Heart Anatomy and Position

Heart Anatomy

• The heart is a four-chambered, hollow, muscular organ, about the size of a clenched fist
• It beats approximately 100,000 times per day
• The heart functions as two separate pumps
• The right side pumps blood to the lungs
• The left side pumps blood to the rest of the body

Location and Position

• It is located in the thoracic cavity, specifically in the mediastinum between the lungs
• The heart sits behind the sternum (breastbone)
• Most of the heart is situated to the left of the midline, with its apex pointing downwards
• The base is the broad, superior end where large blood vessels attach

Pericardium Structure

• The heart is enclosed and protected by the parietal pericardium (pericardial sac)
• The parietal pericardium separates it from other thoracic organs
• It forms the wall of the pericardial cavity, which contains pericardial fluid
• The parietal pericardium has two layers: an outer fibrous layer and an inner serous layer
• The serous layer secretes pericardial fluid

Heart Wall Layers

• The heart wall consists of three distinct layers
• Epicardium: The outer layer
• Myocardium: A thick, middle layer composed of cardiac muscle
• Endocardium: The smooth, inner layer lining the heart and its internal structures

Cardiac Muscle Characteristics

• Exists only in animal hearts and is controlled by the autonomic nervous system
• Fibers are branched and arranged in a striated pattern
• Cardiac muscle cells connect through intercalated discs
• Intercalated discs feature fascia adherens, gap junctions, and desmosomes
• This arrangement enables smooth and repeated muscle contractions

Chambers of the Heart

• The heart is divided into four chambers
• The upper chambers are the right and left atria
• The lower chambers are the right and left ventricles
• The atria are receiving chambers that contract to empty blood into the ventricles
• They are separated by the interatrial septum
• The ventricles are pumping chambers, separated by the thick interventricular septum
• Grooved depressions indicate partitions between chambers and contain cardiac vessels

Right Atrium and Ventricle Function

• The right atrium receives venous blood from the superior and inferior vena cava
• The cardiac cycle includes diastole, when the heart fills with blood, and systole, a period of contraction
• During diastole, blood flows 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 Function

• After gas exchange in the lungs, oxygenated blood passes into the left atrium via the pulmonary veins
• During diastole, blood flows from the left atrium to the left ventricle through the bicuspid (mitral) valve
• During systole, the mitral valve closes to prevent backflow as the left ventricle contracts
• The left ventricle has thicker walls compared to the right ventricle
• Oxygenated blood exits the left ventricle through the aorta
• The aortic semilunar valve prevents backflow from the aorta into the left ventricle

Heart Sounds

• When listening to the heart with a stethoscope, the heart valve sounds can be heard
• S1 (Lub) is the first heart sound
• S2 (Dub) is the second heart sound
• Problems with heart valves can be detected through variations in these sounds, known as heart murmurs

Conduction System Components

• Consists of several key components:
• Sinoatrial (SA) node
• Atrioventricular (AV) node
• Bundle of His
• Purkinje fibers

Conduction System Details

• Each heartbeat begins at the SA node, known as the heart's pacemaker
• The SA node is located in the right atrium
• During diastole, the SA node shows spontaneous depolarisation, the pacemaker potential
• The depolarisation wave spreads across the atria resulting in atrial systole
• Impulses converge at the AV node
• The AV node delays impulses, allowing the atria to empty into the ventricles
• The Bundle of His conducts impulses to the Purkinje fibers, resulting in ventricular systole

Electrocardiogram (ECG)

• Measures the heart's electrical activity during contractions and records it as a trace
• An ECG is non-invasive and provides info about heart rate and rhythm
• The trace includes components such as:
  • P wave
  • QRS complex
  • T wave
• ECGs can detect conditions like arrhythmias, enlarged heart due to hypertension, and myocardial infarctions

Circulatory System Function

• It is also known as the cardiovascular system
• Circulates blood, transports nutrients, oxygen, hormones, and removes waste
• Controls homeostasis

Pulmonary Circulation

• Transports blood from the right ventricle to the lungs
• Returns blood to the left atrium

Systemic Circulation

• Composed of vessels not part of the pulmonary system
• It distributes blood to the remainder of the body

Coronary Circulation

• Provides its own circulation, known as the coronary circulation
• The right and left coronary arteries deliver oxygenated blood to the heart
• Blood from capillaries in the myocardium enters the cardiac veins, which drain into the right atrium

Fetal Circulation

• Pulmonary circulation is unnecessary in utero because the placenta oxygenates fetal blood
• The umbilical cord connects foetus and placenta
• Systemic venous blood returning to the right atrium is deflected into the left atrium through the foramen ovale
• Blood from the left ventricle is diverted away from pulmonary arteries through the ductus arteriosus

Blood Vessels

• Arteries
• Arterioles
• Capillaries
• Venules
• Veins

Arteries

• Carries oxygenated blood away from the heart
• Pulmonary arteries are the exception
• Has thick elastic muscular walls with blood flowing under high pressure
• Consists of three layers
  • Tunica externa
  • Tunica media
  • Tunica intima

Capillaries

• Capillaries are the smallest and most abundant blood vessels in the body
• Walls consist of just an endothelium, and are one cell layer thick
• It permits rapid exchange between blood and tissues
• Capillary beds consist of networks supplying organs
• Precapillary sphincter muscles, constriction and dilation of arterioles determine Blood flow

Types of Capillaries

• Continuous
• Fenestrated
• Discontinuous (sinusoidal)

Veins

• Carry oxygen-depleted blood back to the heart
• Have valves to prevent backflow
• Veins act as a blood reservoir
• Rely on the skeletal muscle pump and respiratory pump
• Possess 3 layered walls which are less muscular and elastic compared to arteries

Blood-Brain Barrier Role

• Selective barrier protecting the brain from foreign substances
• It maintains a constant environment
• Brain capillaries are not fenestrated
• Endothelial cells are tightly packed, preventing large molecules and harmful cells from passing into the central nervous system

Great Vessels - Arteries

• The pulmonary arteries carry blood from the right atrium to the lungs for oxygenation
• The aorta originates from left ventricle as the ascending aorta
• It forms the aortic arch, and becomes the descending aorta

Great Vessels - Veins

• The pulmonary vein returns oxygenated blood from the lungs to the left atrium
• The superior vena cava receives deoxygenated blood from the upper body
• The inferior vena cava receives the venous return from the lower body
• Both drain into the right atrium

Blood Pressure

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