CARDIOVASCULAR SYSTEM: PHYSIOLOGY, ORGANS FUNCTIONS, HISTOLOGY - ANATOMY AND PHYSIOLOGY - 2024

The cardiovascular system is a complex set of blood vessels that transports substances between cells and blood, and between blood and the environment. Its components are the heart, blood vessels and blood.

The functions of the cardiovascular system are: 1) distribute oxygen and nutrients to the tissues of the body; 2) transport carbon dioxide and metabolic waste products from tissues to the lungs and excretory organs; 3) contribute to the functioning of the immune system and thermoregulation.

Source: Edoarado

The heart acts as two pumps, one for the pulmonary circulation and one for the systemic one. Both circulations require the chambers of the heart to contract in an orderly fashion, moving the blood unidirectionally.

Pulmonary circulation is the flow of blood between the lungs and the heart. It allows the exchange of blood gases and the pulmonary alveoli. Systemic circulation is the flow of blood between the heart and the rest of the body, excluding the lungs. It involves the blood vessels inside and outside the organs.

The study of congenital heart diseases has allowed great advances in the understanding of the anatomy of the heart in newborns and adults, and of the genes or chromosomes involved in congenital defects.

A large number of heart diseases acquired during life depend on factors such as age, sex, or family history. A healthy diet, physical exercise, and medications can prevent or control these diseases.

Reliable diagnosis of diseases of the circulatory system has been made possible by technological advances in imaging. Similarly, advances in surgery have allowed most congenital defects, and many non-congenital diseases, to be remedied.

Anatomy and Histology of the Heart

Cameras

The heart has a functionally different left and right side. Each side of the is divided into two chambers, an upper one called the atrium and a lower one called the ventricle. Both chambers are made up primarily of a special type of muscle called the cardiac.

The atria, or upper chambers, are separated by the interatrial septum. The ventricles, or lower chambers, are separated by the interventricular septum. The wall of the right atrium is thin. Three veins discharge blood into its interior: the superior and inferior vena cava, and the coronary sinus. This blood comes from the body.

Parts of the heart. Source: Diagram_of_the_human_heart_ (cropped) _pt.svg: Rhcastilhosderivative work: Ortisa

The left atrium wall is three times thicker than the right one. Four pulmonary veins discharge oxygenated blood into the left atrium. This blood comes from the lungs.

The walls of the ventricles, especially the left, are much thicker than that of the atria. The pulmonary artery starts from the right ventricle, which directs blood to the lungs. The aorta starts from the left ventricle, which directs blood to the rest of the body.

The inner surface of the ventricles is ribbed, with bundles and bands of muscle, called trabeculae carneae. The papillary muscles project into the cavity of the ventricles.

Valves

Each opening of the ventricles is protected by a valve that prevents the return of blood flow. There are two types of valve: the atrioventricular (mitral and tricuspid) and the semilunar (pulmonary and aortic).

The mitral valve, which is bicuspid, connects the left atrium (atrium) with the ventricle on the same side. The tricuspid valve connects the right atrium (atrium) with the ventricle on the same side.

Cusps are leaf-shaped folds of the endocardium (a membrane reinforced with fibrous connective tissue). The cusps and papillary muscles of the atrioventricular valves are joined by structures, called chordae tendinae, in the shape of thin cords.

Semilunar valves are pocket-shaped structures. The pulmonary valve, composed of two leaflets, connects the right ventricle with the pulmonary artery. The aortic valve, composed of three leaflets, connects the left ventricle with the aorta.

A band of fibrous connective tissue (annulus fibrosus), which separates the atria from the ventricles, provides surfaces for muscle attachment and valve insertion.

Wall

The wall of the heart consists of four layers: endocardium (inner layer), myocardium (inner middle layer), epicardium (outer middle layer), and pericardium (outer layer).

The endocardium is a thin layer of cells similar to the endothelium of blood vessels. The myocardium contains the contractile elements of the heart.

The myocardium consists of muscle cells. Each of these cells has myofibrils that form contractile units called sarcomeres. Each sarcomere has actin filaments that project from opposite lines, and are organized around thick myosin filaments.

The epicardium is a layer of mesothelial cells penetrated by coronary vessels leading to the myocardium. These vessels supply arterial blood to the heart.

The pericardium is a loose layer of epithelial cells that rests on connective tissue. It forms a membranous sac in which the heart is suspended. It is attached below to the diaphragm, on the sides to the pleura, and in front of the sternum.

Histology of the vascular system

The great blood vessels share a three-layered structure, namely: tunica intima, tunica media, and tunica adventitia.

The tunica intima, which is the innermost layer, is a monolayer of endothelial cells covered by elastic tissue. This layer controls vascular permeability, vasoconstriction, angiogenesis, and regulates coagulation.

