The cardiovascular system consists of the blood, the heart, and blood vessels. Learn about the pump that circulates it throughout the body—the heart.
For blood to reach body cells and exchange materials with them, it must be pumped continuously by the heart through the body’s blood vessels.
The heart beats about 100,000 times every day, which adds up to about 35 million beats in a year, and approximately 2.5 billion times in an average lifetime.
The left side of the heart pumps blood through an estimated 120,000 km (75,000 mi) of blood vessels, which is equivalent to traveling around the earth’s equator about 3 times. The right side of the heart pumps blood through the lungs, enabling blood to pick up oxygen and unload carbon dioxide.
Even while you are sleeping, your heart pumps 30 times its own weight each minute, which amounts to about 5 liters (5.3 qt) to the lungs and the same volume to the rest of the body.
At this rate, your heart pumps more than about 14,000 liters (3600 gal) of blood in a day, or 5 million liters (1.3 million gal) in a year. You don’t spend all your time sleeping, however, and your heart pumps more vigorously when you are active. Thus, the actual blood volume your heart pumps in a single day is much larger.
The scientific study of the normal heart and the diseases associated with it is cardiology. Explore the structure of the heart and the unique properties that permit it to pump for a lifetime without rest.
Location of the Heart
For all its might, the heart is relatively small, roughly the same size (but not the same shape) as your closed fist.
It is about 12 cm (5 in.) long, 9 cm (3.5 in.) wide at its broadest point, and 6 cm (2.5 in.) thick, with an average mass of 250 g (8 oz) in adult females and 300 g (10 oz) in adult males.
The heart rests on the diaphragm, near the midline of the thoracic cavity. Recall that the midline is an imaginary vertical line that divides the body into unequal left and right sides. The heart lies in the mediastinum (me¯-de¯-as-TI¯-num), an anatomical region that extends from the sternum to the vertebral column, from the first rib to the diaphragm, and between the lungs.
About two-thirds of the mass of the heart lies to the left of the body’s midline. You can visualize the heart as a cone lying on its side. The pointed apex is formed by the tip of the left ventricle (a lower chamber of the heart) and rests on the diaphragm.
It is directed anteriorly, inferiorly, and to the left. The base of the heart is its posterior surface. It is formed by the atria (upper chambers) of the heart, mostly the left atrium.
In addition to the apex and base, the heart has several distinct surfaces and borders (margins). The anterior surface is deep to the sternum and ribs. The inferior surface is the part of the heart between the apex and right border and rests mostly on the diaphragm. The right border faces the right lung and extends from the inferior surface to the base. The left border, also called the pulmonary border, faces the left lung and extends from the base to the apex.
Because the heart lies between two rigid structures—the vertebral column and the sternum—external pressure on the chest (compression) can be used to force blood out of the heart and into the circulation.
In cases in which the heart suddenly stops beating, cardiopulmonary resuscitation or CPR—properly applied cardiac compressions, performed with artificial ventilation of the lungs via mouth-to-mouth respiration—saves lives.
CPR keeps oxygenated blood circulating until the heart can be restarted.
Researchers have found that chest compressions alone are equally as effective as, if not better than, traditional CPR with lung ventilation. This is good news because it is easier for an emergency dispatcher to give instructions limited to chest compressions to frightened, non medical bystanders.
As public fear of contracting contagious diseases such as hepatitis, HIV, and tuberculosis continues to rise, bystanders are much more likely to perform chest compressions alone than treatment involving mouth-to-mouth rescue breathing.
The membrane that surrounds and protects the heart is the pericardium. It confines the heart to its position in the mediastinum, while allowing sufficient freedom of movement for vigorous and rapid contraction.
The pericardium consists of two main parts:
- (1) the fibrous pericardium
- (2) the serous pericardium
The superficial fibrous pericardium is composed of tough, inelastic, dense irregular connective tissue. It resembles a bag that rests on and attaches to the diaphragm; its open end is fused to the connective tissues of the blood vessels entering and leaving the heart.
The fibrous pericardium prevents:
- Overstretching of the heart
- Provides protection
- Anchors the heart in the mediastinum
The fibrous pericardium near the apex of the heart is partially fused to the central tendon of the diaphragm and therefore movement of the diaphragm, as in deep breathing, facilitates the movement of blood by the heart.
The deeper serous pericardium is:
- A more delicate membrane that forms a double layer around the heart.
The outer parietal layer of the serous pericardium is fused to the fibrous pericardium. The inner visceral layer of the serous pericardium, also called the epicardium, is one of the layers of the heart wall and adheres tightly to the surface of the heart.
Between the parietal and visceral layers of the serous pericardium is a thin film of lubricating serous fluid. This slippery secretion of the pericardial cells, known as pericardial fluid, reduces friction between the layers of the serous pericardium as the heart moves. The space that contains the few milliliters of pericardial fluid is called the pericardial cavity.
Inflammation of the pericardium is called pericarditis.
