood volume.
  2. Muscular arteries regulate local blood flow and deliver blood to individual organs. They measure 0.3mm-1cm in diameter and possess more smooth muscle but less elastin than elastic arteries.
  3. Arterioles are the smallest arteries (0.01-0.3mm in diameter). In certain areas, they have all three vascular layers (tunica intima, media, and externa). When they are close to the capillaries they comprise a single smooth muscle layer overlying endothelial cells[5].

Capillaries[edit | edit source]

Blood brain barrier.png

The capillaries can be compared to the smallest branches of a tree and connect arterioles to venules. This thin layer allows for the exchange of nutrients, gases, and waste with tissues and organs.

  • Blood flow into the capillaries is determined by the diameter of the arterioles and can be increased through vasodilation.
  • Capillaries act as a semipermeable membrane allowing the diffusion of gases and the transfer of nutrients and waste products.
  • The single layer of flattened endothelial cells of the capillaries facilitates the exchange of substances between capillaries and tissues.
  • Gases, such as O2 and CO2, metabolic waste products, lactate, glucose, and other nutrients are transferred across the walls of the capillaries through small slits in the endothelial cells known as pores or fenestrations.
  • To prevent capillaries from losing vital substances such as plasma proteins, the slits in the endothelial cells are smaller than these proteins[5].

Veins[edit | edit source]

Transport blood back to the heart, contain valves to prevent the backflow of blood[2]. Veins are:

  • Thin, elastic vessels that act as a reservoir of blood
  • Do not need large amounts of elastin and smooth muscle as they transport low-pressure blood back to the heart
  • Have a large lumen, as well as valves that ensure a one-way flow of blood to the heart.


  • Venules measure 8-100µ in diameter and the largest ones possess a thin tunica externa and a tunica media comprising two or three layers of smooth muscle cells.
  • The venules join to form veins, in which the tunica externa, consisting of thick collagenous bundles, is the largest layer.
  • The largest veins – the superior and inferior venae cava – have a large tunica externa further thickened by smooth muscle bands
  • The venous system is an irregular network that tends to follow the course of the arteries[5].


Types of Circulation of Blood[edit | edit source]

Types of Circulatory Pathways


Systemic Circulation (greater)[edit | edit source]

Left Ventricle ➜ body ➜ Right Atrium

Pulmonary Circulation (lesser)[edit | edit source]

Right Ventricle ➜ lungs ➜ Left Atrium

Portal Circulation[edit | edit source]

The circulation of nutrient-rich blood between the gut and liver is called portal circulation.

It enables the liver to remove any harmful substances that may have been digested before the blood enters the main blood circulation around the body—the systemic circulation.

Blood flow from the abdominal organs that passes through the portal vein, of the liver, and into the hepatic vein before returning to the heart from the inferior vena cava. This pathway permits the liver to process and detoxify substances entering the body from the gastrointestinal tract.

Renal Circulation[edit | edit source]

The renal circulation supplies the blood to the kidneys via the renal arteries, left and right, which branch directly from the abdominal aorta. Despite their relatively small size, the kidneys receive approximately 20% of the cardiac output. Blood mainly comes to the kidney for purification.

First, the kidney receives approximately 20–25% of the cardiac output even though it is less than 1% of body weight. Blood flows through most organs to provide oxygen and nutrients but blood traverses through the kidney primarily to be cleansed.

Cardiac Cycle[edit | edit source]

Cardiac cycle.png[8]

Cardiac Cycle Phases[edit | edit source]

The two main phases of the cardiac cycle are systole (the contraction phase) and diastole (the relaxation phase)

Diastole[edit | edit source]

The diastole phase begins with the relaxation of all the heart muscles. During diastole, blood returns to the heart and begins to fill the atria and ventricles. The lack of pressure in the ventricle allows the mitral and tricuspid valves to open, which allow blood from the atria into the left and right ventricles respectively. This phase of the cardiac cycle can be seen in the image below:



Systole[edit | edit source]

Another wave of contraction starts as the ventricles enter the ventricular systole and begin contracting themselves. The increased pressure in the ventricles closes the mitral and tricuspid valves. The pressure pushes open the aortic and pulmonary valves. This can be seen in the image below. This starts the systole part of the cycle.



Phases of systole and diastole[edit | edit source]

Systole - 0.27(0.3)

Diastole - 0.53(0.5)

Total Cardiac Cycle is 0.27+0.53=0.8 sec

Systole phase[edit | edit source]

Isometric contraction[edit | edit source]

All the valves are closed, the ventricle undergoes isometric contraction, and pressure in the ventricle is increased.

Ejection period[edit | edit source]

Semilunar valves are open, the ventricle contracts, and blood is ejected out.

Diastole Phase[edit | edit source]

Proto diastole[edit | edit source]

In the first stage of diastole, the semilunar valves are closed at the end of this period.

Isometric relaxation[edit | edit source]

All the valves are closed, the ventricle undergoes isometric relaxation, and pressure in the ventricle is reduced.

Rapid & slow filling[edit | edit source]

Atrioventricular valves are opened, the ventricle relaxes, and filling occurs.