The tunica intima of the veins of the arms and legs has valves that prevent the return flow of blood, directing it towards the heart. These valves consist of endothelium and little connective tissue.

The tunica media, which is the intermediate layer, is separated from the intima by an internal elastic sheet, composed of elastin. The tunica media is composed of smooth muscle cells, embedded in an extracellular matrix, and elastic fibers. In the arteries, the tunica media is thick, while in the veins it is thin.

The tunica adventitia, which is the outermost layer, is the strongest of the three layers. It is composed of collagen and elastic fibers. This layer is a limiting barrier, protecting vessels from expansion. In large arteries and veins, the adventitia contains vasa vasorum, small blood vessels that supply the vascular wall with oxygen and nutrients.

Physiology of the heart

Driving system

The regular contraction of the heart is the result of the inherent rhythm of the heart muscle. The contraction begins in the atria. It follows the contraction of the ventricles (atrial and ventricular systole). Relaxation of the atrial and ventricular chambers (diastole) follows.

A specialized cardiac conduction system is responsible for firing the electrical activity and transmitting it to all parts of the myocardium. This system consists of:

- Two small masses of specialized tissue, namely: sinoatrial node (SA node) and atrioventricular node (AV node).

- The His bundle with its branches and the Purkinje system, located in the ventricles.

In the human heart, the SA node is located in the right atrium, next to the superior vena cava. The AV node is located in the right posterior part of the interatrial septum.

Rhythmic cardiac contractions originate from a spontaneously generated electrical impulse at the SA node. The speed of electrical impulse generation is controlled by the pacemaker cells of this node.

The pulse generated in the SA node passes through the AV node. Then, it continues through the bundle of His and its branches towards the Purkinje system, in the ventricular muscle.

Cardiac muscle

Cardiac muscle cells are connected by intercalated discs. These cells are connected to each other in series and in parallel and thus form muscle fibers.

The cell membranes of the intercalated discs fuse with each other to form permeable communicating junctions that allow rapid diffusion of ions and thus electrical current. Because all cells are electrically connected, the heart muscle is said to be functionally an electrical syncytium.

The heart is made up of two syncytics:

- The one of the atrium, constituted by the walls of the atriums.

- The ventricular, made up of the walls of the ventricles.

This division of the heart allows the atria to contract shortly before the ventricles contract, making the heart pump effectively.

Action potential of the heart muscle

The distribution of ions across the cell membrane produces a difference in electrical potential between the inside and outside of the cell, which is known as the membrane potential.

The resting membrane potential of a mammalian heart cell is -90 mV. A stimulus produces an action potential, which is a change in membrane potential. This potential spreads and is responsible for the onset of contraction. The action potential happens in phases.

In the depolarization phase, the cardiac cell is stimulated and the opening of the voltage-gated sodium channels and the entry of sodium into the cell occurs. Before the channels close, the membrane potential reaches +20 mV.

In the initial repolarization phase, sodium channels close, the cell begins to repolarize, and potassium ions leave the cell through potassium channels.

In the plateau phase, the opening of calcium channels and the rapid closure of potassium channels take place. The rapid repolarization phase, the closure of calcium channels, and the slow opening of potassium channels return the cell to its resting potential.

Contractile response

The opening of voltage-dependent calcium channels in muscle cells is one of the events of depolarization that allows Ca ^{+2 to} enter the myocardium. Ca ⁺² is an effector that couples depolarization and cardiac contraction.

After depolarization of cells, Ca ⁺² entry occurs, which triggers the release of additional Ca ⁺² through Ca ^+2- sensitive channels in the sarcoplasmic reticulum. This increases the Ca ⁺² concentration one hundred times.

The contractile response of the heart muscle begins after depolarization. When muscle cells repolarize, the saccoplasmic reticulum reabsorbs excess Ca ⁺². The Ca ⁺² concentration returns to its initial level, allowing the muscle to relax.

The statement of Starling's law of the heart is "the energy released during contraction depends on the length of the initial fiber." At rest, the initial length of the fibers is determined by the degree of diastolic filling of the heart. The pressure that develops in the ventricle is proportional to the volume of the ventricle at the end of the filling phase.

Heart function: cardiac cycle and electrocardiograms

In late diastole, the mitral and tricuspid valves are open and the aortic and pulmonary valves are closed. Throughout diastole, blood enters the heart and fills the atria and ventricles. The filling rate slows as the ventricles expand and the AV valves close.

Contraction of the atrial muscles, or atrial systole, reduces the foramina of the superior and inferior vena cava and the pulmonary vein. Blood tends to be held in the heart by the inertia of the movement of the incoming blood.

