Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Cardiovascular System
Heart

General Structure. The Heart, cor (Greek cardia), is located in the thoracic cavity, within the anterior Mediastinum. The greater part of it lies to the left, and the smaller part to the right of the midline. The human heart is cone-shaped. In size, it is approximately equal to the volume of a clenched fist. The apex of the heart points forward, downward, and to the left (Fig. 12.2). The Base of the organ is directed backward, upward, and to the right, and serves as the site of Water/144.html">Origin of the major vessels. By its anterior (sternocostal) surface, the heart adjoins the chest wall and is partially covered by the Lungs. The inferior (diaphragmatic) surface contacts the Diaphragm in the region of its cardiac impression. Laterally (pulmonary surface), the lungs are adjacent to the heart.

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Fig. 12.2. The heart (anterior surface):

1 — brachiocephalic trunk; 2 — left common carotid artery; 3 — left Subclavian Artery; 4 — Pericardium; 5 — pulmonary trunk; 6 — left auricle; 7 — left atrium; 8 — anterior interventricular sulcus; 9 — left ventricle; 10 — apex; 11 — right ventricle; 12 — right atrium; 13 — base of the heart; 14 — right auricle; 15 — aorta; 16 — SUPERIOR VENA CAVA

The coronary sulcus lies between the atria and the ventricles. Along the anterior and inferior surfaces of the ventricles run the anterior and posterior interventricular sulci, extending toward the apex of the heart.

The heart consists of four chambers: two ventricles and two atria.

The right atrium collects venous Blood from the entire body. The superior and inferior venae cavae empty into it. In addition, blood from the heart walls flows into the right atrium via the coronary sinus. The atrium has a protrusion which, due to its shape, is called the right auricle. In the PROJECTION OF THE right auricle on the internal surface of the heart, special ridges known as pectinate Muscles are visible. On the interatrial septum lies the fossa ovalis, in the area of which an opening is located in the fetus that connects the right atrium with the left and closes after birth. Blood flows from the right atrium through the atrioventricular orifice into the right ventricle (Fig. 12.3).

The right ventricle is a cavity, on the internal surface of which there are numerous muscular bridges known as trabeculae carneae. Papillary muscles protrude into the ventricular cavity, giving rise to chordae tendineae. These are anchored to the cusps of the right atrioventricular (tricuspid) valve (Fig. 12.4), which closes the orifice between the right atrium and the right ventricle. It consists of three cusps formed by the endocardium. During ventricular relaxation, blood flows freely into it from the atrium, pushing the valve cusps inward. During ventricular contraction, blood acts under pressure on the valve, causing it to seal the atrioventricular orifice. The chordae tendineae attached to the cusps become taut, preventing them from protruding into the atrial cavity. Thus, venous blood is expelled from the ventricle into the pulmonary trunk, which leads to the lungs. The orifice leading into the pulmonary trunk is closed by the pulmonary valve, which consists of three pocket-like semilunar cusps (see Fig. 12.4).

Fig. 12.4. Heart Valves:

1, 2, 3 — semilunar cusps of the pulmonary valve; 4 — pulmonary trunk; 5, 16, 17 — semilunar cusps of the aortic valve; 6 — ostium of the right coronary artery; 7, 8, 9 — cusps of the right atrioventricular valve; 10 — right fibrous ring; 11 — right fibrous trigone; 12 — left ventricle; 13, 14 — cusps of the mitral valve; 15 — left fibrous ring

Fig. 12.3. Chambers of the heart:

1 — pulmonary trunk (reflected); 2 — superior vena cava; 3 — brachiocephalic trunk; 4 — left common carotid artery; 5 — left subclavian artery; 6 — aorta; 7 — Pulmonary Veins; 8 — left atrium; 9 — papillary Muscle; 10 — left ventricle; 11 — interventricular septum; 12 — right ventricle; 13 — chordae tendineae; 14 — right atrium

During contraction of the right ventricle, the semilunar valves open. During its relaxation, blood fills the space between the cusps and the wall of the pulmonary trunk, the valve closes, and prevents the backflow of blood from the pulmonary trunk into the right ventricle.

The left atrium is filled with arterial blood flowing from the lungs via four pulmonary veins. In its wall structure, it resembles the right atrium and likewise features an additional space in the form of the left auricle. Blood passes from the left atrium through the atrioventricular orifice into the left ventricle.

The left ventricle has a thicker wall compared to the right. Its internal surface features muscular trabeculae and papillary muscles, from which chordae tendineae originate. The latter attach to the margins of the cusps of the left atrioventricular (bicuspid, mitral) valve. Despite its name, this valve sometimes presents not two, but three cusps. Its MECHANISM OF ACTION is identical to that of the tricuspid valve.

