Anatomy of the Heart

Anatomy of the Heart

• The heart is relatively small, about the size of a closed fist
• The heart measures approximately 12 cm long, 9 cm wide, and 6 cm thick
• The average mass of the heart is 250 g in adult females and 300 g in adult males
• The heart rests on the diaphragm, near the midline of the thoracic cavity
• The heart is situated in the mediastinum and extends from the sternum to the vertebral column, the first rib to the diaphragm, and between the lungs
• Approximately two-thirds of the heart mass lies to the left of the midline
• The heart can be visualized as a cone lying on its side
• The pointed apex of the heart is formed by the left ventricle tip and rests on the diaphragm
• The heart is directed anteriorly, inferiorly, and to the left
• The base of the heart constitutes its posterior surface, majorly formed by the atria, specifically the left atrium

Surfaces and Borders of the Heart

• The heart has distinct surfaces and borders, or margins, in addition to the apex and base
• The anterior surface is deep to the sternum and ribs
• The inferior surface lies between the apex and right border and mainly rests on the diaphragm
• The right border faces the right lung and extends from the inferior surface to the base
• The left border, also known as the pulmonary border, faces the left lung and extends from the base to the apex

Pericardium

• The pericardium is the membrane that surrounds and protects the heart
• The pericardium confines the heart to its mediastinum position, allowing freedom of movement during contractions
• The pericardium has two main parts: the fibrous pericardium and the serous pericardium
• The superficial fibrous pericardium comprises tough, inelastic, dense irregular connective tissue
• The open end resembles a bag attached to the diaphragm, fused to the blood vessels' connective tissues entering and exiting the heart
• The fibrous pericardium prevents overstretching, provides protection, and anchors the heart in the mediastinum
• The deeper serous pericardium, a double-layered membrane around the heart is thinner and more delicate
• The parietal layer of the serous pericardium is fused to the fibrous pericardium
• Also known as the epicardium, the inner visceral layer of the serous pericardium constitutes a heart wall layer tightly adhering to the heart's surface
• A thin film of lubricating serous fluid exists between the parietal and visceral layers of the serous pericardium
• Pericardial cells secrete pericardial fluid to reduce friction as the heart moves
• The pericardial cavity is the space containing a few ml of pericardial fluid

Layers of the Heart Wall

• The heart wall has three layers: the external epicardium, the middle myocardium, and the inner endocardium
• The visceral layer of serous pericardium is the outermost epicardium
• The middle myocardium is cardiac muscle tissue and makes up 95% of the heart and is responsible for pumping
• Cardiac muscle is striated like skeletal muscle but involuntary like smooth muscle
• Cardiac muscle fibers swirl around the heart in bundles
• The innermost endocardium is an endothelium layer over a connective tissue layer
• The endocardium provides a smooth lining for the chambers and covers the valves
• The endocardium is continuous with the endothelial lining in blood vessels attached to the heart, minimizing surface friction

Chambers of the Heart

• The heart has four chambers, two superior atria and two inferior ventricles
• An auricle is a wrinkled pouch-like structure on each atrium's anterior surface
• Each auricle increases an atrium's capacity to blood
• Sulci, or grooves, on the heart's surface contain coronary blood vessels and varying amounts of fat
• Each sulcus marks the border between two heart chambers
• The coronary sulcus encircles most of the heart and marks the boundary between the atria and ventricles
• The anterior inter-ventricular sulcus marks the boundary between the right and left ventricles on the heart's anterior surface
• The posterior inter-ventricular sulcus, as a continuation of the anterior, marks the boundary between ventricles on the heart's posterior aspect

Right Atrium

• The right atrium forms the heart's right border and receives blood from the superior vena cava, inferior vena cava, and coronary sinus
• The right atrium averages 2–3 mm (0.08–0.12 in.) in thickness
• The interatrial septum is a thin partition between the right and left atria
• Blood passes from the right atrium to the right ventricle through the tricuspid valve with three leaflets or cusps
• This valve is also known as the right atrioventricular valve
• The heart's valves consist of dense connective tissue covered by the endocardium