Ventricular contraction, or ventricular systole, begins and the AV valves close. During this phase the ventricular muscle shortens little and the myocardium presses the blood on the ventricle. This is called isovolumic pressure, it lasts until the pressure in the ventricles exceeds the pressure in the aorta and the pulmonary artery and its valves open.

The measurement of the fluctuations in the potential of the cardiac cycle are reflected in the electrocardiogram: the P wave is produced by the depolarization of the atria; the QRS complex is dominated by ventricular depolarization; the T wave is the repolarization of the ventricles.

Functioning of the circulatory system

Components

The circulation is divided into systemic (or peripheral) and pulmonary. The components of the circulatory system are veins, venules, arteries, arterioles, and capillaries.

Venules receive blood from capillaries and gradually merge with large veins. Veins carry blood back to the heart. The pressure in the venous system is low. The vessel walls are thin but muscular enough to contract and expand. This allows them to be a controllable reservoir of blood.

The arteries have the function of transporting blood under high pressure to the tissues. Because of this, the arteries have strong vascular walls and the blood moves at high speed.

The arterioles are small branches of the arterial system, which act as control conduits through which blood is transported to the capillaries. The arterioles have strong muscular walls that can contract or dilate several times. This allows the arteries to alter blood flow as needed.

Capillaries are small vessels in the arterioles that allow the exchange of nutrients, electrolytes, hormones, and other substances between blood and interstitial fluid. Capillary walls are thin and have many pores that are permeable to water and small molecules.

Pressure

When the ventricles contract, the internal pressure of the left ventricle increases from zero to 120 mm Hg. This causes the aortic valve to open and blood flow to be expelled into the aorta, which is the first artery of the systemic circulation. The maximum pressure during systole is called the systolic pressure.

The aortic valve then closes and the left ventricle relaxes, so blood can enter from the left atrium through the mitral valve. The period of relaxation is called diastole. During this period the pressure drops to 80 mm Hg.

The difference between the systolic and diastolic pressure is, therefore, 40 mm Hg, being denominated as pulse pressure. The complex arterial tree reduces the pressure of the pulsations, making, with a few pulsations, the blood flow to the tissues is continuous.

The contraction of the right ventricle, which occurs simultaneously with that of the left, pushes blood through the pulmonary valve and into the pulmonary artery. This is divided into small arteries, arterioles and capillaries of the pulmonary circulation. Pulmonary pressure is much lower (10–20 mm Hg) than systemic pressure.

Circulatory response to bleeding

Bleeding can be external or internal. When they are large, they require immediate medical attention. A significant decrease in blood volume causes a drop in blood pressure, which is the force that moves blood in the circulatory system to provide the oxygen that tissues need to stay alive.

The drop in blood pressure is perceived by baroreceptors, which decrease their discharge rate. The cardiovascular center of the brainstem located at the base of the brain detects decreased activity of basoreceptors, which unleashes a series of homeostatic mechanisms that seek to restore normal blood pressure.

The medullary cardiovascular center increases the sympathetic stimulation of the right sinoatrial node, which: 1) increases the force of contraction of the heart muscle, increasing the volume of blood pumped in each pulse; 2) increases the number of beats per unit of time. Both processes increase blood pressure.

Simultaneously, the medullary cardiovascular center stimulates the contraction (vasoconstriction) of certain blood vessels, forcing part of the blood they contain to move to the rest of the circulatory system, including the heart, increasing blood pressure.

Circulatory response to exercise

During exercise, body tissues increase their need for oxygen. Therefore, during extreme aerobic exercise, the rate of blood pumping through the heart should rise from 5 to 35 liters per minute. The most obvious mechanism to achieve this is the increase in the number of heart beats per unit of time.

The increase in pulsations is accompanied by: 1) arterial vasodilation in the muscles; 2) vasoconstriction in the digestive and renal systems; 3) vasoconstriction of veins, which increases venous return to the heart and, therefore, the amount of blood it can pump. Thus, the muscles receive more blood and therefore more oxygen

The nervous system, particularly the medullary cardiovascular center, plays a fundamental role in these responses to exercise through sympathetic stimulations.

Embryology

In week 4 of human embryonic development, the circulatory system and blood begin to form into "blood islands" that appear in the mesodermal wall of the yolk sac. By this time, the embryo begins to be too large for the distribution of oxygen to be carried out only by diffusion.

The first blood, consisting of nucleated erythrocytes such as those of reptiles, amphibians, and fish, is derived from cells called hemangioblasts, located in "blood islands."

In weeks 6–8, blood production, consisting of typical mammalian nucleusless red blood cells, begins to move to the liver. By month 6, erythrocytes colonize the bone marrow and their production by the liver begins to decline, ceasing in the early neonatal period.