Last Rapid filling[edit | edit source]

Atrial contract and a small amount of blood enter the ventricle.

Heart Sounds[edit | edit source]

  • The first and second heart sounds are known as the classical heart sound and are heard through a stethoscope.
  • Two heart sounds are more prominent and resemble the word 'lub'(or lubb) and 'dub'(or dubb).
  • The third heart sound is mild and not heard by a stethoscope but can be through a microphone.
  • The fourth heart sound is an inaudible sound, it becomes audible in pathological conditions only. This heart sound is studied only by graphical registration i.e. the phonocardiogram.

For more details on the various heart sounds, refer to Auscultation.

Various Heart Sounds


Cardiovascular Disease[edit | edit source]

Coronary heart disease.png

According to the World Health Organization, cardiovascular disease is the leading cause of death for people world-wide. Cardiovascular disease involves disorders of the heart and blood vessels eg coronary artery disease, stroke, hypertension, and heart failure[12].

  • Ageing/Hypertension: Persistently elevated blood pressure (high blood pressure) in the arteries, associated with the development of disorders such as atherosclerosis, heart attack, and stroke, and can cause kidney damage.
    • There is a decrease in compliance with age due to a loss in elasticity (notably with the aorta).
    • Increase in fibrotic changes (an increase in collagen fibers and their cross-linking).
    • With older age, higher blood pressures become prevalent.
    • The pathophysiology of hypertension includes diabetes, hypercholesterolemia, and cigarette smoking (contributing to the arteriosclerotic changes).
  • Coronary artery disease (heart disease): narrowing or blockage in the coronary arteries, which supply blood directly to the heart muscle. Complete blockage of blood flow will cause a heart attack.
  • Stroke is the death of brain cells (neurons) due to the lack of blood supply.
  • Heart failure: the heart is not able to supply enough blood to body tissues. It is caused by conditions such as hypertension, heart disease, and cardiomyopathy (chronic disease of the heart muscle)[12].
  • Vessel Aneurysm:
    • With a loss or defective elastic components of a vessel, the outward transmural pressure (distension) can be unbalanced with an inadequate tension force allowing for an aneurysm to form (susceptible to rupture).
    • The aneurysmal rupture in the brain results in life-threatening subarachnoid hemorrhage.
  • Atherosclerosis:
    • A form of arteriosclerosis characterized by the deposition of atheromatous plaques containing cholesterol and lipids on the innermost layer of the walls of large and medium-sized arteries.
  • Dissection:
    • The abnormal, and usually an abrupt formation of a tear along the inside wall of an artery. As the tear becomes larger, it forms a small pouch which is called a "false lumen." The blood that accumulates inside this false lumen can generate blood clots or otherwise block blood flow, leading to a stroke[3].

Assessment[edit | edit source]

High Blood Pressure Blausen.png

The following items are important for the clinical assessment of the vascular system:

  1. Blood pressure: normal BP -  less than 120 (systolic) and under 80 (diastolic); elevated BP - 120 to 129 for systole and less than 80 for diastole.
  2. Pulses - Palpation in the arterial regions of the carotid, femoral, popliteal, dorsalis pedis, posterior tibial, etc. for the distribution of blood flow.
  3. Bruit – Auscultation for vibration sound localized to an arterial wall that is caused by turbulence and suggests a lesion.
  4. Skin findings – Cyanosis, pallor, edema, and ulceration can indicate a vascular etiology. With ulceration, distal ulceration of the foot is more associated with an arterial etiology compared to ulceration of the malleolar region, which better suggests a venous cause.
  5. Capillary refill – Refers to the amount of time required for refill after compression of a nailbed, and indicates perfusion status[3].

Summary[edit | edit source]

  • The vasculature works with the heart to supply the body with oxygen and nutrients and to remove waste products
  • There are five classes of blood vessels: arteries, arterioles, veins, venules, and capillaries
  • Capillaries allow the diffusion of gases and transfer of nutrients and waste products between blood and tissues
  • Blood flow and blood pressure are regulated by nervous, chemical, and hormonal mechanisms
  • Some organs and tissues can automatically adjust their own blood flow[5]

References[edit | edit source]

  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 Chaudhry R, Rehman A. Physiology, cardiovascular. InStatPearls [Internet] 2020. StatPearls Publishing.Available from: (accessed 28 August 2020)
  3. 3.0 3.1 3.2 Satish M, Tadi P. Physiology, Vascular. InStatPearls [Internet] 2019. StatPearls Publishing. Available from: (accessed 28 August 2020)
  4. Cardiac output in humans is generally 5-6 L/min in an at-rest to more than 35 L/min in elite athletes during exercise.[1]
  5. 5.0 5.1 5.2 5.3 Nursing times. Vascular system. Available from: (accessed 29 August 2020)
  6. Susannaheinze.Cardiovascular System | Summary. Available from:[last accessed 31/8/2020]
  7. [2]
  8. [3]
  9. [4]
  10. [5]
  11. [6]
  12. 12.0 12.1 Thoughtco.Cardiovascular disease. Available from: (accessed 28 August 2020)