Right Ventricle

• The right ventricle, approximately 4–5 mm (0.16–0.2 in.) thick, forms most of the heart's anterior surface
• The inside of the right ventricle contains trabeculae carneae, a series of ridges formed by raised bundles of cardiac muscle fibers
• Chordae tendineae connects the tricuspid valve cusps to papillary muscles, which are cone-shaped trabeculae carneae
• The interventricular septum separates the right ventricle from the left ventricle internally
• Blood flows from the right ventricle into the pulmonary trunk and then into the right and left pulmonary arteries through a pulmonary valve - pulmonary semilunar valve

Left Atrium

• The left atrium is approximately the same thickness as the right atrium and forms most of the heart's base
• It receives blood from the lungs through four pulmonary veins
• Blood passes from the left atrium into the left ventricle through the bicuspid (mitral) valve, which has two cusps
• It is also called the left atrioventricular valve

Left Ventricle

• The left ventricle is the thickest chamber, averaging 10–15 mm (0.4–0.6 in.), and forms the apex
• Like the right ventricle, the left ventricle comprises trabeculae carneae and has chordae tendineae that anchor the bicuspid valve cusps to papillary muscles
• Blood passes from the left ventricle into the ascending aorta through the aortic valve (aortic semilunar valve)

Heart Valves and Blood Circulation

• As each chamber contracts, a volume of blood is pushed into a ventricle or out of the heart into an artery
• Valves open and close responding to pressure changes as the heart contracts and relaxes
• Valves facilitate one-way blood flow by opening to let blood through and then closing to prevent backflow

Operation of the Atrioventricular Valves

• Tricuspid and bicuspid valves are called atrioventricular AV valves, as they are located between an atrium and a ventricle
• An open AV valve results in the rounded ends of the cusps project into the ventricle
• Blood flows from a higher pressure in the atria to a lower pressure in the ventricles through open AV valves as the ventricles are relaxed
• The papillary muscles then relax, and the chordae tendineae are slack
• Blood pressure drives the cusps upward until their edges meet and close the opening when the ventricles contract
• Papillary muscles contract, which pulls on and tightens the chordae tendineae at the same time
• This prevents valve cusps from everting, opening into the atria, in response to high ventricular pressure
• Damage to the AV valves or chordae tendineae may result in blood regurgitating, flowing back into the atria when the ventricles contract

Operation of the Semilunar Valves

• The aortic and pulmonary valves are semilunar SL valves

• Each cusp attaches to the arterial wall via its convex outer margin

• The SL valves facilitate blood ejection from the heart into arteries and prevent backflow into the ventricles

• As ventricles contract, pressure builds up inside the chambers and the free borders of the cusps project into the artery's lumen

• Semilunar valves open when ventricular pressure exceeds arterial pressure, ejecting blood into the pulmonary trunk and aorta

• As ventricles relax, blood starts to flow toward the heart

• The back flowing blood fills valve cusps tightly closing semi-lunar valves

• No valves guard the junctions between the venae cavae and the right atrium or the pulmonary veins and the left atrium

• A small amount of blood flows backward from the atria into these vessels as the atria contract

• Backflow is minimized by atrial muscle contraction compressing and collapsing the venous entry points

Systemic and Pulmonary Circulations

• In postnatal or after birth circulation, the heart pumps blood into two closed circuits

• The circuits are systemic and pulmonary The output of one becomes the input of the other, a series arrangement

• The left side of the heart pumps systemic circulation, receiving bright red, oxygen-rich blood from the lungs

• The left ventricle ejects blood into the aorta

• The blood from the aorta separates into smaller streams, entering progressively smaller systemic arteries that carry it to all organs

• In systemic tissues, arteries branch into smaller-diameter arterioles that lead to extensive beds of systemic capillaries

• Exchange of nutrients and gases occurs across the thin capillary walls

• Blood unloads oxygen and picks up carbon dioxide

• Blood flows through only one capillary and then enters a systemic venule in most cases

• Venules carry deoxygenated, oxygen-poor blood away from tissues, merging to form larger systemic veins

