Heart Wall and Valves

Heart Wall

• Epicardium is the outermost layer, also known as the visceral layer of the serous pericardium.
• Myocardium is the middle layer mainly composed of cardiac muscle, forming the bulk of the heart.
• The fibrous skeleton of the heart is a crisscrossing, interlacing connective tissue layer.
  • It anchors cardiac muscle fibers.
  • It supports great vessels and valves.
  • It limits the spread of action potentials for specific paths.
• Endocardium is the innermost layer of endothelium resting on connective tissue.
  • It lines the heart chambers.
  • It covers the fibrous skeleton of the valves.
  • It is continuous with the endothelial lining of blood vessels.

Heart Valves

• Atrioventricular (AV) valves include the tricuspid (right side) and mitral (left side) valves.
  • Chordae tendineae anchor cusps to papillary muscles, preventing blood backflow into atria.
• Semilunar (SL) valves include the aortic and pulmonary valves.
  • They prevent blood backflow into ventricles.

Pathway of Blood Through the Heart

• Route: Superior vena cava (SVC), inferior vena cava (IVC), coronary sinus -> Right atrium -> Tricuspid valve -> Right ventricle -> Pulmonary semilunar valve -> Pulmonary trunk -> Pulmonary arteries -> Lungs -> Pulmonary veins -> Left atrium -> Mitral valve -> Left ventricle -> Aortic semilunar valve -> Aorta -> Systemic circulation

Coronary Circulation

• The arterial supply includes the right and left coronary arteries.
  • The arteries arise from the base of the aorta.
  • They supply arterial blood to the heart.
• The venous drainage is via cardiac veins into the coronary sinus.

Cardiac Muscle Contraction

• Depolarization opens voltage-gated Na+ channels in the sarcolemma.
  • This positive feedback rapidly opens more Na+ channels, reversing the membrane potential from -90mV to +30mV.
  • Na+ channels subsequently close.
• Depolarization opens slow Ca2+ channels in the sarcolemma, prolonging depolarization (plateau).
  • Ca2+ influx triggers Ca2+-sensitive channels in the SR, releasing bursts of Ca2+.
  • Ca2+ binds to troponin, causing filaments to slide.

Electrical Events

• Sequence: Sinoatrial (SA) node (pacemaker) -> Atrioventricular (AV) node -> Atrioventricular (AV) bundle (bundle of His) -> Right and left bundle branches -> Purkinje fibers

Electrocardiography

• ECG Tracing includes:
  • P wave: atrial depolarization
  • QRS complex: ventricular depolarization
  • T wave: ventricular repolarization
  • P-R interval: atrial excitation to ventricular excitation
  • S-T segment: the entire ventricular myocardium is depolarized
  • Q-T interval: ventricular depolarization through ventricular repolarization

Heart Sounds

• Heart sounds are described as "Lub-dub"
• First sound: AV valves close
• Second sound: SL valves close

Cardiac Cycle

• Systole is the period of heart contraction.
• Diastole is the period of heart relaxation.
• Cardiac Output (CO) is the volume of blood pumped by each ventricle in one minute.
  • CO = Heart rate (HR) x Stroke volume (SV)
  • Normal is 5.25 L/min.

Regulation of Stroke Volume

• Preload is the degree of stretch of cardiac muscle cells before contraction.
• Contractility is the contractile strength at a given muscle length.
• Afterload is the pressure ventricles overcome to eject blood.

Chemical Regulation of Heart Rate

• Hormones affecting heart rate include epinephrine and thyroxine.
• Ions such as Ca2+ and K+ must be maintained at specific concentrations for normal heart function.

Arteries

• Arteries carry blood away from the heart.
  • They have three layers: tunica intima, tunica media, and tunica externa.
  • Types include elastic arteries, muscular arteries, and arterioles.

Capillaries

• Capillaries are the smallest blood vessels.
  • Walls consist of a thin tunica intima (one cell thick).
  • Allow only a single RBC to pass at a time.
  • Types include continuous, fenestrated, and sinusoid capillaries.

Veins

• Veins carry blood toward the heart.
  • They have three layers: tunica intima, tunica media, and tunica externa.
  • They have valves.

Factors Aiding Venous Return

• Muscular pump: contraction of skeletal muscles "milks" blood toward the heart.
• Respiratory pump: pressure changes during breathing move blood toward the heart.
• Vasoconstriction of veins under sympathetic control.

Maintaining Blood Pressure

• Main factors: Cardiac output (CO), Peripheral resistance (PR), and Blood volume

Control of Blood Pressure

• Short-term mechanisms involve nervous and chemical controls.
• Long-term mechanisms involve renal regulation.

Composition of Blood

• Blood consists of plasma and formed elements.
• Formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

Plasma

• Plasma is 90% water.
• Plasma proteins include albumin, globulins, and fibrinogen.
• Nonprotein nitrogenous substances include lactic acid, urea, and creatinine.
• Organic nutrients include glucose, carbohydrates, and amino acids.
• Electrolytes include sodium, potassium, calcium, chloride, and bicarbonate.
• Respiratory gases include oxygen and carbon dioxide.

Erythrocytes

• Erythrocytes are biconcave discs, anucleate, with essentially no organelles.
  • They are filled with hemoglobin (Hb).
  • They function in gas transport.
  • They contain spectrin that gives them flexibility and allows them to change shape.
  • They have a large surface area relative to volume.
  • They are more than 97% hemoglobin (excluding water).
  • They generate ATP anaerobically.
  • Life span: 100-120 days.

Leukocytes

• Types of leukocytes:
  • Granulocytes: neutrophils, eosinophils, basophils
  • Agranulocytes: lymphocytes, monocytes

Platelets

• Platelets are cytoplasmic fragments of megakaryocytes that function in the clotting mechanism.

Steps Involved in Hemostasis

• Vascular spasm
• Platelet plug formation
• Coagulation (blood clotting)