Human Anatomy: The Heart Structure

Layers of the Heart Wall

• The heart wall is made of three layers:
  • Epicardium (outer)
  • Myocardium (middle)
  • Endocardium (inner)

Heart Chambers

• The heart has four chambers:
  • Two atria (superior)
  • Two ventricles (inferior)

Right Atrium

• Receives deoxygenated blood from:
  • Superior vena cava (from upper body)
  • Inferior vena cava (from lower body)
• Pumps blood to the right ventricle

Right Ventricle

• Receives deoxygenated blood from the right atrium
• Pumps deoxygenated blood into the pulmonary trunk
  • Pulmonary trunk branches into the left and right pulmonary arteries

Left Atrium

• Receives oxygenated blood from:
  • Pulmonary veins (from lungs)
• Pumps oxygenated blood to the left ventricle

Left Ventricle

• Receives oxygenated blood from the left atrium
• Pumps oxygenated blood to the aorta, which distributes it to the body.

Left vs Right Ventricle

• Both ventricles eject equal blood volumes.
• The right ventricle pumps blood a short distance to the lungs at lower pressure.
• The left ventricle pumps blood a longer distance to the body at a higher pressure.

Systemic vs Pulmonary Circulation

• Pulmonary circulation
  • Moves blood between the heart and lungs
  • Right side of the heart - leaves via pulmonary arteries
• Systemic circulation
  • Moves blood between the heart and the body
  • Left side of the heart - leaves via the aorta

Electrical Conduction System of the Heart

• Enables coordinated heart contractions
• Components:
  • Sinoatrial (SA) node (pacemaker)
  • Atrioventricular (AV) node
  • Bundle of His
  • Purkinje fibers
• SA node initiates electrical impulses
  • AV node delays impulse
  • Bundle of His transmits impulse to ventricles
  • Purkinje fibers distribute impulse throughout ventricles

Electrocardiogram (ECG)

• A recording of the electrical activity of the heart
• Components:
  • P wave: atrial depolarization
  • QRS complex: ventricular depolarization
  • T wave: ventricular repolarization

Cardiac Cycle

• Complete sequence of events in one heartbeat
• Divided into two phases:
  • Diastole: Relaxation phase - ventricles fill with blood
  • Systole: Contraction phase - blood is ejected from ventricles

Cardiac Output (CO)

• Volume of blood ejected from the left ventricle per minute.
• Calculated by: CO = Heart Rate (beats/min) x Stroke Volume (mL/beat)
• Factors that influence cardiac output:
  • Preload: Amount of stretch of the ventricle before contraction
  • Afterload: Resistance the ventricle must overcome to eject blood
  • Contractility: Force of contraction of the ventricle
  • Autonomic nervous system: Sympathetic (increases HR & contractility) and parasympathetic (decreases HR)
  • Hormonal factors: Epinephrine, norepinephrine, thyroid hormones

Functions of Blood

• Transport:
  • O2 and CO2
  • Nutrients
  • Hormones
• Regulation:
  • pH
  • Temperature
• Protection:
  • Clotting
  • Antibodies and white blood cells

Components of Blood

• Plasma (55%)
  • Suspension medium for formed elements
• Formed elements (45%)
  • Red blood cells (erythrocytes) (99%)
  • White blood cells (leukocytes)
  • Platelets

Red Blood Cells (RBC)

• Transport O2 and CO2
• Biconcave disk shape:
  • Increases surface area
  • Promotes cell flexibility
• Lack a nucleus and other organelles to maximize oxygen carrying capacity.
• Contain hemoglobin: Oxygen-carrying protein

White Blood Cells (WBC)

• Function in immune defense: Combat invading microbes.
• Contain a nucleus and organelles
• Lack hemoglobin
• Increased levels may indicate an infection

Haemostasis

• Sequence of responses that stop bleeding
• Processes involved:
  • Vascular spasm: Smooth muscle in vessel walls contracts to reduce blood flow.
  • Platelet plug formation: Platelets adhere to exposed collagen and aggregate to form a plug.
  • Coagulation: Blood clotting to form a fibrin mesh to reinforce the platelet plug

Blood Pressure

• The pressure exerted by the blood on the walls of a blood vessel
• Generated by ventricular contraction
• Factors that influence blood pressure:
  • Blood volume
  • Cardiac output
  • Vascular resistance

Vascular Resistance

• Opposition to blood flow due to friction between blood and the walls of blood vessels.
• Factors affecting vascular resistance:
  • Size of the lumen
  • Blood viscosity
  • Total vessel length

Test your knowledge on the anatomy of the heart in this quiz. Explore the various layers of the heart wall, the four chambers, and the flow of blood through the right and left atria and ventricles. Understand the crucial differences between the left and right ventricles and their functions.