Cardiovascular System

Heart

• The heart is a hollow muscular organ with four chambers.
• It is located in the mediastinum, behind the sternum, mostly left of the midline.
• The heart is protected by the parietal pericardium, or pericardial sac, that isolates the heart from other thoracic organs, containing pericardial fluid.
• The heart wall has three distinct layers:
  • Epicardium: outer layer
  • Myocardium: thick, middle layer made up of cardiac muscle
  • Endocardium: smooth, inner layer lining the heart and its internal structures
• Cardiac muscle cells are connected by intercalated discs that consist of fascia adherens, gap junctions, and desmosomes, allowing coordinated contractions.
• The heart has four chambers:
  • Right atrium: receives blood from the superior and inferior vena cava.
  • Right ventricle: pumps blood to the lungs through the pulmonary valve.
  • Left atrium: receives oxygenated blood from the lungs through the pulmonary veins.
  • Left ventricle: pumps oxygenated blood to the body through the aortic valve.
• The heart beats around 100,000 times a day, acting as two separate pumps:
  • Right heart: pumps deoxygenated blood to the lungs.
  • Left heart: pumps oxygenated blood to the body.

Conduction System of the Heart

• The heart’s natural pacemaker is the sinoatrial (SA) node.
• The SA node spontaneously depolarizes, generating the pacemaker potential, initiating each heartbeat.
• The wave of depolarization spreads across the atria, causing atrial systole. It converges at the atrioventricular (AV) node.
• The AV node delays the impulse, allowing time for the atria to fully empty into the ventricles.
• The Bundle of His conducts the impulse to the Purkinje fibers of the ventricles.

Electrocardiogram (ECG)

• ECG measures the electrical activity generated by heart contractions.
• It records a trace consisting of the P wave, QRS complex, and T wave.
• ECG can detect abnormalities in heart rate, rhythm, and structure, such as arrhythmias, enlarged heart due to hypertension, and myocardial infarction.

Circulatory system

• The circulatory system, also known as the cardiovascular system, circulates blood, transporting nutrients, oxygen, hormones, and removing waste products.
• It plays a crucial role in maintaining homeostasis.
• The system includes the pulmonary and systemic circulations:
  • Pulmonary circulation: transports blood from the right ventricle to the lungs and then to the left atrium.
  • Systemic circulation: comprises all blood vessels outside the pulmonary system.

Coronary Circulation

• The heart has its own circulation system known as the coronary circulation.
• The right and left coronary arteries deliver oxygenated blood to the heart.
• Blood from the myocardium capillaries enters the cardiac veins and then flows into the right atrium.

Fetal Circulation

• During fetal development, the pulmonary circulation is inactive because the fetus receives oxygen from the placenta.
• The umbilical cord connects the fetus to the placenta.
• Blood from the right atrium is diverted through the foramen ovale to the left atrium, bypassing the pulmonary circulation.
• Blood from the left ventricle flows through the ductus arteriosus, directly to the aorta.

Blood Vessels

• Blood vessels transport blood throughout the body:
  • Arteries: Carry oxygenated blood away from the heart (except for the pulmonary artery). Arteries have thick, elastic, muscular walls, and blood flow under high pressure. They are composed of three layers: tunica externa, tunica media, and tunica intima.
  • Arterioles: Smaller branches from arteries that regulate blood flow into capillaries.
  • Capillaries: Smallest and most abundant blood vessels, facilitating rapid exchange of oxygen, nutrients, and waste between blood and tissues. There are three types: continuous, fenestrated, and discontinuous (sinusoidal).
  • Venules: Small veins that collect blood from capillaries.
  • Veins: Carry deoxygenated blood back to the heart. They have valves to prevent backflow and act as a reservoir of blood. Veins also have three layers, but are less muscular and elastic than arteries, relying on the skeletal muscle pump and respiratory pump for blood flow.

Blood-Brain Barrier

• The blood-brain barrier acts as a boundary between circulating blood and the brain and spinal cord.
• It is highly selective, protecting the brain from foreign substances and maintaining a constant environment for the brain.
• Brain capillaries are not fenestrated and have tightly packed endothelial cells, preventing large molecules, immune cells, bacteria, and viruses from entering the central nervous system.

Major Blood Vessels – The Great Vessels

• Arteries:
  • Pulmonary artery: Carries deoxygenated blood from the right ventricle to the lungs for oxygenation.
  • Aorta: The largest artery in the body; originates from the left ventricle as the ascending aorta, forms the aortic arch, and continues down the body as the descending aorta.
• Veins:
  • Pulmonary vein: Returns oxygenated blood from the lungs to the left atrium.
  • Superior vena cava: Receives deoxygenated blood from the upper body.
  • Inferior vena cava: Receives deoxygenated blood from the lower body.