Heart Anatomy: Location, Chambers, and Valves

Location of the Heart

• The heart resides in the mediastinum, the region in the thorax between the two pleural cavities.
• Approximately the size of a fist.
• The base is situated at the second rib.
• Its apex (distal end) is located at the 5th intercostal space and tilts leftward.

Heart Chambers

• The heart has atria and ventricles.
• The atria receive blood returning to the heart.
• The ventricles pump blood out of the heart through arteries; ventricles are thick and muscular.
• Septa are muscular walls dividing the heart into left and right sides.

Heart Valves

• Valves ensure one-way blood flow, responding to pressure gradients.
• Atrioventricular valves include the tricuspid (right) and mitral/bicuspid (left) valves.
• Semilunar valves include the pulmonary (right ventricle to pulmonary artery) and aortic (left ventricle to aorta) valves.
• Valves sit within the cardiac skeleton, stabilizing positions and preventing overdilation.

Atrioventricular Valves vs. Semilunar Valves

• Atrioventricular valves have chordae tendineae and papillary muscles that prevent eversion into the atria during ventricular systole.
• Semilunar valves form cups that fill with blood when closed, preventing prolapse.

The Pericardium

• The pericardium is a sac-like structure around the heart.
• It has outer fibrous and inner serous layers (parietal and visceral).
• The pericardial cavity contains fluid, reducing friction.
• The heart wall consists of the visceral serous pericardium (epicardium), myocardium, and endocardium.

Cardiac Muscle Tissue

• Cardiomyocytes are cardiac muscle cells.
• They are short, thick, branched, and striated.
• Cardiomyocytes hold a mass of glycogen.
• They are dependent on aerobic metabolism.
• Intercalated discs connect cardiomyocytes with adhesive and gap junctions.
• Desmosomes offer strong cell adhesion.
• Gap junctions create a functional syncytium, allowing signals to pass and enabling synchronized contraction.

Cardiac Muscle Action Potential

• Phase 0: Rapid Na+ influx via open fast Na+ channels.
• Phase1: Transient K+ channels open; K+ efflux returns membrane potential to 0 mV.
• Phase 2: Ca2+ influx balances K+ efflux, creating a plateau phase.
• Phase 3: Ca2+ channels close; rectifier K+ channels open, returning membrane potential to -90 mV.
• Phase 4: Rectifier channels maintain negative resting potential.

Refractory Periods

• Long absolute refractory period prevents rapid, disorganized contractions (fibrillation).
• Prevents a second action potential from being activated until ARP is over.
• The plateau phase allows enough calcium influx for a strong, sustained contraction.
• Ensures ventricles have sufficient time to squeeze blood out with each beat.

The Cardiac Cycle

• It consists of systole (contraction) and diastole (relaxation).
• Atrial systole:
  • Atria contract together and finish filling the ventricles.
• Ventricular systole first phase:
  • Ventricles contract, closing AV valves but without enough pressure to open semilunar valves.
• Ventricular systole second phase:
  • Increasing pressure opens semilunar valves, pushing blood into pulmonary and systemic circuits; atria relaxes.
• Ventricular diastole:
  • Ventricles relax, pressure drops, blood in aorta/pulmonary trunk backflows, closing semilunar valves.
• All chambers relaxed:
  • AV valves open, ventricles fill passively (~70%).
• All valves closed:
  • No volume change; blood passively fills atria due to the pressure created while the ventricles are relaxed.
• Isovolumetric contraction: fibers contract, valves closed, ventricular volume unchanged; builds tension.
• Ventricular ejection: aortic/pulmonary valves open, blood ejected.
• Isovolumetric relaxation: ventricles relax, valves closed, ventricular volume unchanged.
• Ventricular filling (end-diastolic volume (EDV)): blood in ventricle before systole.

