Heart – Overview

• The heart is a cone-shaped, hollow, muscular organ about 10 cm long, weighing ~225g in women and ~310g in men.
• It lies in the thoracic cavity, in the mediastinum (space between the lungs), tilted more to the left.
• The apex (bottom) is near the 5th intercostal space, and the base (top) extends to the 2nd rib.

Surrounding Structures

Inferior: Diaphragm

Superior: Great blood vessels (aorta, pulmonary artery and veins, superior vena cava)

Posterior: Oesophagus, trachea, bronchi, descending aorta, IVC, thoracic vertebrae

Lateral: Lungs (left lung overlaps the heart)

Anterior: Sternum, ribs, intercostal muscles

Heart Wall Layers

Pericardium

• Outer protective sac with two layers:
• Fibrous pericardium (tough, prevents overdistension)
• Serous pericardium (parietal + visceral layers with pericardial fluid in between for smooth movement)

Myocardium

• The middle muscular layer composed of specialized cardiac muscle
• Involuntary, striated, with intercalated discs allowing coordinated contraction
• Thickest in the left ventricle (does most of the pumping)
• Contains specialized conducting fibers for electrical signal transmission
• Secretes atrial natriuretic peptide (ANP) in the atria
• Supported by the fibrous skeleton, which separates atria from ventricles and ensures conduction only via specific pathways

Endocardium

• Inner smooth lining of the heart chambers and valves
• Made of flattened epithelial cells
• Continuous with the blood vessel endothelium

Interior of the Heart

Divided by the septum into right and left sides (no blood flow between them after birth)

Each side has:

Atrium (upper chamber)

Ventricle (lower chamber)

Atrioventricular (AV) valves separate atria from ventricles:

Right AV valve: Tricuspid (3 cusps)

Left AV valve: Mitral or bicuspid (2 cusps)

Valves open/close passively based on pressure differences

Prevent backflow with help of chordae tendineae and papillary muscles

Blood Flow Through the Heart

• Deoxygenated blood from body enters right atrium via superior and inferior venae cavae
• Blood flows to right ventricle -> pumped to lungs via pulmonary artery
• Gas exchange in lungs: CO2 removed, O2 added
• Oxygenated blood returns to left atrium via pulmonary veins
• Blood flows to left ventricle -> pumped into aorta -> distributed to body
• Valves (pulmonary and aortic) prevent backflow during relaxation
• Both atria contract together, followed by both ventricles contracting simultaneously
• Atrial walls are thinner than ventricular walls due to lesser workload
• Pulmonary trunk exits from right ventricle; aorta exits from left ventricle

Coronary (Heart) Circulation

Arterial Supply:

• Right and left coronary arteries arise from the aorta immediately after the aortic valve
• These supply ~5% of cardiac output despite the heart’s small size
• Left ventricle receives the most blood due to its workload

Venous Drainage:

• Most blood is collected by cardiac veins, which drain into the coronary sinus -> opens into right atrium
• Some blood drains directly into chambers via small venous channels

Conducting System of the Heart

i. Autorhythmicity:

• Heart can generate its own electrical impulses.
• Beats independently of nervous/hormonal input.

ii. Nerve Supply Influence:

• Sympathetic -> increases heart rate.
• Parasympathetic (Vagus nerve) -> decreases heart rate.
• Also influenced by hormones like adrenaline and thyroxine.

Key Structures in the Conducting System

Sinoatrial (SA) Node

• Located in the right atrium near superior vena cava.
• Electrically unstable – fires spontaneously (60–80 bpm).
• Sets the heart’s pace -> pacemaker.
• Initiates atrial contraction.

Atrioventricular (AV) Node

• Located in atrial septum near AV valves.
• Delays impulse by 0.1 s -> allows atrial contraction to complete.
• Can act as secondary pacemaker (40–60 bpm).