The most common type, acute pericarditis, begins suddenly and has no known cause in most cases but is sometimes linked to a viral infection. As a result of irritation to the pericardium, there is chest pain that may extend to the left shoulder and down the left arm (often mistaken for a heart attack) and pericardial friction rub (a scratchy or creaking sound heard through a stethoscope as the visceral layer of the serous pericardium rubs against the parietal layer of the serous pericardium).
Acute pericarditis usually lasts for about 1 week and is treated with drugs that reduce inflammation and pain, such as ibuprofen or aspirin.
Chronic pericarditis begins gradually and is long-lasting. In one form of this condition, there is a buildup of pericardial fluid. If a great deal of fluid accumulates, this is a life-threatening condition because the fluid compresses the heart, a condition called cardiac tamponade.
As a result of the compression, ventricular filling is decreased, cardiac output is reduced, venous return to the heart is diminished, blood pressure falls, and breathing is difficult.
Most causes of chronic pericarditis involving cardiac tamponade are unknown, but it is sometimes caused by conditions such as cancer and tuberculosis.
Treatment consists of draining the excess fluid through a needle passed into the pericardial cavity.
Layers of the Heart Wall
The wall of the heart consists of three layers:
- the epicardium (external layer)
- the myocardium (middle layer)
- the endocardium (inner layer)
The epicardium is composed of two tissue layers. The outermost, as you just learned, is called the visceral layer of the serous pericardium.
This thin, transparent outer layer of the heart wall is composed of mesothelium. Beneath the mesothelium is a variable layer of delicate fibroelastic tissue and adipose tissue. The adipose tissue predominates and becomes thickest over the ventricular surfaces, where it houses the major coronary and cardiac vessels of the heart.
The amount of fat varies from person to person, corresponds to the general extent of body fat in an individual, and typically increases with age.
The epicardium imparts a smooth, slippery texture to the outermost surface of the heart and contains:
- Blood vessels
- Vessels that supply the myocardium
The middle myocardium is responsible for the pumping action of the heart and is composed of cardiac muscle tissue.
It makes up approximately 95% of the heart wall. The muscle fibers (cells), like those of striated skeletal muscle tissue, are wrapped and bundled with connective tissue sheaths composed of endomysium and perimysium.
The cardiac muscle fibers are organized in bundles that swirl diagonally around the heart and generate the strong pumping actions of the heart. Although it is striated like skeletal muscle, recall that cardiac muscle is involuntary like smooth muscle.
The innermost endocardium is a thin layer of endothelium overlying a thin layer of connective tissue. It provides a smooth lining for the chambers of the heart and covers the valves of the heart. The smooth endothelial lining minimizes the surface friction as blood passes through the heart. The endocardium is continuous with the endothelial lining of the large blood vessels attached to the heart.
Myocarditis and Endocarditis
Myocarditis is an inflammation of the myocardium that usually occurs as a complication of a viral infection, rheumatic fever, or exposure to radiation or certain chemicals or medications.
Myocarditis often has no symptoms. However, if they do occur, they may include fever, fatigue, vague chest pain, irregular or rapid heartbeat, joint pain, and breathlessness.
Myocarditis is usually mild and recovery occurs within 2 weeks. Severe cases can lead to cardiac failure and death. Treatment consists of avoiding vigorous exercise, alow-salt diet, electrocardiographic monitoring, and treatment of the cardiac failure.
Endocarditis refers to an inflammation of the endocardium and typically involves the heart valves.
Most cases are caused by bacteria (bacterial endocarditis). Signs and symptoms of endocarditis include fever, heart murmur, irregular or rapid heartbeat, fatigue, loss of appetite, night sweats, and chills.
Treatment is with intravenous antibiotics.
Chambers of the Heart
The heart has four chambers.
- The two superior receiving chambers are the atria ( entry halls or chambers)
- and the two inferior pumping chambers are the ventricles ( little bellies).
The paired atria receive blood from blood vessels returning blood to the heart, called veins, while the ventricles eject the blood from the heart into blood vessels called arteries.
On the anterior surface of each atrium is a wrinkled pouchlike structure called an auricle (OR-i-kul; auri- ear), so named because of its resemblance to a dog’s ear. Each auricle slightly increases the capacity of an atrium so that it can hold a greater volume of blood.
Also on the surface of the heart are a series of grooves, called sulci (SUL-sı¯), that contain coronary blood vessels and a variable amount of fat. Each sulcus (SUL-kus; singular) marks the external boundary between two chambers of the heart.
The deep coronary sulcus (coron- resembling a crown) encircles most of the heart and marks the external boundary between the superior atria and inferior ventricles.
The anterior interventricular sulcus is a shallow groove on the anterior surface of the heart that marks the external boundary between the right and left ventricles. This sulcus continues around to the posterior surface of the heart as the posterior interventricular sulcus, which marks the external boundary between the ventricles on the posterior aspect of the heart.
The right atrium forms the right border of the heart and receives blood from three veins:
- Superior vena cava
- Inferior vena cava
- Coronary sinus
(Veins always carry blood toward the heart.)