Heartbeat Sounds

• Turbulence causes heart sounds,.
• S1 ("Lubb"): AV valves close.
• S2 ("Dupp"): semilunar valves close.
• S3: ventricular filling sound.
• S4 sound indicate valves problems: atrial gallop.
• Abnormal sounds (murmurs) indicate valve problems.
• Valve problems can cause leaks (regurgitation) or narrowing (stenosis), leading to turbulent flow.

Cells of the Heart

• Heart cells include smooth muscle, endothelial, epicardium, fibroblasts, cardiomyocytes, pacemaker, and Purkinje fibers.
• Endothelial cells line the inner surface and vessels.
• Fibroblasts produce and maintain the extracellular matrix (ECM).
• Cardiomyocytes are responsible for contraction.
• Pacemaker cells generate electrical impulses.
• Purkinje fibers rapidly conduct electrical impulses

The Electrical Conduction System

• The sinoatrial (SA) node (pacemaker) generates impulses, spreading through atria, allowing the heart to beat on its own.
• Impulses spread to the left atrium.
• Impulses reach the atriorenticular AV node, which delays the impulse.
• Delayed impulses travel through the Bundle of His and Purkinje fibers.
• Other conduction tissue takes over as pacemaker if SA node fails, based on depolarization speed.

Cardiac Action Potentials

• Muscles of atria/ventricles (contractile cells) exhibit a plateau phase driven by open voltage gated Ca2+ channels.
• Automaticity is also due to the cell start leaking Na+ back into the cell
• SA node (conducting cells) shows spontaneous depolarization due to unstable resting membrane potential caused by leaky sodium channels.

Electrocardiogram (ECG/EKG)

• An electrocardiogram records the heart's electrical activity via electrodes on the body surface.
• A normal ECG shows P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization).
• A deflection is called a wave if it passes by the baseline.

Electrical Vectors

• Electrical vectors represent the average direction of electrical impulse as it travels through heart muscle.
• Right atrium first, depolarization spreads to the left atrium.
• Vectors from ventricular free walls generate the QRS complex.

Heart Rate Vocabulary

• Heart rate: beats per minute; normal resting is 60-100.
• Heart rhythm: the electrical activity that drives function.
• Fibrillation: uncoordinated contraction.
• Sinus tachycardia: >100 bpm.
• Sinus bradycardia: <60 bpm.
• Sinus arrhythmia: unequal intervals between heartbeats.

ANS Regulation

• The Medulla oblongata is involved.
• Sympathetic innervation (cardio-acceleratory center) uses adrenergic receptors.
• Parasympathetic innervation (cardioinhibitory center) uses muscarinic receptors via the vagus nerve.
• Summary of the effects are stated as increased/positive, or decreased/negative

The Aorta/Vena Cavae

• Arteries pump through the upper limbs and head/brain.
• The vena cavae pumps from the lower limbs and organ systems.

Blood Flow Through the Heart

• Deoxygenated blood returns to the heart through the superior and inferior vena cava into the right atrium.
• It passes through the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve into the pulmonary arteries toward the lungs.
• Oxygenated blood returns from the lungs through the pulmonary veins into the left atrium.
• It passes through the mitral valve into the left ventricle, which pumps it through the aortic valve into the aorta to the rest of the body.

Pressure Gradients

• Pressure gradients propel blood, created mainly by heart contraction/relaxation.
• ΔP = P1 - P2

Stroke Volume

• It is affected by preload.
• A greater volume is produced when the heart is in diastole
• It influences heart rate and blood pressure.

Factors Influencing Stroke Volume

• Stroke volume can be affected by factors causing increased heart rate and blood pressure.
• Factors include preload and the Frank Starling Law.
• Increased contractility/strength of myocardial contraction will impact stroke volume.

Ejection Fraction

• The fraction of blood pumped out of the ventricles with every contraction indicates cardiac efficiency, normalized for heart size.
• It can indicate subtypes of HE.