The aorta emerges from the left ventricle. Located in the orifice leading from the left ventricle into this vessel is the aortic valve, which consists of three semilunar cusps. Directly above the valve are two openings leading to the right and left coronary Arteries, which supply the heart.

Blood from the heart walls drains into the coronary sinus, located in the coronary sulcus. From the sinus, it flows into the right atrium.

STRUCTURE OF THE Heart Wall. The wall of the organ consists of three layers. The inner layer, the endocardium, is formed by flat Cells and has the appearance of a thin membrane. The cuspid and semilunar valves, as well as the chordae tendineae, are composed of the endocardium. The middle layer, the myocardium, is the thickest layer of the heart wall and is represented by Cytology/practical/59.html">Striated cardiac muscle tissue. In the ventricles, the myocardium consists of three layers: outer and inner longitudinal layers, and a middle circular layer. In the atria, the muscular tunic is represented by two layers: an outer circular layer and an inner longitudinal layer. The outer coat of the heart, the epicardium, is a serous membrane fixed to the myocardium. The ventricular wall is significantly thicker than the atrial wall: the thickness of the atria is 2–3 mm, whereas the wall of the left ventricle (about 1 cm) is substantially thicker than the wall of the right ventricle (5–7 mm).

Fig. 12.5. Projection of heart valves and their Auscultation sites (explained in the text):

1 — pulmonary valve; 2 — mitral valve; 3 — apex of the heart; 4 — tricuspid valve; 5 — aortic valve; Roman numerals indicate Ribs

At the level of the large Blood Vessels, the epicardium transitions into the pericardial sac, or pericardium. Between the pericardium and the epicardium lies the pericardial cavity. It is filled with a small amount of serous fluid, which reduces friction during heart contractions.

The soft Skeleton of the heart consists of four fibrous rings located in the region of the atrioventricular orifices, at the ostia of the aorta and pulmonary trunk, as well as the adjacent right and left fibrous trigones. The fibrous rings serve as the attachment sites for the valves and the muscular tunic.

Borders of the heart. The upper, lower, right, and left borders of the heart are distinguished. The upper border is projected onto the anterior thoracic wall at the level of the upper edge of the cartilages of the 3rd pair of ribs (Fig. 12.5). The right border runs along the right parasternal line from the 3rd to the 5th rib. The lower border extends transversely from the Cartilage of the 5th right rib to the projection of the apex of the heart, located in the fifth intercostal space 1 cm medial to the left midclavicular line. The left border runs from the cartilage of the 3rd left rib to the apex of the heart. In clinical practice, the borders of the heart are determined by Percussion (tapping).

In approximately 50% of healthy individuals, the apex of the heart contacts the anterior thoracic wall in the 5th intercostal space. Here, 1.0–1.5 cm medial to the left midclavicular line, the apical impulse may be palpated. The apical impulse is a rhythmic pulsation of the chest wall caused by the contraction of the heart.

Projection of the heart valves onto the anterior thoracic wall. The projection of the heart valves onto the anterior thoracic wall must be known for their examination using ultrasound techniques and cardiac auscultation (listening). The right atrioventricular orifice (tricuspid valve) is projected behind the Sternum along an oblique line connecting the sternal ends of the cartilages of the 4th left and 5th right ribs. The left atrioventricular orifice (bicuspid/mitral valve) is projected at the left edge of the sternum at the attachment site of the 4th rib cartilage. The aortic valve is located behind the sternum on the right at the level of the 2nd intercostal space. The pulmonary valve is projected at the left edge of the sternum at the attachment site of the 3rd rib cartilage.

The sites for auscultation (listening) of the heart valves using a phonendoscope (stethoscope) differ somewhat from their projection points. The auscultation point for the mitral valve corresponds to the projection of the heart apex. The second point (aortic valve) is located In the second intercostal space at the right sternal border. The third point (pulmonary valve) is in the second intercostal space at the left sternal border. The fourth point (tricuspid valve) is at the base of the xiphoid process. The fifth point is also a site for auscultation of the mitral valve; it is located in the area of its anatomical projection (the attachment site of the 4th left rib cartilage to the sternum).

Conduction system of the Heart. Inside the heart, There is a set of structures capable of independently generating nerve impulses, conducting them, and transmitting them from one part of the organ to the heart muscle. This is the conduction system, which consists of nodes and bundles formed by atypical cardiomyocytes (Fig. 12.6).