The right atrium is about 2–3 mm (0.08–0.12 in.) in average thickness. The anterior and posterior walls of the right atrium are very different. The inside of the posterior wall is smooth; the inside of the anterior wall is rough due to the presence of muscular ridges called pectinate muscles (PEK-ti-na¯t; pectin comb), which also extend into the auricle.
Between the right atrium and left atrium is a thin partition called the interatrial septum (inter- between; septum a dividing wall or partition). A prominent feature of this septum is an oval depression called the fossa ovalis, the remnant of the foramen ovale, an opening in the interatrial septum of the fetal heart that normally closes soon after birth).
Blood passes from the right atrium into the right ventricle through a valve that is called the tricuspid valve because it consists of three leaflets or cusps. It is also called the right atrioventricular valve. The valves of the heart are composed of dense connective tissue covered by endocardium.
The right ventricle is about 4–5 mm (0.16–0.2 in.) in average thickness and forms most of the anterior surface of the heart.
The inside of the right ventricle contains a series of ridges formed by raised bundles of cardiac muscle fibers called trabeculae carneae. Some of the trabeculae carneae convey part of the conduction system of the heart.
The cusps of the tricuspid valve are connected to tendonlike cords, the chordae tendineae, which in turn are connected to cone-shaped trabeculae carneae called papillary muscles.
Internally, the right ventricle is separated from the left ventricle by a partition called the interatrial septum.
Blood passes from the right ventricle through the pulmonary valve (pulmonary semilunar valve) into a large artery called the pulmonary trunk, which divides into right and left pulmonary arteries and carries blood to the lungs. Arteries always take blood away from the heart.
The left atrium is about the same thickness as the right atrium and forms most of the base of the heart. It receives blood from the lungs through four pulmonary veins.
Like the right atrium, the inside of the left atrium has a smooth posterior wall. Because pectinate muscles are confined to the auricle of the left atrium, the anterior wall of the left atrium also is smooth.
Blood passes from the left atrium into the left ventricle through the bicuspid (mitral) valve, which, as its name implies, has two cusps. The term mitral refers to the resemblance of the bicuspid valve to a bishop’s miter (hat), which is two-sided. It is also called the left atrioventricular valve.
The left ventricle is the thickest chamber of the heart, averaging 10–15 mm (0.4–0.6 in.) and forms the apex of the heart. Like the right ventricle, the left ventricle contains trabeculae carneae and has chordae tendineae that anchor the cusps of the bicuspid valve to papillary muscles.
Blood passes from the left ventricle through the aortic valve (aortic semilunar valve) into the ascending aorta. Some of the blood in the aorta flows into the coronary arteries, which branch from the ascending aorta and carry blood to the heart wall.
The remainder of the blood passes into the arch of the aorta and descending aorta (thoracic aorta and abdominal aorta). Branches of the arch of the aorta and descending aorta carry blood throughout the body.
During fetal life, a temporary blood vessel, called the ductus arteriosus, shunts blood from the pulmonary trunk into the aorta. Hence, only a small amount of blood enters the nonfunctioning fetal lungs. The ductus arteriosus normally closes shortly after birth, leaving a remnant known as the ligamentum arteriosum, which connects the arch of the aorta and pulmonary trunk.
Myocardial Thickness and Function
The thickness of the myocardium of the four chambers varies according to each chamber’s function.
The thin-walled atria deliver blood under less pressure into the adjacent ventricles. Because the ventricles pump blood under higher pressure over greater distances, their walls are thicker.
Although the right and left ventricles act as two separate pumps that simultaneously eject equal volumes of blood, the right side has a much smaller workload. It pumps blood a short distance to the lungs at lower pressure, and the resistance to blood flow is small.
The left ventricle pumps blood great distances to all other parts of the body at higher pressure, and the resistance to blood flow is larger.
Therefore, the left ventricle works much harder than the right ventricle to maintain the same rate of blood flow. The anatomy of
the two ventricles confirms this functional difference—the muscular wall of the left ventricle is considerably thicker than the wall of the right ventricle.
Note also that the perimeter of the lumen (space) of the left ventricle is roughly circular, in contrast to that of the right ventricle, which is somewhat crescent-shaped.
Fibrous Skeleton of the Heart
In addition to cardiac muscle tissue, the heart wall also contains dense connective tissue that forms the fibrous skeleton of the heart.
Essentially, the fibrous skeleton consists of four dense connective tissue rings that surround the valves of the heart, fuse with one another, and merge with the interventricular septum.
In addition to forming a structural foundation for the heart valves, the fibrous skeleton prevents overstretching of the valves as blood passes through them. It also serves as a point of insertion for bundles of cardiac muscle fibers and acts as an electrical insulator between the atria and ventricles.
HEART VALVES AND CIRCULATION OF BLOOD
- Describe the structure and function of the valves of the heart
- Outline the flow of blood through the chambers of the heart and through the systemic and pulmonary circulations
- Discuss the coronary circulation
As each chamber of the heart contracts, it pushes a volume of blood into a ventricle or out of the heart into an artery. Valves open and close in response to pressure changes as the heart contracts and relaxes. Each of the four valves helps ensure the one-way flow of blood by opening to let blood through and then closing to prevent its backflow.
