Heart Anatomy and Blood Flow: Key Structures and Electrical Conduction

**Superior Vena Cava

Vein returning deoxygenated blood from upper body to right atrium**

**Inferior Vena Cava

Vein returning deoxygenated blood from lower body to right atrium**

**Right Atrium

Chamber receiving deoxygenated blood from vena cavae**

**Tricuspid Valve

One‑way valve between right atrium and right ventricle**

**Right Ventricle

Chamber pumping deoxygenated blood to lungs**

**Pulmonary Valve

Valve controlling blood flow from right ventricle into pulmonary artery**

**Pulmonary Artery

Only artery carrying deoxygenated blood; transports blood to lungs**

**Pulmonary Veins

Veins carrying oxygenated blood from lungs to left atrium**

**Left Atrium

Chamber receiving oxygenated blood from pulmonary veins**

**Mitral (Bicuspid) Valve

Valve between left atrium and left ventricle**

**Left Ventricle

Thickest chamber; pumps oxygenated blood to entire body**

**Aortic Valve

Valve controlling blood flow from left ventricle into aorta**

**Aorta

Largest artery; distributes oxygenated blood to body**

**Coronary Arteries

Vessels supplying oxygenated blood to heart muscle**

**Blood Flow Step 1

Deoxygenated blood enters right atrium from superior and inferior vena cava**

**Blood Flow Step 2

Blood passes through tricuspid valve into right ventricle**

**Blood Flow Step 3

Right ventricle pumps blood through pulmonary valve into pulmonary artery**

**Blood Flow Step 4

Blood travels to lungs for gas exchange**

**Blood Flow Step 5

Oxygenated blood returns via pulmonary veins to left atrium**

**Blood Flow Step 6

Blood passes through mitral valve into left ventricle**

**Blood Flow Step 7

Left ventricle pumps blood through aortic valve into aorta**

**Blood Flow Step 8

Oxygenated blood is delivered to body tissues**

**SA Node (Sinoatrial Node)

Natural pacemaker located in right atrium; initiates heartbeat**

**Atrial Pathways

Spread impulse across atria causing atrial contraction**

**AV Node (Atrioventricular Node)

Delays impulse to allow ventricles to fill**

**Bundle of His (AV Bundle)

Conducts impulse from AV node into interventricular septum**

**Right and Left Bundle Branches

Carry impulse down septum toward apex**

**Purkinje Fibers

Spread impulse through ventricles causing contraction from bottom up**

**Conduction Sequence

SA Node → Atria → AV Node → Bundle of His → Bundle Branches → Purkinje Fibers**

**EKG (ECG)

Recording of electrical activity of the heart**

**P Wave

Atrial depolarization and atrial contraction**

**PR Segment

AV node delay**

**QRS Complex

Ventricular depolarization and ventricular contraction; atrial repolarization hidden**

**T Wave

Ventricular repolarization and relaxation**

**Endocardium

Smooth inner lining of heart chambers and valves; continuous with blood vessels**

**Myocardium

Thick middle layer of cardiac muscle responsible for pumping action**

**Epicardium

Outermost layer; also visceral layer of serous pericardium; contains vessels and nerves**

**Tricuspid Valve

Prevents backflow from right ventricle to right atrium**

**Pulmonary Valve

Prevents backflow from pulmonary artery to right ventricle**

**Mitral (Bicuspid) Valve

Prevents backflow from left ventricle to left atrium**

**Aortic Valve

Prevents backflow from aorta to left ventricle**

**Cardiac Cycle

Complete sequence of events in one heartbeat including atrial and ventricular systole and diastole**

**Ventricular Filling

Ventricles fill during diastole; AV valves open; semilunar valves closed**

**Atrial Systole

Atria contract to push final blood into ventricles**

**Isovolumetric Contraction

Ventricles contract with all valves closed; pressure rises but no blood moves**

**Ventricular Ejection

Semilunar valves open; blood pumped into aorta and pulmonary artery**

**Isovolumetric Relaxation

Ventricles relax with all valves closed; pressure falls**

**Passive Filling

AV valves reopen and ventricles fill passively before atrial systole**

**Neutrophils

Most abundant WBC; multilobed nucleus; first responders to bacterial infection**

**Lymphocytes

Large nucleus; responsible for immune responses (B cells and T cells)**

**Monocytes

Largest WBC; become macrophages that phagocytose pathogens and debris**

**Eosinophils

Bilobed nucleus; attack parasites and modulate allergic reactions**

**Basophils

Rarest WBC; release histamine and heparin during inflammation**

**Plasma

Liquid portion of blood containing water, proteins, nutrients, hormones, and waste**

**Formed Elements

Cellular components including RBCs, WBCs, and platelets**

**Red Blood Cells (Erythrocytes)

Carry oxygen using hemoglobin; no nucleus**

**White Blood Cells (Leukocytes)

Immune cells that defend against pathogens**

**Platelets (Thrombocytes)

Cell fragments essential for blood clotting**

**Antigen

Surface protein on RBCs that determines blood type**

**Antibody

Protein in plasma that attacks foreign antigens**

**Type A Blood

A antigens; anti‑B antibodies**

**Type B Blood

B antigens; anti‑A antibodies**

**Type AB Blood

A and B antigens; no antibodies; universal recipient**

**Type O Blood

No antigens; anti‑A and anti‑B antibodies; universal donor**

**Rh Factor

Determines positive or negative blood type; Rh+ has antigen, Rh- does not**

**Safe Transfusion Rule

Donor antigens must not react with recipient antibodies**

**Hemostasis

Process that stops bleeding**

**Vascular Spasm

Immediate vasoconstriction to reduce blood loss**

**Platelet Plug Formation

Platelets stick to damaged vessel and to each other**

**Coagulation

Fibrin mesh forms to stabilize the clot**

**RBC Lifespan

About 120 days before removal by spleen and liver**

**Hemoglobin Recycling

Iron reused; heme → bilirubin; globin → amino acids**

**Erythropoiesis

Production of RBCs in red bone marrow**

**Erythropoietin (EPO)

Kidney hormone that stimulates RBC production when oxygen is low**

**Reticulocyte

Immature RBC released into blood before maturing**

**Pectinate Muscles

Ridges in atria that increase contraction efficiency**

**Papillary Muscles

Ventricular muscles that anchor chordae tendineae and prevent valve prolapse**

**Trabeculae Carneae

Irregular ridges in ventricles that prevent suction and aid contraction**