The sinuatrial node (sinus node, SA node, Keith–Flack node) lies in the region of the right auricle. It is the primary generator of impulses, which is why it is also called the pacemaker. The frequency of impulses it generates is 60–80 per minute. This node transmits excitation to the atria. In addition, from it, the impulse travels via Bachmann's bundle to the atrioventricular node (AV node, Aschoff–Tawara node), which is located in the upper part of the interventricular septum. It is also capable of automatically reproducing impulses at a rate of about 40 per minute. Extending from the atrioventricular node is

Fig. 12.6. Conduction System of the heart:

1 — superior vena cava; 2 — sinuatrial node; 3 — Bachmann's bundle; 4 — atrioventricular node; 5 — bundle of His; 6 — mitral valve; 7 — bundle branches of His; 8 — left ventricle; 9 — interventricular septum; 10 — Purkinje fibers of the right ventricle; 11 — tricuspid valve; 12 — INFERIOR VENA CAVA

the atrioventricular bundle (bundle of His). It runs within the interventricular septum and divides into the left and right Branches of the atrioventricular bundle (bundle branches of His), which terminate in the ventricular myocardium as fine fibers (Purkinje fibers).

The conduction system of the heart allows it to function relatively autonomously. Neural and HUMORAL INFLUENCES ON the organ merely coordinate the work of the conduction system. If the nodes and bundles of the conduction system are damaged, arrhythmias occur.

Properties of the heart muscle. The heart muscle possesses A number of properties. The main ones are: excitability, automaticity, conductivity, and contractility.

Excitability is The ability to enter a state of excitation under METABOLISM/18.html">The Influence of stimuli (Filling of the atria with blood), during which the electrical activity of the heart changes.

Automaticity is the ability of the nodes of the heart conduction system to independently enter a state of excitation (generate an impulse) at strictly regular intervals.

Conductivity is the ability of the heart conduction system to conduct the generated impulse to all areas of the myocardium.

Contractility is the ability of the heart muscle to respond with a contraction to an incoming impulse.

The first three properties are due to the presence of atypical cardiomyocytes in the myocardium that form the conduction system. Contractility is provided by typical cardiomyocytes.

Cardiac cycle. The human heart works continuously throughout life. It constantly exhibits rhythmic, sequential contractions (systole) and relaxations (diastole) of the atria and ventricles. This ensures the continuous Circulation of blood in the body.

The uniform sequence of systole and diastole of the heart chambers is called the cardiac cycle. Given that the heart rate averages 60–80 beats per minute, one cardiac cycle takes 0.8–1.0 s. To get a clear picture of how the heart works, it is necessary to examine its individual phases sequentially.

The first phase is called atrial systole and ventricular diastole. When the atria contract, the tricuspid and bicuspid (mitral) valves open, and blood is pumped into the ventricles, which are in a relaxed state. This phase takes about 0.1 s.

The second phase is ventricular systole and atrial diastole. During this period, the ventricular myocardium contracts, leading to a significant increase in pressure within the ventricular cavities. Under its influence, the tricuspid and bicuspid valves snap shut. Subsequently, the semilunar valves open; blood is pushed from the left ventricle into the aorta, and from the right into the pulmonary trunk. At this time, the atria enter the diastole phase: they relax and begin to fill with blood. The duration of this phase is 0.3 s.

The third phase is general diastole. After blood is ejected from the ventricles, the myocardium relaxes, the semilunar valves of the aorta and pulmonary trunk close, and blood enters the atria: into the left from the pulmonary veins, and into the right from the superior and inferior venae cavae. A pause common to the myocardium of all heart chambers occurs—diastole. At this time, blood fills not only the atria but also the ventricles: under the force of gravity, the atrioventricular valves open, and blood moves from the atria into the ventricles. Then the entire cycle repeats. The duration of the general diastole phase is 0.4–0.6 s.

The number of heart contractions per minute is called the heart rate (HR). On average, this indicator is 60–90 beats per minute. In one cycle, the heart ejects 70–100 ml of blood from the left ventricle into the aorta and the same amount from the right ventricle into the pulmonary trunk. This amount is conventionally designated as stroke volume (SV). The volume of blood ejected by the heart per minute is called Cardiac Output (CO). It can be determined by multiplying the heart rate by the stroke volume. Under conditions of physical rest, the cardiac output is about 4–6 L/min. It varies depending on gender, age, physical development, and training status. In 1 hour, the heart pumps 250–600 L of blood, and in a day, 12–15 tons.

A slower heart rhythm (less than 60 beats per minute) is called bradycardia. During intense physical exertion and neuro-emotional stress, the heart rate increases to 90–120 or more beats per minute. This heart rhythm is called tachycardia.

Each cardiac cycle is accompanied by distinct sounds known as heart sounds. There are two main sounds: systolic and diastolic. The first sound (systolic) occurs during the ejection of blood from the ventricles. It is caused by the snapping shut of the atrioventricular valves. Its duration is about 0.1–0.15 s. The second sound (diastolic) lasts about 0.1 s. It results from the tension of the cusps of the closing aortic and pulmonary valves, as well as vibrations of the aorta and pulmonary trunk. At the apex of the heart, the 1st sound is heard best, while at the base, the 2nd sound is louder. During auscultation in healthy individuals, the following sound sequence is detected: first, the 1st sound is heard, then a short pause (ventricular systole), the 2nd sound, and a long pause (diastole). There is a method for graphically recording heart sounds called phonocardiography.