Operation of the Atrioventricular Valves
Because they are located between an atrium and a ventricle, the tricuspid and bicuspid valves are termed atrioventricular (AV) valves
When an AV valve is open:
- the rounded ends of the cusps project into the ventricle
When the ventricles are relaxed:
- the papillary muscles are relaxed
- the chordae tendineae are slack
- The blood moves from a higher pressure in the atria to a lower pressure in the ventricles through open AV valves.
When the ventricles contract:
- the pressure of the blood drives the cusps upward until their edges meet and close the opening
- At the same time, the papillary muscles contract
- which pulls on and tightens the chordae tendineae
- This prevents the valve cusps from everting (opening into the atria) in response to the high ventricular pressure.
If the AV valves or chordae tendineae are damaged, blood may
regurgitate (flow back) into the atria when the ventricles contract.
Operation of the Semilunar Valves
The aortic and pulmonary valves are known as the semilunar (SL) valves (sem-e¯-LOO-nar; semi- half; -lunar moonshaped) because they are made up of three crescent moon–shaped cusps. Each cusp attaches to the arterial wall by its convex outer margin.
The SL valves allow ejection of blood from the heart into arteries but prevent backflow of blood into the ventricles. The free borders of the cusps project into the lumen of the artery.
When the ventricles contract:
- pressure builds up within the chambers
- The semilunar valves open when pressure in the ventricles exceeds the pressure in the arteries
- permitting ejection of blood from the ventricles into the pulmonary trunk and aorta.
As the ventricles relax:
- blood starts to flow back toward the heart
- This backflowing blood fills the valve cusps
- which causes the free edges of the semilunar valves to contact each other tightly and close the opening between the ventricle and artery.
Surprisingly perhaps, there are no valves guarding the junctions between the venae cavae and the right atrium or the pulmonary veins and the left atrium.
As the atria contract, a small amount of blood does flow backward from the atria into these vessels. However, backflow is minimized by a different mechanism;
as the atrial muscle contracts, it compresses and nearly collapses
the venous entry points.
Systemic and Pulmonary Circulations
In postnatal (after birth) circulation, the heart pumps blood into two closed circuits with each beat:
- Systemic circulation
- Pulmonary circulation
The two circuits are arranged in series: The output of one becomes the input of the other, as would happen if you attached two garden hoses.
The left side of the heart is the pump for systemic circulation; it receives bright red oxygenated (oxygen-rich) blood from the lungs.
- The left ventricle ejects blood into the aorta.
- From the aorta, the blood divides into separate streams, entering progressively smaller systemic arteries that carry it to all organs throughout the body—except for the air sacs (alveoli) of the lungs, which are supplied by pulmonary circulation.
- In systemic tissues, arteries give rise to smaller-diameter arterioles, which finally lead into extensive beds of systemic capillaries.
- Exchange of nutrients and gases occurs across the thin capillary walls. Blood unloads O2 (oxygen) and picks up CO2 (carbon dioxide). In most cases, blood flows through only one capillary and then enters a systemic venule.
- Venules carry deoxygenated (oxygen-poor) blood away from tissues and merge to form larger systemic veins.
- Ultimately the blood flows back to the right atrium.
The right side of the heart is the pump for pulmonary circulation; it receives all the dark-red deoxygenated blood returning from the systemic circulation.
- Blood ejected from the right ventricle flows into the pulmonary trunk,
- which branches into pulmonary arteries that carry blood to the right and left lungs.
- In pulmonary capillaries, blood unloads CO2, which is exhaled, and picks up O2 from inhaled air.
- The freshly oxygenated blood then flows into pulmonary veins and returns to the left atrium.
Heart Valve Disorders
When heart valves operate normally, they open fully and close completely at the proper times.
A narrowing of a heart valve opening that restricts blood flow is known as stenosis (ste-NO¯ -sis a narrowing); failure of a valve to close completely is termed insufficiency (in-su-FISH-en-se¯ ) or incompetence.
In mitral stenosis, scar formation or a congenital defect causes narrowing of the mitral valve. One cause of mitral insufficiency, in which there is backflow of blood from the left ventricle into the left atrium, is mitral valve prolapse (MVP).
In MVP one or both cusps of the mitral valve protrude into the left atrium during ventricular contraction. Mitral valve prolapse is one of the most common valvular disorders, affecting as much as 30% of the population. It is more prevalent in women than in men, and does not always pose a serious threat.
In aortic stenosis the aortic valve is narrowed, and in aortic insufficiency there is backflow of blood from the aorta into the left ventricle.
Certain infectious diseases can damage or destroy the heart valves. One example is rheumatic fever, an acute systemic inflammatory disease that usually occurs after a streptococcal infection of the throat. The bacteria trigger an immune response in which antibodies produced to destroy the bacteria instead attack and inflame the connective tissues in joints, heart valves, and other organs. Even though rheumatic fever may weaken the entire heart wall, most often it damages the mitral and aortic valves.
