Cardiovascular system

Heart Overview

• The heart is a hollow, four-chambered, muscular organ.
• It acts as two pumps, the right side pumps blood to the lungs and the left side pumps blood to the body.

Heart Location and Structure

• The heart is located in the mediastinum, a region in the thoracic cavity between the lungs.
• The heart sits behind the sternum and mostly lies left of the midline.
• The heart is enclosed by the parietal pericardium, which is a fibrous sac that separates the heart from other thoracic organs and forms the wall of the pericardial cavity.
• The pericardial cavity contains pericardial fluid.
• The heart wall is composed of three layers:
  • Epicardium (outer layer)
  • Myocardium (thick middle layer made of cardiac muscle)
  • Endocardium (smooth inner layer lining the heart and its internal structures)

Cardiac Muscle

• Cardiac muscle is only found in animal hearts.
• It is controlled by the autonomic nervous system.
• Cardiac muscle fibers are branched and arranged in a striated pattern.
• Cardiac muscle cells are connected by intercalated discs that contain fascia adherens, gap junctions, and desmosomes.

Heart Chambers

• The heart has four chambers:
  • Two upper chambers: the right and left atria, receiving chambers that contract and empty into the ventricles.
  • The atria are separated by a thin muscular interatrial septum.
  • Two lower chambers: the right and left ventricles, pumping chambers separated by a thick muscular interventricular septum.
• Grooves on the heart surface indicate the partitions between the chambers and also contain cardiac vessels that supply blood to the heart wall.

Right Atrium and Ventricle

• The right atrium receives venous blood from the superior and inferior vena cava.
• The cardiac cycle consists of diastole (relaxation when the heart fills with blood) and systole (contraction).
• During diastole, blood passes from the right atrium to the right ventricle through the tricuspid valve.
• During systole, the tricuspid valve closes to prevent backflow into the atrium.

Left Atrium and Ventricle

• Oxygenated blood is passed from the lungs to the left atrium via pulmonary veins.
• During diastole, blood passes from the left atrium to the left ventricle through the mitral valve.
• During systole, the mitral valve closes to prevent backflow of blood into the atrium.
• The left ventricle has thicker walls than the right ventricle.
• Oxygenated blood leaves the left ventricle via the aorta. The aortic semilunar valve prevents backflow from the aorta into the left ventricle.

Heart Sounds

• Sounds heard with a stethoscope are created by the turbulence caused by the closing of heart valves.
• S1 (Lub) is the first heart sound
• S2 (Dub) is the second heart sound.
• Variations in heart sounds can indicate valve problems, referred to as heart murmurs.

Conduction System of the Heart

• The heart’s conduction system consists of:
  • Sinoatrial (SA) node (pacemaker)
  • Atrioventricular (AV) node
  • Bundle of His
  • Purkinje fibers.

Conduction System Functions

• Each heartbeat begins in the SA node, located in the right atrium.
• Spontaneous depolarization in the SA node (pacemaker potential) initiates the heartbeat.
• Depolarization spreads across the atria, causing atrial systole, and converges at the AV node.
• The AV node delays impulses to allow the atria to empty into the ventricles.
• The Bundle of His conducts the impulses to the Purkinje fibers in the ventricles, leading to ventricular systole.

Electrocardiogram (ECG)

• An ECG records the electrical activity of the heart.
• It provides information about heart rate and rhythm.
• ECG traces consist of:
  • P wave
  • QRS complex
  • T wave
• ECG can help detect arrhythmias, an enlarged heart, heart attacks, and other conditions.

The Circulatory System Overview

• The circulatory system (cardiovascular system) circulates blood, transports nutrients, oxygen, and hormones, removes waste products, and regulates homeostasis.

Pulmonary Circulation

• Pulmonary circulation transports blood from the right ventricle to the lungs for gas exchange, and then to the left atrium of the heart.

Systemic Circulation

• Systemic circulation includes all vessels outside of the pulmonary system.

Coronary Circulation

• This system provides oxygenated blood to the heart muscle itself and includes the right and left coronary arteries and cardiac veins.

Fetal Circulation

• Pulmonary circulation is unnecessary in utero because fetal blood is oxygenated by the placenta.
• The umbilical cord connects the fetus to the placenta.
• Venous blood returning to the right atrium is diverted through the foramen ovale to the left atrium.
• Blood from the left ventricle is diverted away from the pulmonary arteries and to the aorta through the ductus arteriosus.

Blood Vessels

• Blood vessels include:
  • Arteries
  • Arterioles
  • Capillaries
  • Venules
  • Veins

Arteries

• Arteries carry oxygenated blood (except in the pulmonary system) away from the heart.
• They have thick, elastic, muscular walls and carry blood under high pressure.
• Arteries consist of three layers:
  • Tunica externa
  • Tunica media
  • Tunica intima.

Veins

• Veins carry deoxygenated blood back to the heart.
• They contain valves to prevent blood backflow.
• They act as a reservoir for blood.
• Veins rely on the skeletal muscle pump and respiratory pump to move blood.
• Veins have three layered walls, but they are less muscular and elastic than arteries.

Capillaries

• The smallest and most abundant blood vessels.
• They have walls composed of endothelium only, one cell layer thick.
• They allow rapid exchange of oxygen, nutrients, and waste products between blood and tissues.
• Capillary beds supply organs.
• Blood flow through capillaries is regulated by precapillary sphincter muscles and the constriction or dilatation of arteries and arterioles
• Capillaries are classified into three types:
  • Continuous
  • Fenestrated
  • Discontinuous (sinusoidal)

Blood-Brain Barrier

• This barrier, located between the blood and the brain and spinal cord, is highly selective. It prevents foreign substances from entering the brain and protects it.
• It maintains a constant environment for the brain.
• Brain capillaries are not fenestrated and their endothelial cells are tightly packed. This prevents large molecules, immune cells, bacteria, and viruses from entering the CNS.

Major Blood Vessels

• Arteries:
  • Pulmonary artery: Carries blood from the right ventricle to the lungs.
  • Aorta:
    • Ascending aorta: Originates from the left ventricle.
    • Aortic arch: Connects to the descending aorta.
    • Descending aorta: Continues down through the body.
• Veins:
  • Pulmonary vein: Returns oxygenated blood from the lungs to the left atrium.
  • Superior vena cava: Receives deoxygenated blood from the upper body and drains into the right atrium.
  • Inferior vena cava: Receives venous return from the lower body and drains into the right atrium.

Blood Pressure

• Blood pressure is the force exerted by circulating blood against the artery walls.
• Measured in millimeters of mercury (mmHg).
• Expressed as a systolic/diastolic reading (e.g., 120/80 mmHg).