Circulatory System & Thoracic Cavity

• The circulatory system consists of the blood and the lymphatic systems.
• The cardiovascular system includes the heart, arteries, capillaries, and veins.
• The lymphatic vascular system comprises lymphatic capillaries that start the system and return fluid to the blood.

Thoracic Cavity

• The chest, or thoracic cavity, is bounded by the chest wall and the diaphragm
• The thoracic cavity extends upward into the root of the neck, approximately one fingerbreadth above the clavicle on each side.
• The chest cavity can be divided into the mediastinum (a median partition) and the pleurae and lungs (laterally placed).
• The mediastinum is a movable median partition that extends from the thoracic outlet and root of the neck superiorly to the diaphragm inferiorly.
• The mediastinum extends anteriorly to the sternum and posteriorly to the vertebral column.
• The diaphragm is a thin muscle that separates the chest from the abdominal viscera, together with the pleura and peritoneum.

Mediastinum

• The mediastinum is a narrow, vertically oriented structure residing between the medial parietal pleural layers of the lungs.
• It extends from the sternum to the vertebrae and from the superior thoracic aperture to the diaphragm.
• The mediastinum contains thymus gland, pericardial sac, heart, trachea, and major arteries and veins.

Divisions of the Mediastinum

• The mediastinum has superior and inferior divisions.
• The inferior mediastinum is further divided into anterior, middle, and posterior sections.
• The anterior mediastinum is anterior to the pericardial sac and posterior to the sternum.
• The middle mediastinum contains the pericardial sac and its contents.
• The posterior mediastinum is posterior to the pericardial sac and diaphragm and anterior to the vertebrae.

Blood Vascular System

• The blood vascular system distributes nutrients, gases, and hormones to all parts of the body.
• It collects wastes produced during cellular metabolism.
• The blood vascular system consists of arteries, arterioles, capillaries, venules, veins, and the muscular pump, the heart.

Lymphatic Vascular System

• Lymphatic capillaries are the starting point and are blind-ended tubules that anastomose to form vessels of increasing size
• These capillaries terminate in the blood vascular system and empty into large veins near the heart.
• The lymphatic vascular system returns fluid to the blood from tissue spaces, and this fluid contributes to the lymph’s liquid part.
• Lymphocytes and immunologic factors contribute to this fluid in lymphoid organs.

Blood Vessel Wall Layers

• The blood vessel/vessel walls contain three major layers/tunics
• The tunica intima is one layer of the vessel wall
• The tunica intima contains: endothelium, the subendothelial layer, and the internal elastic lamina (in arteries and arterioles).
• The tunica media is the second layer of the vessel wall and contains the external elastic lamina (arteries).
• The tunica adventitia, is also known as the tunica externa is the third layer of the vessel wall.
• The lumen is the interior space within the blood vessel.

Heart Anatomy

• On the superior surface of the diaphragm the heart is located, left of the midline, anterior to the vertebral column, and posterior to the sternum.

Coverings of the Heart-Anatomy

• The pericardium is a double-walled sac around the heart
• The pericardium is composed of a superficial fibrous pericardium (outer) and a deep two-layered pericardium (inner).
• The parietal layer lines the fibrous pericardium's internal surface, while the visceral layer or epicardium lines the heart's surface.
• These layers are separated by the fluid-filled pericardial cavity.
• The pericardium protects and anchors the heart.
• The pericardium prevents overfilling of the heart with blood.
• The pericardium allows the heart to work in a relatively friction-free environment.

Heart Wall

• The epicardium is fused with the visceral layer of the serous pericardium.
• The myocardium is a cardiac muscle layer.
• The myocardium forms the bulk of the heart and is the middle/thick portion.
• The endocardium is the endothelial lining of the inner myocardial surface.
• The endocardium makes direct contact with blood in the heart chambers.

Heart Structure

• The heart is divided by vertical septa into four chambers - the right and left atria and the right and left ventricles.

• The atrial portion of the heart has relatively thin walls.

• The atrial (interatrial) septum divides the atria into right and left sides.

• The septum runs backward and to the right from the anterior wall.

• The ventricular portion of the heart has thick walls.

• The ventricular septum (interventricular) divides the heart into right and left ventricles.

• The septum is placed obliquely, with one surface facing forward and to the right and the other facing backward and to the left.

• The septum's position is indicated by anterior and posterior interventricular grooves on the heart's surface.

• The lower part of the interventricular septum is thick and muscular.

• The smaller upper part of the septum is thin and membranous and attached to the fibrous skeleton of the heart.

• The superior vena cava and inferior vena cava are categorized as veins returning blood to the heart.

• The right and left pulmonary veins are categorized as veins returning blood to the heart.

• The pulmonary trunk, which splits into the right and left pulmonary arteries vessels categorized as arteries carrying blood away from the heart.

• The ascending aorta vessels are categorized as arteries carrying blood away from the heart.

Atria and Ventricles

• The atria are the receiving chambers of the heart, and each has a protruding auricle.
• Pectinate muscles mark atrial walls.
• Blood enters the right atrium from the superior and inferior venae cavae and the coronary sinus.
• Blood enters the left atrium from the pulmonary veins.
• Ventricles are the discharging chambers and papillary muscles and trabeculae carneae muscles mark ventricular walls.
• The right ventricle pumps blood into the pulmonary trunk.
• The left ventricle pumps blood into the aorta.