If daily activities are affected by symptoms and if a heart valve cannot be repaired surgically, then the valve must be replaced. Tissue valves may be provided by human donors or pigs; sometimes, mechanical replacements are used. In any case, valve replacement involves open heart surgery. The aortic valve is the most commonly replaced heart valve.
Nutrients are not able to diffuse quickly enough from blood in the chambers of the heart to supply all the layers of cells that make up the heart wall.
For this reason, the myocardium has its own network of blood vessels, the coronary or cardiac circulation (coron- crown). The coronary arteries branch from the ascending aorta and encircle the heart like a crown encircles the head.
While the heart is contracting, little blood flows in the coronary arteries because they are squeezed shut.
When the heart relaxes, however, the high pressure of blood in the aorta propels blood through the coronary arteries, into capillaries, and then into coronary veins.
Two coronary arteries, the right and left coronary arteries, branch from the ascending aorta and supply oxygenated blood to the myocardium.
The left coronary artery passes inferior to the left auricle and divides into the:
- Anterior interventricular (Both Ventricles)
- Circumflex branches (Left Ventricle & Left Atrium)
The anterior interventricular branch or left anterior descending (LAD) artery is in the anterior interventricular sulcus and supplies oxygenated blood to the walls of both ventricles.
The circumflex branch (SER-kum-fleks) lies in the coronary sulcus and distributes oxygenated blood to the walls of the left ventricle and left atrium.
The right coronary artery supplies small branches (atrial branches) to the right atrium. It continues inferior to the right auricle and ultimately divides into the:
- Posterior interventricular (Both Ventricles)
- Marginal branch (Right Ventricle)
The posterior interventricular branch follows the posterior interventricular sulcus and supplies the walls of the two ventricles with oxygenated blood.
The marginal branch beyond the coronary sulcus runs along the right margin of the heart and transports oxygenated blood to the myocardium of the right ventricle.
Most parts of the body receive blood from branches of more than one artery, and where two or more arteries supply the same region, they usually connect.
These connections, called anastomoses), provide alternate routes, called collateral circulation, for blood to reach a particular organ or tissue.
The myocardium contains many anastomoses that connect branches of a given coronary artery or extend between branches of different coronary arteries. They provide detours for arterial blood if a main route becomes obstructed. Thus, heart muscle may receive sufficient oxygen even if one of its coronary arteries is partially blocked.
After blood passes through the arteries of the coronary circulation, it flows into capillaries, where it delivers oxygen and nutrients to the heart muscle and collects carbon dioxide and waste, and then moves into coronary veins.
Most of the deoxygenated blood from the myocardium drains into a large vascular sinus in the coronary sulcus on the posterior surface of the heart, called the coronary sinus (A vascular sinus is a thin walled vein that has no smooth muscle to alter its diameter.)
The deoxygenated blood in the coronary sinus empties into the right atrium. The principal tributaries carrying blood into the coronary sinus are the following:
- Great cardiac vein in the anterior interventricular sulcus, which drains the areas of the heart supplied by the left coronary artery (left and right ventricles and left atrium)
- Middle cardiac vein in the posterior interventricular sulcus, which drains the areas supplied by the posterior interventricular branch of the right coronary artery (left and right ventricles)
- Small cardiac vein in the coronary sulcus, which drains the right atrium and right ventricle
- Anterior cardiac veins, which drain the right ventricle and open directly into the right atrium
When blockage of a coronary artery deprives the heart muscle of oxygen, reperfusion, the reestablishment of blood flow, may damage the tissue further.
This surprising effect is due to the formation of oxygen free radicals from the reintroduced oxygen. Free radicals are molecules that have an unpaired electron. These unstable, highly reactive molecules cause chain reactions that lead to cellular damage and death.
To counter the effects of oxygen free radicals, body cells produce enzymes that convert free radicals to less reactive substances. Two such enzymes are superoxide dismutase (dis-MU¯ -ta¯s) and catalase (KAT-a-la¯s).
In addition, nutrients such as vitamin E, vitamin C, beta-carotene, zinc, and selenium serve as antioxidants, which remove oxygen free radicals from circulation. Drugs that lessen reperfusion damage after a heart attack or stroke are currently under development.
Myocardial Ischemia and Infarction
Partial obstruction of blood flow in the coronary arteries may cause myocardial ischemia (is-KE¯-me¯ -a; ische- to obstruct; -emia in the blood), a condition of reduced blood flow to the myocardium.
Usually, ischemia causes hypoxia (hı¯-POKS-e¯ -a reduced oxygen supply), which may weaken cells without killing them.
Angina pectoris (an-JI¯-na, or AN-ji-na, PEK-to¯ -ris), which literally means “strangled chest,” is a severe pain that usually accompanies myocardial ischemia. Typically, sufferers describe it as a tightness or squeezing sensation, as though the chest were in a vise. The pain associated with angina pectoris is often referred to the neck, chin, or down the left arm to the elbow.
Silent myocardial ischemia, ischemic episodes without pain, is particularly dangerous because the person has no forewarning of an impending heart attack.