Embryonic blood vessels are formed by three mechanisms:

- Coalescence in situ (vasculogenesis).

- Migration of endothelial precursor cells (angioblasts) towards the organs.

- Development from existing vessels (angiogenesis).

The heart arises from the mesoderm and begins to beat in the fourth week of gestation. During the development of the cervical and cephalic regions, the first three branchial arches of the embryo form the carotid arterial system.

Diseases: partial list

Aneurism. Widening of a weak segment of an artery caused by blood pressure.

Arrhythmia. Deviation from the normal regularity of the heart rhythm due to a defect in the electrical conduction of the heart.

Atherosclerosis. Chronic disease caused by deposition (plaques) of lipids, cholesterol, or calcium on the endothelium of large arteries.

Congenital defects. Abnormalities of genetic or environmental origin of the circulatory system present at birth.

Dyslipidemias. Abnormal blood lipoprotein levels. Lipoproteins transfer lipids between organs.

Endocarditis. Inflammation of the endocardium caused by a bacterial and sometimes fungal infection.

Cerebrovascular disease. Sudden damage due to reduced blood flow in part of the brain.

Valvular disease. Mitral valve failure to prevent improper blood flow.

Failure heart. Inability of the heart to contract and relax effectively, reducing its performance and compromising circulation.

Hypertension. Blood pressure greater than 140/90 mm Hg. Produces atherogenesis by damaging the endothelium

Infarct. Death of part of the myocardium caused by interruption of blood flow by a thrombus stuck in a coronary artery.

Varicose veins and hemorrhoids. A chickenpox is a vein that has been distended by blood. Hemorrhoids are groups of varicose veins in the anus.

References

1. Aaronson, PI, Ward, JPT, Wiener, CM, Schulman, SP, Gill, JS 1999. The cardiovascular system at a glance Blackwell, Oxford.
2. Artman, M., Benson, DW, Srivastava, D., Joel B. Steinberg, JB, Nakazawa, M. 2005. Cardiovascular development and congenital malformations: molecular and genetic mechanisms. Blackwell, Malden.
3. Barrett, KE, Brooks, HL, Barman, SM, Yuan, JX-J. 2019. Ganong's review of medical physiology. McGraw-Hill, New York.
4. Burggren, WW, Keller, BB 1997. Development of cardiovascular systems: molecules to organisms. Cambridge, Cambridge.
5. Dzau, VJ, Duke, JB, Liew, C.-C. 2007. Cardiovascular genetics and genomics for the cardiologist, Blackwell, Malden.
6. Farmer, CG1999. Evolution of the vertebrate cardio-pulmonary system. Annual Review of Physiology, 61, 573–592.
7. Gaze, DC 2012. The cardiovascular system - physiology, diagnostics and clinical implications. InTech, Rijeka.
8. Gittenberger-de Groot, AC, Bartelings, MM, Bogers, JJC, Boot, MJ, Poelmann, RE 2002. The embryology of the common arterial trunk. Progress in Pediatric Cardiology, 15, 1–8.
9. Gregory K. Snyder, GK, Sheafor, BA 1999. Red blood cells: centerpiece in the evolution of the vertebrate circulatory system. American Zoologist, 39, 89–198.
10. Hall, JE 2016. Guyton and Hall textbook of medical physiology. Elsevier, Philadelphia.
11. Hempleman, SC, Warburton, SJ 2013. Comparative embryology of the carotid body. Respiratory Physiology & Neurobiology, 185, 3–8.
12. Muñoz-Chápuli, R., Carmona, R., Guadix, JA, Macías, D., Pérez-Pomares, JM 2005. The origin of the endothelial cells: an evo-devo approach for the invertebrate / vertebrate transition of the circulatory system. Evolution & Development, 7, 351–358.
13. Rogers, K. 2011. The cardiovascular system. Britannica Educational Publishing, New York.
14. Safar, ME, Frohlich, ED 2007. Atherosclerosis, large arteries and cardiovascular risk. Karger, Basel.
15. Saksena, FB 2008. Color atlas of local and systemic signs of cardiovascular disease. Blackwell, Malden.
16. Schmidt-Rhaesa, A. 2007. The evolution of organ systems. Oxford, Oxford.
17. Taylor, RB 2005. Taylor's Cardiovascular Diseases: A Handbook. Springer, New York.
18. Topol, EJ, et al. 2002. Textbook of Cardiovascular Medicine. Lippincott Williams & Wilkins, Philadelphia.
19. Whittemore, S., Cooley, DA 2004. The circulatory system. Chelsea House, New York.
20. Willerson, JT, Cohn, JN, Wellens, HJJ, Holmes, DR, Jr. 2007. Cardiovascular medicine. Springer, London.