AV Bundle (Bundle of His)

• Originates from AV node.
• Crosses fibrous ring -> divides into right & left bundle branches.
• Ends in Purkinje fibers -> carry impulse to ventricles -> ventricular contraction begins at apex and moves upward.

Nerve Supply to the Heart

Parasympathetic (Vagus nerve)

• Acts on SA, AV nodes and atrial muscle.
• Reduces rate and force of heartbeat.

Sympathetic nerves

• Act on SA, AV nodes and both atria/ventricles.
• Increase rate and force of heartbeat.

Factors Affecting Heart Rate

• Gender
• Age
• Activity & exercise
• Emotional states
• Temperature
• Autonomic nervous system
• Circulating hormones (e.g. adrenaline, thyroxine)
• Baroreceptor reflex
• Body position

Cardiac Cycle

Definition: Sequence of events in one heartbeat (0.8 s at 74 bpm).

Phases:

• Atrial systole (0.1 s): atria contract -> blood to ventricles.
• Ventricular systole (0.3 s): ventricles contract -> blood to aorta & pulmonary artery.
• Complete cardiac diastole (0.4 s): all chambers relax.

Valves operate based on pressure differences:

• AV valves open during atrial filling -> close at ventricular contraction start.
• Aortic & pulmonary valves open during ventricular contraction -> close during relaxation.

Heart Sounds

• 1st Sound (“lub”): Closure of AV valves – start of ventricular systole.
• 2nd Sound (“dup”): Closure of aortic & pulmonary valves – start of ventricular diastole.
• Heard best slightly below and medial to the left nipple.

Electrical Activity & ECG

ECG (Electrocardiogram) records electrical impulses:

• P wave: Atrial depolarization (SA node -> atria).
• QRS complex: Ventricular depolarization (AV node -> ventricles).
• T wave: Ventricular repolarization.
• Atrial repolarization hidden in QRS complex.

Sinus Rhythm: Normal rhythm from SA node (60–100 bpm).

• 100 bpm = Tachycardia
• <60 bpm = Bradycardia

Cardiac Output (CO)

Formula:

Cardiac Output (L/min) = Stroke Volume (mL) × Heart Rate (bpm)

At rest:

• Stroke Volume = 70 mL
• Heart Rate = 72 bpm -> CO ~ 5 L/min
• During exercise: CO rises to 25–35 L/min (cardiac reserve).

Stroke Volume – Determinants

Preload (VEDV): Volume in ventricles before contraction.

Venous return: More return = increased preload =  increased stroke volume.

Myocardial contractility: Strength of ventricular contraction.

Blood volume: More blood -> higher preload.

Afterload: Resistance due to arterial pressure; higher afterload = decreased stroke volume.

Factors Affecting Venous Return

Body position:

• Upright: gravity helps from head/neck, hinders lower body return.
• Lying down: even flow.

Skeletal muscle pump:

• Muscle contraction squeezes veins -> helps blood flow to heart.

Respiratory pump:

• Inhalation lowers thoracic pressure, raises abdominal pressure -> pulls blood toward heart.

Heart Rate – Additional Influences

Autonomic system:

• Sympathetic increases rate
• Parasympathetic decreases rate

Chemicals:

• Adrenaline, noradrenaline, thyroxine  increases  rate
• Electrolyte imbalances (e.g. high potassium decreases rate)
• Certain drugs (e.g. beta-blockers decreases rate)

Other factors:

• Emotional state
• Gender (higher in women)
• Age (higher in infants)
• Body temp (increased temp causes increased rate)
• Baroreceptor reflex (responds to blood pressure changes)

Blood Pressure – Definition & Significance

• Blood pressure (BP) is the force blood exerts on vessel walls.
• Systemic arterial BP maintains organ perfusion.
• Too high -> risk of vessel damage, clots, rupture.
• Too low -> inadequate blood supply to vital organs (brain, heart, kidneys).

Systolic & Diastolic Pressure

Systolic BP: Pressure during left ventricular contraction -> ~120 mmHg.

