The Heart: Anatomy and Function

• Circulatory system function: transports nutrients/gases and removes waste
• The circulatory system consists of the heart, blood vessels, and blood

The Heart

• The heart is a hallow, muscular pump that moves blood through blood vessels by repeated, rhythmic contraction followed by relaxation.
• Embryonic hearts start beating 21 days post-conception
• The heart is situated left of the thorax's middle, beneath the sternum

Heart Structure

• Pericardium: a double-walled sac enclosing the heart and the roots of great vessels, surrounded by lungs. Reduce friction and protect heart.

• Myocardium: muscular heart tissue; Adult mass: 250-350g, diseased: up to 1000g and is composed of specialized cardiac muscle cells able to conduct electricity. Coronary arteries supply blood to the myocardium; if occluded by atherosclerosis or thrombosis, it can cause angina pectoris or myocardial infarction and some viruses may lead to inflammation of the myocardium, or myocarditis.

• Endocardium: innermost heart layer covering the muscular tissue

Chambers and Valves

• The heart has 4 chambers: 2 upper atria and 2 lower ventricles
• Septum: divides the right atrium/ventricle from the left atrium/ventricle, preventing blood passage between the and the foramen ovale is a hole in interatrial septum, which allows blood flow from the right to left atrium during fetal life, which closes right after birth.
• Valves maintain coordinated unidirectional flow of blood from atria to ventricles and from ventricles to big arteries.
• Atrioventricular valves maintain unidirectional flow from atria to ventricles.
• Tricuspid valve: the right atrioventricular valve with three cusps or flaps
• Dicuspid/Mitral valve: the left atrioventricular valve with two cusps or flaps
• Aortic and pulmonary semilunar valves are at artery roots with half-moon shaped cusps.

Function of the Heart

• The heart's right side collects deoxygenated blood and sends it to the lungs for CO2 release and O2 pick-up (diffusion)
• The heart's left side collects oxygenated blood from the lungs and pumps it to the body
• Ventricles are thicker than atria
• The left ventricle (1.3-1.5 cm thick) is more muscular than the right (0.3-0.5 cm thick) to overcome vascular pressure for whole body blood pumping and the right ventricle requires less muscle to pump only to the lungs
• The superior vena cava collects blood from the upper body, the inferior vena cava from the lower body, both drain into the right atrium
• Blood flows through the tricuspid valve into the right ventricle, which pumps deoxygenated blood to the lungs via the pulmonary artery, where it uptakes oxygen and releases carbon dioxide
• Oxygenated blood flows through the pulmonary veins to the left atrium, passes through the bicuspid or mitral valve into the left ventricle, and is pumped through the aorta to the body
• The aorta is the largest artery arising from the heart's left ventricle
• Lungs receive blood supply from the aorta via bronchial arteries

Systemic and Pulmonary Circulation

• Systemic Circulation: the left ventricle pumps blood into the aorta, which sends blood to arteries for tissues/organs which then enter capillaries then returns deoxygenated blood through venules and veins into the superior and inferior vena cava and back to the right atrium
• Pulmonary Circulation: blood circulation towards the lungs where red blood cells uptakes oxygen and releases carbon dioxide via the right ventricle pumping blood into the pulmonary trunk then pulmonary arteries then lungs. Oxygenated blood returns to the left atrium via pulmonary veins.
• The heart acts as two seperated pumps for both circulations.
• The right side pumps deoxygenated blood into pulmonary circulation, while the left side pumps oxygenated blood into systemic circulation.
• Blood in one circuit has to go through the heart to enter the other circuit.
• Blood circulates 2-3 times per minute, totaling 19,000 km daily

Coronary Circulation

• Coronary circulation: blood vessels supply blood to/remove blood from the heart
• Coronary arteries supply oxygen-rich blood and branch off from the aorta
• Cardiac veins remove deoxygenated blood
• Epicardial coronary arteries run on the heart's surface and maintain coronary blood flow via autoregulation. However, these vessels narrow and become blocked, causing a heart attack

Cardiac Cycle Facts

• Cardiac means "related to the heart" (Greek: kardia)

• The heart beats ~70 times/minute, undergoing 3 billion+ contractions in a lifetime

• Cardiac cycle: Diastole: The whole heart is resting, the blood of the large veins enters the right atrium, and the blood of the pulmonary veins enters the left atrium. The time is about 0.4 seconds. Atrial systole: Is very transient, and the contraction of the atria takes place, and by doing it, the ventricles are completely filled with blood. The time is about 0.1 seconds. ( During atrial diastole 70% of ventricles are filled with blood and During atrial systole 30% ) Ventricular systole: Is the contraction of the ventricles and blood is sent to all parts of the body through the arteries. The time is about 0.3 seconds

• Blood flows from high to low pressure areas, with the heart generating pressure. Blood flows from veins to atria to ventricles to arteries.