A complete obstruction to blood flow in a coronary artery may result in a myocardial infarction (MI) (in-FARK-shun), commonly called a heart attack. Infarction means the death of an area of tissue because of interrupted blood supply.
Because the heart tissue distal to the obstruction dies and is replaced by noncontractile scar tissue, the heart muscle loses some of its strength. Depending on the size and location of the infarcted (dead) area, an infarction may disrupt the conduction system of the heart and cause sudden death by triggering ventricular fibrillation.
Treatment for a myocardial infarction may involve injection of a thrombolytic (clot-dissolving) agent such as streptokinase or t-PA, plus heparin (an anticoagulant), or performing coronary angioplasty or coronary artery bypass grafting. Fortunately, heart muscle can remain alive in a resting person if it receives as little as 10–15% of its normal blood supply.
Foetal Circulation & Development of the Cardiovascular System
How does the fetal circulatory system work?
During pregnancy, the unborn baby (fetus) depends on its mother for nourishment and oxygen. Since the fetus doesn’t breathe air, his or her blood circulates differently than it does after birth:
The placenta is the organ that develops and implants in the mother’s womb (uterus) during pregnancy. The unborn baby is connected to the placenta by the umbilical cord.
All the necessary nutrition, oxygen, and life support from the mother’s blood goes through the placenta and to the baby through blood vessels in the umbilical cord.
Waste products and carbon dioxide from the baby are sent back through the umbilical cord blood vessels and placenta to the mother’s circulation to be eliminated.
While the baby is still in the uterus, his or her lungs are not being used. The baby’s liver is not fully developed. Circulating blood bypasses the lungs and liver by flowing in different pathways and through special openings called shunts.
Blood flow in the unborn baby follows this pathway:
Oxygen and nutrients from the mother’s blood are transferred across the placenta to the fetus through the umbilical cord.
This enriched blood flows through the umbilical vein toward the baby’s liver. There it moves through a shunt called the ductus venosus.
This allows some of the blood to go to the liver. But most of this highly oxygenated blood flows to a large vessel called the inferior vena cava and then into the right atrium of the heart.
Here is what happens inside the fetal heart:
When oxygenated blood from the mother enters the right side of the heart it flows into the upper chamber (the right atrium). Most of the blood flows across to the left atrium through a shunt called the foramen ovale.
From the left atrium, blood moves down into the lower chamber of the heart (the left ventricle). It is then pumped into the first part of the large artery coming from the heart (the ascending aorta).
From the aorta, the oxygen-rich blood is sent to the brain and to the heart muscle itself. Blood is also sent to the lower body.
Blood returning to the heart from the fetal body contains carbon dioxide and waste products as it enters the right atrium. It flows down into the right ventricle, where it normally would be sent to the lungs to be oxygenated. Instead, it bypasses the lungs and flows through the ductus arteriosus into the descending aorta, which connects to the umbilical arteries.
From there, blood flows back into the placenta. There the carbon dioxide and waste products are released into the mother’s circulatory system. Oxygen and nutrients from the mother’s blood are transferred across the placenta. Then the cycle starts again.
At birth, major changes take place. The umbilical cord is clamped and the baby no longer receives oxygen and nutrients from the mother. With the first breaths of air, the lungs begin to expand, and the ductus arteriosus and the foramen ovale both close. The baby’s circulation and blood flow through the heart now function like an adult’s.
- What are the functional differences between the umbilical arteries and the umbilical vein?
- 2 umbilical arteries supply de-oxygenated blood from the foetus to the placenta.
- Where as the umbilical vein carries oxygenated blood from the mother to the foetus.
- Note that anatomy is described in terms of the foetus (i.e. blood from it’s heart is in an artery and blood going towards it’s heart is in a vein)
- How is foetal blood replenished with oxygen?
- Blood is oxygenated in the placenta, which acts like a respiratory system for the foetus, instead of alveoli, the placenta uses chorionic villi where oxygen diffuses from the mothers blood to the foetal blood.
- Which maternal blood vessels are responsible for exchanging oxygen and waste with foetal blood?
- The Uterine Arteries and the chorionic villi
- What are the names of the two connections that exist in foetal circulation to ensure that the majority of oxygenated blood bypasses the lungs?
- Ductus arteriosus (aorta to pulmonary artery)
- Foramen ovale (right to left atrium)
- State the THREE vessels in foetal circulation that undergo changes upon childbirth
- Ductus arteriosus
- Foramen ovale close, the pulmonary vessels function normally for the first time
- What is the path of oxygenated blood from the placenta to the aorta of the foetus?
- Chorionic villi – Umbilical Vein – IVC – Right Atrium – Left Atrium – Left Ventricle – Ascending Aorta – Arch… etc.
- Which germ layer is responsible for the development of the Cardiovascular System?
- The primitive heart tube is formed by which structures?
- Endocardial tubes
- Briefly describe the development of the heart and cardiovascular system from the formation of blood islands to 28 days post fertilisation
- Blood island grow and form into endocardia tubes, which merge into the primitive heart tube, the differentiates into the Truncus arteriosus, the blubus cordis, and the primitive artrium and ventricle, the heart begins to loop and forms the sinus venosus, the heart begins to differentiate into four chambers. First spontaneous heartbeat at approximately 28 days.
