Heart Valves and Circulatory System

The Circulatory (Cardiovascular) System

• Cells need nutrients and gases, while wastes must be removed
• The circulatory system transports nutrients from the digestive system and oxygen from the lungs to cells, also removes wastes and carbon dioxide
• The components of the circulatory system includes the the heart, blood vessels, and blood itself

The Heart

• The heart is a hollow, muscular organ responsible for pumping blood through blood vessels via rhythmic contractions followed by relaxations
• The human embryonic heart starts beating approximately 21 days after conception
• The heart is usually located slightly left of the midline of the thorax, underneath the sternum

Structure

• The pericardium is a double-walled sac that encloses the heart and contains the roots of great vessels
• The pericardium helps lubricate the heart, preventing friction resulting from of heart activity
• The myocardium is the muscular tissue of the heart, weighing around 250-350g in normal adults, and up to 1000g in extremely diseased hearts
• The myocardium has specialized cardiac muscle cells with electrical conduction abilities unlike other muscles
• The myocardium's blood supply comes from the coronary arteries. If they are blocked by atherosclerosis or thrombosis, it leads to angina pectoris or myocardial infarction
• Certain viruses can cause inflammation of the myocardium, also knows as as myocarditis
• The endocardium is the innermost layer covering the heart, responsible for its contraction

Chambers and Valves

• The heart has four chambers: two upper atria and two lower ventricles
• A septum divides the right atrium and ventricle from the left atrium and ventricle, which prevents blood from passing between them
• During fetal life, there is a in the interatrial septum called the "foramen ovale," to allow blood to directly flow from the the right atrium to the left atrium
• The foramen ovale closes soon after birth, and failing to do so results in ASD (Atrial Septal Defect)

Valves

• Valves between atria and ventricles (atrioventricular valves) ensure unidirectional blood flow from the atria into the ventricles
• The right atrioventricular valve has three cusps or flaps, commonly called the tricuspid valve
• The left atrioventricular valve has two cusps or flaps, it is the bicuspid or mitral valve
• At the the root of the aorta and the pulmonary artery, the aortic and pulmonary semilunar valves can be found, the cusps appear half-moon shaped
• Semilunar valves separate each ventricle (right and left) from the connecting artery (pulmonary artery and aorta

Double Circulatory System

• The human heart functions as part of a double circulatory system that comprises the pulmonary and systemic circulations
• The adult human heart is made up of two separate pumps
• The right side consists of the right atrium/ventricle, transports deoxygenated blood into the pulmonary circulation
• The left side consists of the left atrium/ventricle, transports oxygenated blood into the systemic circulation
• Blood in one circuit must pass through the heart

Systemic Circulation

• During systemic circulation, the left ventricle pumps blood into the aorta
• The aorta then delivers blood to arteries that carry blood to tissues/organs in the body
• To carry deoxygenated blood to the heart, blood flows through venules via capillaries into veins towards the superior and inferior vena cava which enters the heart after systemic circulation

Pulmonary Circulation

• The pulmonary circulation is a blood circulation circuit in the cardiovascular system, toward the lungs
• Here, red blood cells collect oxygen and release carbon dioxide during respiration
• Deoxygenated blood is pumped out from the right ventricle of the heart into the pulmonary trunk then into the pulmonary arteries towards the lungs
• Via pulmonary veins, oxygenated blood is then drained to the left atrium for systemic circulation

Heart and Body Circulation

• Blood circulates through the body two to three times each minute, totaling around 19,000 km each day
• The heart's muscle cardiac tissue does not fatigue, due tot he its contractile nature

Coronary Circulation

• Coronary circulation involves blood vessels supplying blood to, and removing it from, the heart
• Vessels delivering highly oxygenated blood are the coronary arteries, which branch off from the aorta
• Vessels removing deoxygenated blood are known as cardiac veins
• The coronary arteries running on the heart's surface are called epicardial coronary arteries
• These arteries, when in good health, can achieve autoregulation to maintain coronary blood flow by the heart muscle's needs
• Relatively narrow vessels are commonly affected by atherosclerosis, and can become blocked, possibly leading to a heart attack

Cardiac Cycle

• "Cardiac" from the Greek "cardia" refers to the heart
• The heart beats or contracts 70 times per minute
• The human heart will undergo over 3 billion contraction cycles during normal lifetime
• The cardiac cycle is made up of two parts: systole (heart muscle contraction) and diastole (heart muscle relaxation)
• Atria contracts while ventricles relax
• The heart muscle contraction is caused by the heart’s own pacemaker system
• The heartbeat is modulated by the autonomic nervous system
• Blood flow is maintained by pressure differences in the chambers
• Blood flows from higher to lower pressure
• The heart is the muscle that generates pressure
• Blood flows through the heart from veins into the atria/ventricles into arteries in a single direction
• One heartbeat or cardiac cycle includes relaxation and contraction