Diastolic BP: Pressure during heart relaxation -> ~80 mmHg.

Pulse Pressure = Systolic – Diastolic.

Expressed as: BP = 120 / 80 mmHg or 16/11kPa

Factors Affecting Blood Pressure

• Time of day, posture, age, gender.
• Lower at rest/sleep; higher in older adults and women.

Elasticity of Arterial Walls

• Arteries expand during systole and recoil during diastole to maintain pressure.
• Loss of elasticity with age caused increased BP.

Determinants of Blood Pressure

1. Cardiac Output (CO) BP

• CO = Heart Rate × Stroke Volume.
• Increased CO -> BP (both systolic and diastolic).
• increases Stroke Volume causes increased Systolic BP more than diastolic.

2. Peripheral Resistance

• Controlled by arterioles (via vasoconstriction/dilation).
• Vasoconstriction causes increased BP
• Vasodilation causes decreased BP
• With age, muscle in arterioles replaced by fibrous tissue causes increased resistance.

Autoregulation

• Organs self-regulate their own blood supply regardless of systemic BP.
• Especially important in kidneys and brain to prevent damage from pressure fluctuations.

Control of Blood Pressure

Short-Term Control

• Managed by the Cardiovascular Centre (CVC) in the medulla & pons.
• Inputs from:

- Baroreceptors (stretch/pressure)

- Chemoreceptors (CO2, O2, pH)

- Higher brain centres (emotion, temperature)

Baroreceptors

• Located in aortic arch and carotid sinuses.
• Increased BP causes increased baroreceptor activity causes decreased heart rate + vasodilation leads to decreased BP
• Decreased BP causes decreased baroreceptor activity causes increased heart rate + vasoconstriction leads to increased BP
• This mechanism is called the baroreceptor reflex.

Chemoreceptors

• Found in aortic and carotid bodies.
• Sense low O2, high CO2, low pH -> stimulate sympathetic response leads to increased BP.
• Only activated during respiratory failure or very low BP (<80 mmHg).
• Similar receptors in the medulla monitor cerebrospinal fluid.

Higher Brain Centres

• Emotions (fear, anger, pain) influence the CVC -> changes in BP.
• Hypothalamus adjusts BP to help regulate body temperature.

Long-Term Control

• Renin–Angiotensin–Aldosterone System (RAAS) and ADH regulate blood volume -> affect BP.
• Atrial Natriuretic Peptide (ANP) from the heart causes fluid loss -> lowers BP.
• These act over hours to days to maintain stable BP.

Pulmonary Blood Pressure

• Much lower than systemic BP due to extensive capillary network in lungs.
• Normal pressure prevents fluid leakage into alveoli.
• If pulmonary capillary pressure >25 mmHg -> pulmonary oedema (fluid in lungs).

Pulse

• Felt as the expansion of superficial arteries during systole.
• Pulse reflects the heart rate, normally 60–80 bpm at rest.
• Commonly felt where arteries lie close to bone (e.g., wrist, neck).

Information from the Pulse

• Rate: Heartbeats per minute.
• Regularity: Even spacing between beats.
• Volume/Strength: Indicates stroke volume and vessel wall condition.
• Tension: Arterial wall pliability gives info about BP and vascular tone.

Inequalities in Pulse and Heart Rate

Occurs when:

• Arteries are narrowed/blocked -> weak or absent pulse despite heartbeat.
• Cardiac arrhythmias (e.g., atrial fibrillation) -> heart beats but doesn't eject enough blood to generate a palpable pulse.