Cardiac Output

• Cardiac output is a measure of the heart's effectiveness as a pump.
• SV x HR= cardiac output
• Changes to meet the needs of tissues.
• Factors include increases in returning venous pressure due to SNS, heart rate due to parasympathetic innervation, pre load due to ateriolar elasticity and increased stroke volume
• SNS increases venous return, contractility, and HR.

Arteries, Capillaries, and Veins

• Veins thinner, less muscular vessels carrying blood to heart; capacitance vessels; contain valves.
• Arteries are strong, elastic vessels that carry blood away; smaller arteries (arterioles) connect to capillaries.
• Capillaries are permit exchange of material (oxygen, nutrients) between blood and tissues.

Tunics of Blood Vessels

• Tunica intima contains a layer of endothelium.
• Tunica media contains smooth muscle.
• Tunica externa connects tissue.

Capillary Beds

• They are supplied by a single metarteriole.
• There is a precapillary sphincter in the structure.

Precapillary Sphincter in Autoregulation

• Blood flow can be diverted through certain pathways, depending on whether sphincters are open or closed.

Starling Forces

• Filtration/reabsorption is caused by certain pressures.

Types of Capillaries

• Continuous:
  • Small intercellular clefts that wrap around.
• Fenestrated:
  • Small holes through which fluid passes
• Sinusoidal;
  • large gaps

Factors Aiding in Venous Return

• Blood returning to the heart from the veins; also called one-way flow.
• This can be caused by muscle contractions.

Variations in Circulatory Pathways

• Pathways include venous and aterial anastomeses, the simplest pathway, and portal system.

Fetal Circulation

• Oxygenated umbilical vein serous blood directly to the venous system in the inferior vena cava.

Blood Pressure

• Blood pressure is influenced by certain factors, and can increase with exercise and high heart rate.

Ohm's Law of Hemodynamics

• Blood flow is proportional between pressure and resistance.

Factors Affecting Vascular Resistance

• Factors such as viscosity an radius influence resistance.

Factors Affecting Peripheral Resistance

• Factors such as humoral, neural, and others influence resistance.

Pressure Changes

• Higher preasure must be maintained to reduce damage to vessel walls.

Pulse/Mean Arterial Pressure

• Pulse pressure must be maintained with adequate flow.
• Represents average pressure throughout a system during single cardiac cycle.

Arterial Sense Receptors

• Includes a list of the bodies with some action to the blood stream.

The Baroreceptor Reflex

• Homeostatic negative feedback response to changes in vessel pressure.

Composition

• Body tissues, blood cells, and blood composition may change as the person grows, exercises, or becomes ill.

Composition of Plasma

• There are many variables of contents involved in plasma's function.

Erythrocytes

• They are an essential element of body function.

Breakdown of Hemoglobin

• There are 3 basic steps in the function.

Hemopoietic Stem Cell

• This is different in different organisms.

Erythrocytes

• Formed from myeloid stem cells.

Leukopoiesis

• Derived from lymphoid stem cells.

Colony-Stimulating Factors

• Proteins that stimulate growth of the myeloid stem cells

Types of Leukocytes

• There are innate and adaptive immune stem cells.

Granulocytes

• Act as defensive force in specialized and inflammatory conditions.

Agranulocytes

• Monocytes and certain types of T-Cells, etc.

Platelets

• Very specific to the cell's function.

Hemostasis

• This is the process of bleeding at the source of damage.

Primary Hemostasis

• This results from the formation of a platelet plug

Secondary Hemostasis

• Stabilization of the plug in the form of more stable material like clotting and platelets.

Clot Retraction

• More specifically the constritcion of platelets and their organization post clot.

Coagulation Cascade

• Process of breaking down more cells

Groups of cells that determine vessel structure.

Rh group classification

Describes factors the result of the structure.

Explore the heart's anatomy, including its position in the mediastinum and the structure of its chambers. Learn about the roles of atria, ventricles, and septa. Understand the function of heart valves like the tricuspid, mitral, pulmonary, and aortic valves in maintaining unidirectional blood flow.