• Heart valves limit flow to one direction

• Windkessel effect of the aorta/pulmonary artery: ensures continuous blood flow when there is no pressure in ventricles. During ventricular systole, blood is pumped into the aorta/pulmonary artery, using big arteries elastic walls that distend, increasing pumping efficiency

Heart Sounds

• Contraction and valve action produces "lub-dub" sounds:
• Lub: AV valve closing
• Dub: semilunar valve closing

Cardiac Muscle

• Cardiac muscle is striated containing sarcomeres with action and myosin but has unique features for pumping blood

  • Cardiac muscle: single cells with a single nucleus and branched myofibrils

  • Sarcoplasmic reticulum: Developed endoplasmic reticulum in cardiac muscle cells

  • Intercalated discs connect cardiac muscle cells by membrane junctions ( adherenes and gap junctions ), facilitating fast ion/impulse transmission leading to unit contraction of individual cells.

  • Action potential passes from fiber to fiber through gap junctions.

  • Action potentials that trigger heartbeat are generated within the heart and motor nerves modulate rate/strength; if nerves are destroyed, the heart continues to beat

  • Fibrilation: Anything that interferes with synchronous waves of heart can cause fibers to beat at random. Its reversal is the function of defibrillators .

• Tetanus is not possible. The refractory period is longer than the period for contraction/relaxation
• Cardiac mucsle has a richer supply of mitochondria than skeletal muscles and depends on cellular respiration for ATP.
• Cardiac muscle has little glycogen and limited benefit from glycolysis during oxygen deficiency. which anything that interrupts blood flow to heart leads to damage, as in heart attacks.

Cardiac Action Potential

• Normal resting potential ranges between -85 to -90 mV while the action potential is 105 mV
• Atrial membranes depolarize for ~0.2 seconds, ventricles for 0.3 seconds
• Long depolarization for 2 reasons: slow calcium and sodium channels, and decreased potassium permeability at action potential onset
• Cardiac muscle is refractory and difficult to restimulate during action potential, refractory period ~0.25-0.3 seconds

Heart Refractory Periods

• Ventricular muscle is incapable of summation in contractions and tetanic contractions do not occur, this lets the heart function as a pump
• Absolute refractory period exists during the time the membrane cannot be re-excited
• Cardiac muscle: refractory period lasts almost the entire contraction (~250ms)

Cardiac Pacemakers

• Cardiac muscle tissue is able to contract without neural or hormonal stimulation.
• Cardiac contractions are coordinated by 1) nodal cells, which set the rate, and 2) conducting cells, which distribute the contractile stimulus.

Sinoatrial Node (SA node)

• SA node: a group of modified cardiac muscle cells in the right atrium wall near superior vena cava entrance that have some contractile filaments but do not contract, initiates impulses.
• All heart muscle cells can generate electrical impulses, but the SA node generally initiates that because it generates it faster than others naturally discharges at 70-80 times/minute
• SA node is innervated by parasympathetic/sympathetic fibers, is sensitive to autonomic influences
• Parasympathetic nerves: decrease the SA node rate
• Sympathetic system: increases the SA node rate

Atrioventricular Node

• AV node is between atria and ventricles.
• The AV node conducts electrical impulses from atria to ventricles.
• The cells depolarize spontaneously generating 40-60 action potentials/minute
• when stimulus reach AV node, it slows down, which is AV delay that allows atrial contraction before ventricles
• AV nod will become the heart's pacemaker if the SA Node is not functioning or impulses are blocked and can't move down.

Bundle of hiss:

• Bundle of His: specialized heart muscle cells that conduct electrical activity from the AV node to the heart apex
• Bundle of His splits into the Bundle Branch in interventicular septum called the Left and Right Bundle Branches
• Left bundle branch: activates the left ventricle
• Right bundle branch: activates the right ventricle

Purkinje Fibers

• Inner ventricular walls, beneath the endocardium
• Specialized myocardial fibers to conduct electrical stimuli for coordinated contraction
• Stimulate individual groups of myocardial cells to contract.
• They work with the SA and AV nodes to control the heart rate
• During ventricular contraction: carry impulses to myocardium, causing ventricular tissue to contract and force blood into pulmonary or systemic circulation.

Cardiac Muscle

• When a cell of cardiac muscle is placed next to another that will beat in unison.
• The fibers of bundle of hiss allow electrical conduction more easily and more quickly than other cardiac muscle cells.
• Bundle of hiss is an important part of the electrical conduction system as it transmits impulses from the AV node.

Electrocardiography (ECG)

• Records total heart electrical activity
• Electrocardiogram (ECG): measures changes in electrical potential across the heart, detects contraction pulses
• SA node electrical activity between 60-100 beats/minute is normal sinus rhythm
• PR interval represents AV Node, on the ECG
• QRS complex: ventricular depolarization T wave: represents ventricular reporalization
• ECG monitors heart functioning and displays electrical activity flowing through the atrial myocardium