• The blood flows back to the right atrium

• The right side of the heart pumps pulmonary circulation, receiving all the dark red, deoxygenated blood returning from systemic circulation

• Blood flows into the pulmonary trunk branching into pulmonary arteries carrying blood to the lungs by the right ventricle

• Blood unloads carbon dioxide, exhaled, and picks up inhaled oxygen in pulmonary capillaries

• The freshly oxygenated blood then flows into pulmonary veins and returns to the left atrium

Coronary Circulation

• Nutrients cannot diffuse quickly enough from blood in the chambers to get to the heart wall
• The myocardium has its network of blood vessels, the coronary or cardiac circulation for this reason
• Branching from the ascending aorta, the coronary arteries encircle the heart like a crown
• Coronary arteries receive minimal blood flow during heart contraction because of being squeezed shut
• The high pressure of blood in the aorta propels through the coronary arteries into capillaries into coronary veins when the heart relaxes

Coronary Arteries

• Two coronary arteries, the right and left, branch from the ascending aorta, supply oxygeneted blood to the myocardium
• The left coronary artery divides into the anterior interventricular and circumflex branches after passing inferior to the left auricle
• Located in the anterior interventricular sulcus, the anterior interventricular branch or left anterior descending LAD artery supplies oxygenated blood to the walls of both ventricles
• The circumflex branch lies in the coronary sulcus distributing oxygenated blood to the walls of the left ventricle and atrium
• The right coronary artery, supplying small atrial branches to the right atrium, continues inferiorly to the right auricle
• It then divides into the posterior interventricular and marginal branches.
• The posterior interventricular branch follows the posterior interventricular sulcus, supplying the walls of both ventricles with oxygenated blood
• The marginal branch in the coronary sulcus transports oxygenated blood to the right ventricle's myocardium

Coronary Veins

• Most deoxygenated blood from the myocardium drains into a large vascular sinus in the coronary sulcus on the posterior surface, the coronary sinus
• 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 draining the right atrium and right ventricle
• Anterior cardiac veins drain the right ventricle

Cardiac Muscle Tissue Histology

• Compared with skeletal muscle fibers, cardiac muscle fibers are shorter in length and less circular in transverse section
• Cardiac muscle fibers branch giving a "stair-step" appearance
• A typical cardiac muscle fiber is 50–100 μm long and has a diameter of about 14 μm
• Usually a single centrally located nucleus is present, but an occasional cell may have two nuclei
• The ends of cardiac muscle fibers connect to neighboring fibers by irregular, transverse thickenings of the sarcolemma, the intercalated discs
• The discs contain desmosomes holding the fibers together and gap junctions enabling muscle action potentials to conduct from one muscle fiber to its neighbors
• Gap junctions allow the entire myocardium of the atria or ventricles to contract as a single, coordinated unit
• Mitochondria are larger and more numerous in cardiac muscle fibers than in skeletal muscle fibers
• Cardiac muscle fibers take up 25% of cytosolic space compared to skeletal muscle fibers which only take up 2%

Auto-rhythmic Cells: The Conduction System

• Inherent and rhythmical electrical activity initiates the hearts lifelong beat

• Specialized cardiac muscle fibers call auto-rhythmic fibers are electrical source and self-excitable

• Auto-rhythmic fibers repeatedly generate action potentials that trigger heart contractions

• These fibers stimulate the heart to beat even when removed from the body and all nerves have been cut

• Cardiac action potentials propogate through the condution system in the following order

• 1. Cardiac excitation normally begins in located in the right atrial wall just inferior and lateral to the opening of the superior vena cava in the sinoatrial SA node

• Action potential from SA propagates across both atria from gap junctions and atrial muscles, this triggers contraction

• 2)By conducting along atrial muscle fibers, ,the action potential reaches the atrioventricular AV node interatrial septum anterior to sinus

• 3. AV bundle from the AV node, aka bundle of His, is the only place action potentials flow from the atria to ventricles

• 4)The action potential travels along the AV bundle to the right and left bundle branches that spread to apex via the interventricular septum

• 5. Then Purkinje fibers conduct the action potential quickly from apex up to ventricular myocardium and ventricles contract