Cardiovascular System Function

• A major function of the cardiovascular system maintains hydrostatic pressure to transport nutrients, gases, and waste products.
• Another function is to aid in homeostasis, fluid balance, body temperature, and protection from infection and blood loss.
• The cardiovascular systems of humans are closed so that blood never leaves the network of blood vessels.
• Oxygen and nutrients diffuse and enter interstitial fluid, with the fluid carrying oxygen and nutrients to target cells.
• Carbon Dioxide and waste move in the opposite direction of the cardiovascular system.
• A closed circuit can be divided into systemic and pulmonary circulation.
• Systemic circulation has low resistance and carries oxygenated blood from the heart to tissues and deoxygenated blood from the tissues back to the heart.
• Pulmonary circulation has high resistance, carrying deoxygenated blood from the heart to the lungs and then oxygenated blood from the lungs back to the heart.

Blood Flow Through The Heart

• Blood flows as follows
  • Right atrium > tricuspid valve > right ventricle
  • Right ventricle > pulmonary semilunar valve > pulmonary arteries > lungs
  • Lungs > pulmonary veins > left atrium
  • Left atrium> bicuspid valve > left ventricle
  • Left ventricle > aortic semilunar valve > aorta.
  • Aorta > systemic circulation

Heart Valves

• Heart valves ensure unidirectional blood flow through the heart.
• Atrioventricular (AV) valves lie between the atria and the ventricles
• AV valves prevent backflow into the atria when the ventricles contract.
• The chordae tendineae anchor the AV valves to the papillary muscles.
• The right AV valve is known as the tricuspid valve, and the left AV valve, the bicuspid valve, is also called the mitral valve.
• The aortic semilunar valve lies between the left ventricle and the aorta.
• The pulmonary semilunar valve lies between the right ventricle and the pulmonary trunk.
• Semilunar valves prevent backflow of blood into the ventricles
• AV valves open and semilunar valves close during relaxation.
• AV valves close and semilunar valves open during contraction.

How the Heart Contracts

• the Heart generating an electrical signal causes it to contract.
• The autonomic nervous stimulate, but does not initiate, heart beats..
• Myocardial tissue's myogenic activity is able to make electrical signals.
• The right atrium contains sinoatrial nodes, which can cause the heart to contract.
• Sinatrial node cells can create action potential cause the atrium to depolarise and therefore contract.

Electrical Activity & Intrinsic Conduction System.

• Aurorhythmic cells initiate action potentials and have unstable resting potentials called pacemaker potentials.

Sequence of Excitation & Heart Physiology

• The sinoatrial (SA) node generates impulses about 75 times per minute, setting the pace for the heart.
• The atrioventricular (AV) node delays the impulse by approximately 0.12 seconds.
• The impulse passes from the atria to the ventricles via the atrioventricular bundle (bundle of His).
• The AV bundle splits into two pathways in the interventricular septum (bundle branches)
• Bundle branches carry the impulse toward the apex of the heart, and Purkinje fibers carry the impulse to the heart apex and into the ventricular walls.

Electrocardiography

• Electrical activity is recorded by electrocardiography (ECG) using electrodes.
• The P wave corresponds to depolarization of the SA node.
• The QRS complex corresponds to ventricular depolarization.
• The T wave corresponds to ventricular repolarization.
• Atrial repolarization record is masked by the larger QRS complex.

ECG Interpretations

• When interpreting ECGs, consider the following:
• Heart rate (faster is tachycardia, slower is bradycardia)
• Rhythm (regular or irregular)
• Presence of normal waves
• If there is one QRS complex for each P wave and the P-R segment has constant length
• Assess for pathologies like cardiac arrhythmias.

Heart Sounds

• Common heart sounds of lub-dup are associated with the closing of valves.
• The first heart sound occurs as atrioventricular (AV) valves close, signifying the beginning of Systole.
• The second heart sound occurs when semilunar (SL) valves close at the beginning of ventricular diastole.

Heart Murmurs

• A heart murmur is a whooshing, blowing, or raspy sound heard during a heartbeat.
• This sound often occurs when there is rough or turbulent blood flow through the heart valves or near the heart.
• Murmurs can happen when a valve doesn't close properly or blood flows through a hardened valve.
• Many heart murmurs are harmless and called innocent murmurs. They will not cause any issues. Other heart murmurs may indicate an abnormality in the heart.

Arterial Supply

• The arterial supply comes mainly from the pericardiophrenic artery.
• A smaller contribution is supplied by the musclophrenic artery, bronchial artery, esophageal, superior phrenic artery, and coronary artery.

Venous Drainage

• Venous drainage drains through the pericardiophrenic veins.
• The Azygos is a part of the venous system

Blood Supply

• Brachiocephalic (Brs) of the internal thoracic (Int Th) artery, musculophrenic artery, and descending thoracic aorta

Nerve Supply

• The frontal plane & posterior plane (FP & PP) & are supplied by the Phrenic nerve, causing it to be pain sensitive.
• The Vertical plane (VP) id supplied by the autonomic nervous system (ANS): Cardiac plexus/ pain insensitive

Applied

• Pericarditis is inflammation of the pericardium.
• Pericardial effusion is excess fluid within the pericardial sac, which may potentially lead to cardiac tamponade because the liquid will compress the heart.

An overview of the circulatory system including the cardiovascular and lymphatic systems. The thoracic cavity and the mediastinum partition are discussed. Focus is given to the chest wall, diaphragm and the lungs.