ELECTRICAL PHENOMENA IN the heart. Electrocardiography. Atypical cardiomyocytes are characterized by the ability to generate and conduct impulses that trigger heart Muscle contraction. These processes are driven by the movement of ions (Ca²⁺, Na⁺, K⁺, Cl^-) across the cardiomyocyte membrane. The Resting Potential—the electrical charge difference between the outer and inner Cell membrane surfaces present in the absence of an impulse—is replaced by an Action Potential. The latter represents A change in this charge difference.

At any given moment during the cardiac cycle, not just one, but a multitude of cardiomyocytes are in a state of excitation. Normally, excitation sweeps through the heart sequentially: it first spreads across the atria and then through the ventricles. A potential difference arises between the excited and yet-unexcited areas. Because body Tissues are electrically conductive, these processes can be recorded at a distance from the heart.

The recording of electrical processes occurring in the heart is called electrocardiography (ECG). Special devices known as electrocardiographs are used for this purpose. This diagnostic method is widely utilized to detect various conditions, including rhythm and conduction disturbances, as well as coronary artery disease.

To perform an Electrocardiogram, electrodes are placed on various PARTS OF THE body. The presence of points on the body surface that differ in the magnitude and sign of their electrical charge makes it possible to record the potential difference between them. The connection of two such points is called an electrocardiographic lead.

An ECG examination involves recording data from 12 leads: three standard, three augmented unipolar, and six chest (precordial) leads. The standard leads were introduced by the pioneer of electrocardiography, W. Einthoven. To record standard and augmented leads, electrodes are attached to the limbs (in the region of the lower third of the forearm and lower leg), and the potential difference between various points is measured. For chest leads, electrodes are placed at specific points on the chest wall. These leads differentially record potentials from different Regions of the heart wall.

The electrocardiogram appears as a jagged waveform. Each cardiac cycle is recorded as a set of characteristic waves (Fig. 12.7). The distances between them are called intervals. Cardiac cycles repeat at a specific frequency; consequently, the normal sequence of waves repeats at the same frequency.

Fig. 12.7. Electrocardiographic complex in a standard lead (explanation in text)

The waves of the electrocardiogram are designated by the Latin letters P, Q, R, S, and T. The P wave is known as the atrial complex. It reflects the spread of excitation across the atria. The PQ interval represents the time it takes for excitation to travel from the sinuatrial node to the ventricular myocardium. The QRS complex is called the ventricular complex; it is generated during the excitation of the ventricles. The T wave reflects potential recovery processes and corresponds to the general diastole phase.

REGULATION OF CARDIAC activity. The heart possesses automatism. At the same time, the organ is influenced by humoral factors and The Nervous system. It has been established that the force of cardiac contraction directly depends on the filling of the ventricles with blood during diastole. When a large volume of blood enters the heart chambers, the wall stretches to some extent, stretching the cardiomyocytes as well. During systole, they contract more vigorously than they would under a normal venous blood return. Thus, the greater the volume of blood filling the ventricles before systole, the stronger the contraction of the heart muscle. This phenomenon is known as the Frank-Starling law. It is worth noting that the heart is highly sensitive to Changes in the volume of blood filling its chambers; even a 2% change in incoming blood volume triggers a corresponding reaction. This mechanism plays a key role in increasing cardiac output during physical exertion, when chamber filling increases to meet the heightened demands of The Muscular System. In response, the force of contraction increases, along with the stroke volume and cardiac output.

During blood loss, the volume of blood within the vascular bed decreases, leading to a corresponding reduction in venous return to the heart. As a result, the myocardium contracts with less force. Ultimately, cardiac arrest may occur.

The nervous system, particularly its autonomic division, plays a crucial role in regulating heart activity. The Medulla Oblongata houses the vasomotor center, which is linked to the Vagus nerve, whose branches provide parasympathetic innervation to the heart. Sympathetic Innervation of the heart is supplied by fibers from the Sympathetic trunk.

The parasympathetic nervous system decreases heart rate and contractile force, and inhibits impulse conduction through atypical cardiomyocytes. Conversely, activation of the sympathetic nervous system leads to an increased heart rate, enhanced myocardial contractility, and elevated excitability and conductivity.

The effects of adrenaline, noradrenaline, and dopamine on the heart mirror those of the sympathetic nervous system. THYROID Hormones (thyroxine, triiodothyronine) increase the heart rate. Acetylcholine produces effects similar to those of the parasympathetic nervous system.

The heart is also sensitive to the ionic composition of Blood Plasma. Ca²⁺ and K⁺ ions enhance myocardial cell excitability. A deficiency in these ions (particularly K⁺) leads to various arrhythmias and, in some cases, cardiac arrest.