Anatomy and Function of the Heart

• The heart and blood vessels transport food molecules, water, and oxygen to cells and remove waste products, such as carbon dioxide, forming the circulatory system.
• The circulatory system comprises the heart, arteries, veins, capillaries, blood, and the lymphatic system.

Cardiovascular System Functions

• Transports oxygen to tissues and removes carbon dioxide.
• Transports nutrients to cells.
• Removes metabolic waste products.
• Regulates body temperature.
• Distributes hormones throughout the body.
• Facilitates immune function by transporting white blood cells and antibodies.
• Initiates the clotting mechanism to prevent excessive blood loss.
• Maintains blood pressure for efficient circulation.
• Ensures tissue perfusion, delivering oxygenated blood where needed.

Heart Structure

• The heart is a muscular, four-chambered organ that circulates blood.
• It is positioned in the thorax within a cavity known as the mediastinum.
• The hearts conical shape lies in a slightly angled position, with the base cranio-dorsally above the apex, slightly to the left of midline, and near the sternum.
• The heart is enclosed within a double-layered membranous sac called the pericardium.
• The inner layer of the pericardium is the visceral pericardium or epicardium, a serous membrane in contact with the heart wall surface.
• Between the two layers of the pericardium (the pericardial cavity) is a serous fluid that lubricates the heart.

Heart Layers

• Epicardium: The outer layer and also the inner layer of the pericardium.
• Myocardium: The middle layer made of muscle.
• Endocardium: The inner layer.
• The middle layer (myocardium) is made of unique cardiac muscle cells that do not fatigue following repeated contractions.
• Cardiac muscle cells can have rhythmic contractions without nerve supply due to the intercalated discs that lie between branching muscle cells, allowing contraction of one cell to stimulate adjacent cells.
• The heart has four chambers and a wall of cardiac muscle known as the septum separates the right and left sides.
• Each side is divided into two chambers: the atrium (upper chamber that receives blood) and the ventricle (lower chamber that pumps blood out).

Left Side of Heart

• The left side pumps blood into the systemic circulation, requiring a thicker muscle wall.
• The left side receives oxygenated blood from the lungs via the pulmonary vein with blood moving from the pulmonary vein into the left atrium.
• There are two sets of valves within the left side of the heart.
• One is a mitral valve which is also known as the bicuspid valve or left atrio-ventricular valve
• The bicuspid valve has two flaps (cusps) attached to the papillary muscles of the ventricular walls by fibres known as Chordae Tendineae.
• The muscles contract, pulling the fibres and opening the valve.

Aortic Valve

• Also known as semi-lunar valves
• Allows blood to move out of the heart and prevents backflow into the ventricle.
• Oxygenated blood moves from the left side of the heart and travels to the rest of the body to supply tissues with oxygen.

Exchanges

• The exchange of gases takes place in the capillaries and waste products, including deoxygenated blood, are returned to the circulation with blood traveling back to the heart and entering through the right side.
• The right side of the heart pumps blood into the pulmonary circulation, and deoxygenated blood enters via the cranial and caudal vena cava into the right atrium.
• There are two sets of valves within in the right side of the right atrium
• Tricuspid valve (or right atrio-ventricular valve) made of 3 flaps (cusps) pulmonary valve (or semi-lunar valve) between the pulmonary artery and the ventricle which prevents backflow.
• Blood moves from the atrium into the ventricle, and contraction of the heart muscle forces blood to leave the ventricle via the pulmonary artery to the lungs.

Blood Vessels

• Arteries carry oxygenated blood away from the heart, except for the pulmonary artery.
• Veins carry deoxygenated blood towards the heart, except for the pulmonary vein.