Circulation of Blood – Overview

• Blood circulation is continuous but divided into:
• Pulmonary circulation (heart -> lungs -> heart)
• Systemic (general) circulation (heart ->body -> heart)

Pulmonary Circulation

Blood flows from right ventricle -> lungs -> left atrium

Purpose:

• CO2 is removed
• O2 is absorbed

Steps:

• Pulmonary trunk leaves right ventricle and splits into right & left pulmonary arteries
• These arteries branch into lobar arteries, then into arterioles and capillaries
• Gas exchange happens in alveolar capillaries
• Oxygenated blood is returned via 4 pulmonary veins (2 from each lung) to left atrium

Systemic Circulation

Blood flows from left ventricle -> aorta -> body -> right atrium

Main vessels:

• Aorta: carries oxygenated blood out of heart
• Venae cavae (superior & inferior): return deoxygenated blood to heart

Aorta – Main Artery of the Body

Divided into:

A. Thoracic Aorta

i. Ascending aorta:

• Begins at left ventricle, ~5 cm long
• Branches: Right & Left Coronary Arteries (supply heart muscle)

ii. Arch of the aorta:

• Arches over heart and gives off 3 major branches:

- Brachiocephalic artery -> Right common carotid + Right subclavian

- Left common carotid artery

- Left subclavian artery

iii. Descending thoracic aorta:

• Runs along spine; gives off paired branches to thoracic organs/walls

B. Abdominal Aorta

• Begins after passing diaphragm (T12 vertebra)
• Ends at L4, dividing into right & left common iliac arteries
• Gives off many branches:

- Paired: e.g. Renal arteries

- Unpaired: e.g. Coeliac artery

Venae Cavae – Major Veins

Superior vena cava:

• Formed by right & left brachiocephalic veins
• Drains above diaphragm

Inferior vena cava:

• Formed by right & left common iliac veins (L5)
• Drains below diaphragm
• Passes through diaphragm (T8) into thorax

Circulation in the Head and Neck

A. Arterial Supply

Main arteries: Common carotid and Vertebral arteries

Common Carotid Arteries

Right: from brachiocephalic artery

Left: directly from aortic arch

Each divides into:

• External carotid artery (supplies face & scalp)
• Internal carotid artery (supplies brain)

Key Branches of External Carotid:

• Superior thyroid artery – thyroid gland
• Lingual artery – tongue, mouth floor
• Facial artery – face muscles
• Occipital artery – scalp (posterior)
• Temporal artery – scalp (anterior); pulse in front of ear
• Maxillary artery – jaw muscles, skull interior

Internal Carotid Artery

• Supplies brain, eyes, forehead, nose
• Major contributor to circle of Willis

Circle of Willis (Circulus Arteriosus)

Ensures consistent blood supply to brain even if one artery is blocked

Formed by:

• 2 Internal carotid arteries
• 2 Vertebral arteries -> Basilar artery
• 2 Anterior cerebral arteries
• 1 Anterior communicating artery
• 2 Posterior cerebral arteries
• 2 Posterior communicating arteries

Supplies:

• Anterior brain – anterior cerebral
• Lateral brain – middle cerebral
• Posterior brain – posterior cerebral
• Brainstem – branches of basilar artery

Venous Return from Head and Neck

A. Superficial Veins

• Named after arteries
• Join to form the external jugular vein
• Path: over sternocleidomastoid -> behind clavicle -> subclavian vein

B. Deep Veins & Sinuses

• Blood from brain drains into dural venous sinuses
• Formed by dura mater + endothelium
• Key sinus: Superior sagittal sinus (drains superior brain)

Ageing and the Cardiovascular System

A. Heart Changes

• Cardiac output decreases
• Conduction system less efficient
• Fewer cardiac muscle cells but size increases (hypertrophy)
• Heart becomes less compliant (stiffer) due to fibrous skeleton
• Reduced response to adrenaline/noradrenaline
• Increased risk of heart failure
• Exercise slows cardiovascular decline

B. Blood Vessels Changes

• Less efficient vasoconstriction/vasodilation
• Stiffer arterial walls causes increased blood pressure
• Increased smooth muscle in artery walls leads to decreased compliance
• Reduced blood flow to organs, but metabolic rate also falls
• Baroreceptor reflex weakens -  may cause postural hypotension
• Neuronal ageing also affects response