Ventricles

• Normal cardiac cycles (at rest) take 0.8 seconds
• During systole, heart muscle contracts to creates higher pressure
• Difference in ventricular pressure sends blood to the arteries
• As left ventricle generates greater pressure here (120 Hgmm), than the right ventricle (25 Hgmm), blood is pumped to the whole body
• During diastole, muscle cells relax and there is no pressure in the ventricles
• The "Windkessel effect," of the the aorta and pulmonary artery maintains continuous blood flow when there is no pressure in the ventricles
• During ventricular systole blood is pumped into the the aorta and the pulmonary artery Big arteries like the aorta/pulmonary artery are elastic, this distends blood to be pumped into them
• Distension is the Windkessel effect, allowing the heart to be efficient and can provide continuous blood flow

Atria

• Blood from the body flows through the vena cava veins toward the right atrium
• Oxygenated blood empties from the pulmonary veins to the left atrium, simultaneously
• Atrial diastole fills atria with blood from the vena cavae and pulmonary veins
• The atria are 70% full when atrial systole begins
• Atria muscles contract and empty the remaining 30% blood to the ventricles
• Ventricular systole opens the semilunar valves while forcing blood through the aorta/pulmonary artery
• The term "lub" corresponds to diastole
• The term "dub" corresponds to systole

Cardiac Action Potential

• Normal resting potential falls between -85 to -90 mV in the cardiac muscle cells
• The action potential is 105 mV
• Membrane atrial depolarization lasts for ~0.2 seconds, while ventricular depolarization last ~0.3 seconds
• Prolonged cardiac muscle depolarization happens due to: slower Ca2+ and Na+ channels (same as nerve and skeletal tissues), and decreased potassium permeability at action potential onset
• Cardiac muscle is refractory to re-stimulation during AP
• The refractory period in cardiac muscle is almost as long as its systolic action potential (~0.25 to 0.3 seconds in the ventricles)

Refractory Period of the Heart

• The ventricle muscle is incapable of summation in contractions, prolonged tetanic contractions do not occur during normal conditions, so the heart can function as a pump
• The heart does not generate tetanic contractions due to a long refractory period in the cells
• In excitable membranes, an action potential has a refractory period, and cannot be re-excited, also know as the absolute refractory period
• This is almost as long as contraction and the muscle cannot be excited in time, which helps to produce summation
• The absolute refractory period of cardiac muscle lasts ~250 ms because of the long plateau

Heart Sounds

• The heart’s sounds are characterized by the "lub-dub" sound
• "Lub" is associated with closing atrioventricular valves
• "Dub" is associated with closing the semilunar (SL) valves

Cardiac Muscle

• Cardiac muscle has resembles skeletal muscle in some ways, due to it being striated and how its cells contain sarcomeres with sliding actin/myosin filaments
• Cardiac muscle's structure reflects its function of pumping blood
• Cardiac muscle is made up of single branched cells, each with a single nucleus
• Cardiac muscle's cells have abundant sarcoplasmic reticulum
• Cardiac muscle cells are bound to each other with intercalated disks (adherens junctions and gap junctions), which allows for fast flow of ions and individual cells to contract together

Cardiac Tissue Distinctions

• Action potentials trigger the heartbeat and is generated within the heart
• Motor nerves modulate the heartbeat (increase or decrease)
• Beats continue even when the nerves are destroyed
• The action potential that drives contraction passes from fiber to fiber via gap junctions
• Synchronized contraction of the heart drives blood flow to the ventricles
• Damage to the heart can disrupt the synchronized contraction, causing the heart to fibrillate
• Fibrillation eventually causes death
• Defibrillators are used to reverse fibrillation
• The heart is a well vascularized organ and refractory period lasts longer than other areas
• Tetanus cannot happen
• The cardiac muscle contains a high amount of mitochondria, due to its constant need for cellular respiration for ATP
• Cardiac muscle does not contain much glycogen, and gets limited benefit from the glycogen supply in the event of oxygen deficit
• Interruption in the flow of oxygenated blood causes heart attacks

This lesson covers the function of heart valves, including the mitral, tricuspid, aortic, and pulmonary valves. It discusses blood flow between the heart chambers and the role of atrioventricular valves. Also covered are the primary functions of the circulatory system.