Heart Diagram Key

• Aorta
• Superior/cranial vena cava
• Right pulmonary artery
• Pulmonary veins
• Right atrium
• Tricuspid valve / atrioventricular valve
• Right ventricle
• Inferior vena cava
• Pulmonary artery
• Pulmonary veins
• Left atrium
• Mitral valve/ bicuspid valve/ bicuspid valve
• Aortic valve/ semi luna valve
• Left ventricle
• Aorta

Additional valves

• Right Atrioventricular/Tricuspid valve
• Left Atrioventricular/Mitral/Bicuspid valve

Heart Stimulation

• The heart contracts through electrical wave stimulation arising spontaneously by depolarization of specialized muscle cells called the Sinoatrial (SA) node in the wall of the right atrium.
• The SA node is known as the pacemaker.
• As the wave reaches the Atrioventricular (AV) node, it travels along structures known as the Bundle of His, spreading down the septum and out along the Purkinje fibres that radiate upwards through both ventricles and the ventricles then contract.

Cardiac rhythm regulators

• The heart rate is stimulated by the autonomic nervous system.
• The Vagus nerve (10th cranial nerve) slows the heart rate and decreases the power of contraction through its effect on the SA node.
• Areas of the brain detect blood composition and pressure to stimulate an increase or decrease in the frequency and force of contraction, output.

The ECG

• An electrocardiogram (ECG) represents the electrical activity of the heart as recorded from the body surface.
• Electrodes are placed at certain points usually with paper and an electrical ink pen.

ECG Waveform

• P wave – Represents atrial depolarisation (contraction of the atria).
• QRS complex – Represents ventricular depolarisation (contraction of the ventricles).
• T wave – Represents ventricular repolarisation (relaxation of the ventricles).
• PR interval – The time between the atria and ventricles.
• QT interval – The time it takes to contract and then relax ventricles.
• Auscultation allows you to hear 2 sounds; the first and second sounds
• “Lub dub” is the sound of closing valves.
• The first sound "lub" is the sound of atrio-ventricular valves closing.
• The 'dub' is the sound of semilunar valves closing.
• The heartbeat or cardiac cycle is described in two phases, a contraction phase known as systole and a relaxation phase known as diastole.
• When contracting coronary arteries must supply the heart with blood, if it is is blocked it causes myocardial infarction (MI), or a heart attack.

Blood Flow through the Heart

• Deoxygenated blood enters the heart through the superior and inferior vena cava into the right atrium.
• The right atrium contracts and pushing the blood through the right atrio-ventricular valve (tricuspid valve) into the right ventricle.
• The right ventricle contracts and pumps the blood through the pulmonary semi-lunar valve into the pulmonary artery.
• The pulmonary artery carries the blood to the lungs where it is oxygenated and returned to the lungs via the pulmonary veins, then enters the left atrium.
• The left atrium contracts and pushed the blood through the left atrio-ventricular valve (bicuspid or mitral valve) into the left ventricle.
• The left ventricle contracts and pumps the oxygenated blood through the aortic semi-lunar valve into the aorta, which distributes the blood to the rest of the body.

The Circulation

• Systemic Circulation: Carries oxygenated blood around the body, and returns deoxygenated blood to the heart
• Pulmonary Circulation: Carries deoxygenated blood from the heart to the lungs, and returns oxygenated blood to the heart, also known as a double circulation.

Arterial Supply

• Oxygenated blood leaves the left ventricle of the heart in the aorta which branches taking blood to various parts of the body.

Main Branch Arteries (Upper Body)

• Coronary arteries: Supplies the heart muscle.
• Common carotid arteries: Supplies the head.
• Right subclavian artery: Supplies the right forelimb.
• Left subclavian artery: Supplies the left forelimb.
• Brachial artery: Follows on from subclavian artery in each forelimb.

Pulmonary Circulation Arteries

• Deoxygenated blood is pumped from the right ventricle to the lungs in the pulmonary artery, then divides into fine capillaries that wrap the alveoli.
• Oxygen in the inspired air diffuses into the blood, CO2 diffuses into the air within the alveoli.
• Newly oxygenated blood is carried to the left atrium by the pulmonary veins, and around the body via the systemic circulation.

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