Action potentials (electrical impulses) in the heart originate in specialized cardiac muscle cells, called autorhythmic cells. These cells are self‐excitable, able to generate an action potential without external stimulation by nerve cells.
The autorhythmic cells serve as a pacemaker to initiate the cardiac cycle (pumping cycle of the heart) and provide a conduction system to coordinate the contraction of muscle cells throughout the heart.
The autorhythmic cells are concentrated in the following areas:
- The sinoatrial (SA) node, located in the upper wall of the right atrium.
- Do not have a stable resting potential, constantly depolarize to threshold – Pacemaker Potential
- Upon reaching threshold, initiates the cardiac cycle by generating an action potential that spreads through both atria through the gap junctions of the cardiac muscle fibers.
- The atrioventricular (AV) node, located near the lower region of the interatrial septum, receives the action potential generated by the SA node.
- A slight delay of the electrical transmission occurs here, allowing the atria to fully contract and empty its blood before the action potential is passed on to the ventricles.
- The atrioventricular (AV) bundle (bundle of His) receives the action potential from the AV node
- Transmits the impulse to the ventricles by way of the right and left bundle branches.
- Only site that can conduct action potentials from the atria. Otherwise atria is electrically insulated from the ventricles.
- The Purkinje fibers are large‐diameter fibers that conduct the action potential from the interventricular septum, down to the apex, and then upward through the ventricles.
- The ventricles then contract pushing the blood towards the semilunar valves
Autorhythmic cells in the SA node initiate an action potential every 0.6 second (100 times a minute), so sets the rhythm for heart contraction – The Natural Pacemaker
Cardiac Muscle Contraction
- Depolarization: Sodium enters muscle down electrochemical gradient
- Plateau: Calcium enters muscle and binds to filaments that slide producing contraction. Potassium stops leaking.
- Repolarization: Potassium leaves muscle. Balance achieved but Sodium & potassium on opposite sides
- Long refractory period: Pumps restore Sodium & potassium to original side. No new contraction until complete to allow for blood to refill.
- Rapid depolarization occurs when fast‐opening Na + channels open and allow an influx of Na + ions into the cardiac muscle cell. The Na + channels rapidly close.
- Movement occurs because :
Cytosol of contractile fibers is electrically more negative than interstitial fluid and Na + concentration is higher in interstitial fluid
- A plateau phase occurs during which Ca 2+ enters the muscle cell through slow‐opening Ca 2+channels.
- Ca 2+ binds to troponin, which triggers the sliding of actin filaments past myosin filaments. The sliding of the filaments produces cell contraction.
- At the same time that the Ca 2+ channels open, K + channels, which normally leak small amounts of K + out of the cell, become more impermeable to K + leakage.
- The combined effects of the prolonged release of Ca 2+ and the restricted leakage of K + lead to an extended depolarization that appears as a plateau when membrane potential is plotted against time.
- Repolarization occurs as K + channels open and K + diffuses out of the cell. At the same time, Ca 2+ channels close.
- These events restore the membrane to its original polarization, except that the positions of K + and Na + on each side of the sarcolemma are reversed.
- A refractory period follows, during which concentration of K + and Na + are restored to their appropriate sides by Na +/K + pumps.
- The muscle cell cannot contract again until Na + and K + are restored to their resting potential states.
- The refractory period of cardiac muscle is dramatically longer than that of skeletal muscle. This prevents tetanus from occurring and ensures that each contraction is followed by enough time to allow the heart chamber to refill with blood before the next contraction.
P-Wave – onset of atrial contraction
- The P wave is a small wave that represents the depolarization of the atria. During this wave, the muscles of the atria are contracting.
QRS complex – onset of ventricular contraction/atrial relaxation
- The QRS complex is a rapid down‐up‐down movement. The upward movement produces a tall peak, indicated by R. The QRS complex represents the depolarization of the ventricles.
T-wave – occurs just before ventricles relax
- The T wave represents the repolarization of the ventricles. Electrical activity generated by the repolarization of the atria is concealed by the QRS complex.
Analysis of an ECG also involves measuring the time spans between waves, which are called intervals or segments.
For example, the P–Q interval is the time from the beginning of the P wave to the beginning of the QRS complex. It represents the conduction time from the beginning of atrial excitation to the beginning of ventricular excitation.
Put another way, the P–Q interval is the time required for the action potential to travel through the atria, atrioventricular node, and the remaining fibers of the conduction system. As the action potential is forced to detour around scar tissue caused by disorders such as coronary artery disease and rheumatic fever, the P–Q interval lengthens.
The S–T segment, which begins at the end of the S wave and ends at the beginning of the T wave, represents the time when the ventricular contractile fibers are depolarized during the plateau phase of the action potential. The S–T segment is elevated (above the baseline) in acute myocardial infarction and depressed (below the baseline) when the heart muscle receives insufficient oxygen.