• The pushing of blood upward moves towards the semilunar valves

Electrical Changes in the Heart

• Electrical activity, detected by surface electrodes, displays the pattern on a oscilloscope screen showing voltage trace called electrocardiogram, also known as ECG
• The ECG tracings show five waves referred to as P, Q, R, S, & T
• The P wave begins when the impulse from the SA node sweeps atria
• The QRS complex demonstrates quick impulse spread from the AV node spreads to AV bundles through the Purkinje fibers and the electrical activity in the ventricular muscle
• The T wave means the relaxation of muscle that was ventricular
• A sinus rhythm which originates at the SA node contains the ECG and has rate from 60 -100 BPM

Physiology of Cardiac Conduction

• Cardiac electrical activity results from movement of ions such as, sodium, potassium, and calcium.

• Electrical change is recorded within a single cell called cardiac action potential

• The cardiac action shows 5 stages:

  • Phase 0: Rapid depolarization caused by significant movement of sodium flowing into a cell
  • Phase 1: Sodium channels close and stop moving
  • Phase 2: Plateau phase when calcium flows in while potassium start moving
  • Phase 3: Calcium closes while potassium rapidly exits a cell
  • Phase 4: A resting phase is maintained as pumps work restore sodium and potassium inside

Nerve Supply to the Heart

• Autonomic nerves within the cardiovascular center in medulla oblangata influence heart by connecting wuth autonomic nervous system
• Parasympathetice and sympathetic nerves control body antagonistically
• Parasympathetic nerves supply the SA and AV nodes as well as Atrial muscles, that lower the rate of stimulations reducing beat rate and force
• Sympathetic nerves which supply SA and AV nodes of the myocardial muscle the atria that incrase the rate and force of action

Heart Rate and Factors That Affect It

• Heart rate is the speed of the beats which measured by number contractions in a minute
• Autonomic nervous system balances sympathetic versus parasympathetic which is main factor dictating heart beat rate.
• Chemicals such adrenaline and nonadrenaline has same impact, along with thyriod hormones
• Uprightherat rate is normally higher then a patient laying down, especially with exercise
• Heart increases with states of fear, anxiety
• Women have heart that is higher, alongside with babies and childern
• Temperature rises the rate at which beats come in

The Cardiac Cycle

• The goal is constant volume which the heart achieves cycle phases of systole (contraction) and diastole (relaxion)
• Cycle occurs arround 60-100 times per minute, taking 75 beats per min as example means that cycle lasts around 8 seconds and includes:
• 1. Atrical Systole: is contrating around the Atria
• 2. Ventricular systole contrating around ventricular
• 3. complete Diastole complete relaxion of atria and ventricles
• Superior and inferior vena transport dexoygenated blood into the atria at same moment pulmonary veins deposit oxygenated blood left atrium
• Atrioventricular valves open and blood passes through Ventricles
• Signal to contract reaches AV then the AV signal propagates muscles to all ventricule
• Blood pressure is highest at aortic therefore forcing atrioventriuler valves close
• After the complete contract ventricular enters time known as diastole which is complete rest for the myocardium. After rest they can contract again because atria refills preparing it for the next beat

Heart Sounds

• Unless a medical device such as stethoscope is used person isn't aware of regular heart sounds
• There distinguisheable sounds dubbed lub dup
• Lub: indicates loudness and contract from ventricles
• Dup softer sounds correspond with the contract of atria.

Cardiac Ouput

• Amount of blood let out every contraction
• Liter per minute is what describes the level
• Stroke rate dictates the volume alongside beats per minute(Heart Rate)= Cardiac Output
• Stroke is at average 70 liters, paired with average heart comes 5 l per minute. Increase and decrease based factors mentioned previosuly
• Important factors involve diastolic rate, volume, and amount of hormones or sympathetic activity

Factors Affecting Cardiovascular Cardiac Volume

• Diastolic: Volume of blood in ventricules at end of diastole
• Return heart receives by both superior vena and inferior
• Stimulating the heart's sympathetic system which has increase on harmones