The Q–T interval extends from the start of the QRS complex to the end of the T wave. It is the time from the beginning of ventricular depolarization to the end of ventricular repolarization. The Q–T interval may be lengthened by myocardial damage, myocardial ischemia (decreased blood flow), or conduction abnormalities.
Sometimes it is helpful to evaluate the heart’s response to the stress of physical exercise (stress testing).
Although narrowed coronary arteries may carry adequate oxygenated blood while a person is at rest, they will not be able to meet the heart’s increased need for oxygen during strenuous exercise. This situation creates changes that can be seen on an electrocardiogram.
Abnormal heart rhythms and inadequate blood flow to the heart may occur only briefly or unpredictably. To detect these problems, continuous ambulatory electrocardiographs are used. With this procedure, a person wears a battery-operated monitor that records an ECG continuously for 24 hours. Electrodes attached to the chest are connected to the monitor, and information on the heart’s activity is stored in the monitor and retrieved later by medical personnel.
The Cardiac Cycle
- contraction = systole
- relaxation = diastole
- Systolic and diastolic activities of the atria and ventricles
- The blood volume and pressure changes within the heart
- The action of the heart valves.
Pressure and Volume Changes during the Cardiac Cycle
In each cardiac cycle, the atria and ventricles alternately contract and relax, forcing blood from areas of higher pressure to areas of lower pressure.
As a chamber of the heart contracts, blood pressure within it increases. The pressures on the right side are considerably lower than the left. Each ventricle, however, expels the same volume of blood per beat, and the same pattern exists for both pumping chambers.
To examine and correlate the events taking place during a cardiac cycle, we will begin with atrial systole.
During atrial systole, which lasts about 0.1 sec, the atria are contracting. At the same time, the ventricles are relaxed.
Depolarization of the SA node causes atrial depolarization, marked by the P wave in the ECG. Atrial depolarization causes atrial systole. As the atria contract, they exert pressure on the blood within, which forces blood through the open AV valves into the ventricles.
Atrial systole contributes a final 25 mL of blood to the volume already in each ventricle (about 105 mL). The end of atrial systole is also the end of ventricular diastole (relaxation). Thus, each ventricle contains about 130 mL at the end of its relaxation period (diastole). This blood volume is called the end-diastolic volume (EDV).
The QRS complex in the ECG marks the onset of ventricular
Ventricular depolarization causes ventricular systole.
During ventricular systole, which lasts about 0.3 sec, the ventricles are contracting. At the same time, the atria are relaxed in atrial diastole.
As ventricular systole begins, pressure rises inside the ventricles and pushes blood up against the atrioventricular (AV) valves, forcing them shut. For about 0.05 seconds, both the SL (semilunar) and AV valves are closed. This is the period of isovolumetric contraction.
Continued contraction of the ventricles causes pressure inside the chambers to rise sharply. When left ventricular pressure surpasses aortic pressure at about 80 millimeters of mercury (mmHg) and right ventricular pressure rises above the pressure in the pulmonary trunk (about 20 mmHg), both SL valves open.
At this point, ejection of blood from the heart begins. The period when the SL valves are open is ventricular ejection and lasts for about 0.25 sec. The pressure in the left ventricle continues to rise to about 120 mmHg, whereas the pressure in the right ventricle climbs to about 25–30 mmHg.
The left ventricle ejects about 70 mL of blood into the aorta and the right ventricle ejects the same volume of blood into the pulmonary trunk. The volume remaining in each ventricle at the end of systole, about 60 mL, is the end-systolic volume (ESV).
Stroke volume, the volume ejected per beat from each ventricle, equals end-diastolic volume minus end-systolic volume:
- SV = EDV – ESV.
At rest, the stroke volume is about 130 mL 60 mL 70 mL (a little more than 2 oz).
The T wave in the ECG marks the onset of ventricular repolarization.
During the relaxation period, which lasts about 0.4 sec, the atria and the ventricles are both relaxed. As the heart beats faster and faster, the relaxation period becomes shorter and shorter, whereas the durations of atrial systole and ventricular systole shorten only slightly.
Ventricular repolarization causes ventricular diastole. As the ventricles relax, pressure within the chambers falls, and blood in the aorta and pulmonary trunk begins to flow backward toward the regions of lower pressure in the ventricles.
Backflowing blood catches in the valve cusps and closes the SL valves. The aortic valve closes at a pressure of about 100 mmHg. Rebound of blood off the closed cusps of the aortic valve produces the dicrotic wave on the aortic pressure curve.
After the SL valves close, there is a brief interval when ventricular blood volume does not change because all four valves are closed. This is the period of isovolumetric relaxation.
As the ventricles continue to relax, the pressure falls quickly. When ventricular pressure drops below atrial pressure, the AV valves open, and ventricular filling begins. The major part of ventricular filling occurs just after the AV valves open. Blood that has been flowing into and building up in the atria during ventricular systole then rushes rapidly into the ventricles.
At the end of the relaxation period, the ventricles are about three quarters full. The P wave appears in the ECG, signaling the start of another cardiac cycle.