Week 3: Cardiovascular System

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1. HEART ANATOMY AND THE FUNCTION OF THE CARDIOVASCULAR SYSTEM

• Consists of heart, blood vessels, and blood.

• Transports oxygen, hormones, nutrients to cells.

• Removes CO2 and waste from the body.

• Regulates body temperature via vasodilation and vasoconstriction.

• The heart: location and size

• Located in the thoracic cavity, under the sternum.

• Size of a clenched fist; weighs less than two cricket balls.

• Sits on diaphragm, flanked by lungs.

• Base near the 2nd rib; apex points to left hip at about the 5th rib.

1.2 The four chambers of the heart

• Right and left atrium; right and left ventricle.

• Atria act as low-pressure collecting chambers.

• Ventricles act as high-power pumps.

• Interatrial septum separates right and left atria.

• Interventricular septum separates right and left ventricles.

• Prevents oxygen-rich and oxygen-poor blood from mixing.

1.2.1 Blood flow

• Right atrium: Receives deoxygenated blood via vena cava.

• Right ventricle: Pumps blood to lungs.

• Left atrium: Receives oxygenated blood from lungs.

• Left ventricle: Pumps blood to body through aorta.

• Both ventricles contract almost simultaneously.

1.3 The heart valves

• Four valves control one-way blood flow.

1.3.1 Atrioventricular valves (AV valves)

• Located between atria and ventricles.

• Open: blood flows from atria to ventricles.

• Closed: prevent backflow into atria.

• Right AV valve: tricuspid (three cusps).

• Left AV valve: bicuspid/mitral (two cusps).

1.3.2 Chordae tendinae & papillary muscles

• Fibrous cords attached to valve cusps.

• Contract with ventricles to prevent valve inversion.

1.3.3 Semilunar valves

• Control flow from ventricles to lungs/body.

• Pulmonary valve: right side, leads to lungs.

• Aortic valve: left side, leads to body.

• Three "half-moon" cusps, self-supporting when closed.

1.4 Blood flow through the heart

1. Right atrium: Deoxygenated blood enters from inferior and superior vena cava.

2. Tricuspid valve: Opens due to pressure; right ventricle fills passively. Atrial contraction completes filling.

3. Right ventricle: Contracts, tricuspid valve closes, pulmonary valve opens. Blood enters pulmonary artery.

4. Lungs: Blood oxygenated in capillaries.

5. Left atrium: Oxygenated blood returns via pulmonary veins.

6. Mitral valve: Opens due to pressure; left ventricle fills passively. Atrial contraction completes filling.

7. Left ventricle: Contracts, mitral valve closes, aortic valve opens. Blood enters aorta.

8. Aorta: Oxygenated blood distributed to body.

1.5 Walls of the heart

• Enclosed in fluid-filled pericardium.

• Three layers:

1. Epicardium: Outer layer, inner layer of pericardial sac.

2. Myocardium: Middle layer, 2/3 of heart muscle.

3. Endocardium: Innermost layer, lines chambers and valves.

1.6 The cardiac cycle

• Two phases:

• Diastole (relaxation): chambers fill with blood.

• Systole (contraction): chambers pump blood out.

• Cycle duration: Approx. 0.8 seconds.

• Time split: 2/3 in diastole, 1/3 in systole.

1.7 Cardiac output

• Blood pumped per minute.

• Calculated: Heart rate × stroke volume.

• for 70 kg male: 72 bpm, 70 ml/beat → 5,040 ml/min.

• Influenced by body size, exercise, emotions, diet, activity.

1. REGULATION OF THE HEART

Three mechanisms: one internal, two external.

2.1 Internal control of heart rate

• Pacemaker sinoatrial (SA) node: located in right atrium.

• Sets default rate: 60-100 bpm.

2.2 External control of heart rate

2.2.1 Cardiac centre in medulla oblongata

• Receives input from autonomic nervous system

• Adjusts heart rate, contraction strength, stroke volume.

• Baroreceptors: monitor blood pressure.

• Parasympathetic nervous system: dominant at rest, slows heart rate.

• Sympathetic nervous system: active during stress/exercise, increases rate.

2.2.2 Endocrine system

• Influenced by emotions, physical activity.

• Fight or flight: limbic system activates sympathetic branch.

• Parasympathetic releases acetylcholine; sympathetic releases noradrenaline.

2.3 Control of stroke volume

Three main factors: Preload, Contractility, Afterload.

(i) Preload: stretch of ventricle muscles during diastole.

(ii) Contractility: heart muscle's ability to contract.

• Influenced by preload.

(iii) Afterload: pressure to overcome aorta resistance.

2.4 The conduction system

Includes nodal tissue and conduction fibres.

SA Node: Initiates impulse.

AV Node: Delays impulse ~1/10 second.

Bundle of His, Left/Right bundle branches: Carry impulse to ventricles.

Purkinje Fibres: Trigger ventricular contraction.

2.5 Electrocardiogram (ECG)

Records electrical activity.

• P wave: SA node firing, atrial contraction.

• QRS Complex: impulses to Purkinje fibres, ventricular contraction.

• T wave: ventricular relaxation, chamber filling.

1. BLOOD VESSELS AND THE CIRCULATION

3.1 Blood vessels

• Types: arteries, capillaries, veins.

• Subdivisions: arterioles, venules.

3.1.1 Three layers in blood vessel walls

(i) Tunica intima: smooth, frictionless surface.

(ii) Tunica media: smooth muscle, controls vasodilation/vasoconstriction.

Influences blood pressure.

(iii) Tunica externa: fibrous tissue, adds support.

3.1.2 Differences: arteries vs. veins

• Arteries:

• Thick, strong walls.

• Carry blood away from the heart.

• Withstand high pressure.

• Veins:

• Thinner, less elastic walls.

• Carry blood toward the heart.

• House ~2/3 of body's blood.

• One-way valves assist in venous return.

• Skeletal muscles and respiratory system assist in pumping blood back.

3.1.3 Capillaries

• Smallest blood vessels, 0.0025-0.25 cm.

• Red blood cells pass in single file.

Function:

• Site for O2 and CO2 gas exchange.

• Pores in kidneys, liver, etc., allow passage of hormones, white blood cells.

Blood flow control:

• Precapillary sphincter controls blood flow.

• Reacts to local chemical conditions like pH, O2, CO2, temperature.

During exercise:

• Blood flow to muscles increases from 15% to 85%.

Capillary beds:

• Interconnected network.

• Merge with venules, then larger veins, leading back to the heart.

3.2 Circulation: blood flow in the body

Cardiopulmonary circulation consists of:

• Pulmonary circulation (heart to lungs)

• Systemic circulation (heart to body)

3.2.1 Sequence in pulmonary circulation

• Entry: deoxygenated blood enters right atrium via inferior/superior venae cavae.

• Right atrium to right ventricle: through tricuspid valve.

• Pulmonary valve: right ventricle contracts, blood goes to pulmonary artery.

• Pulmonary artery: splits into smaller arteries to lungs.

• Arterioles and capillaries: merge in lungs.

• Oxygenation: blood oxygenated in lungs, moves to venules.

• To heart: via four pulmonary veins to left atrium.

• Left atrium to left ventricle: through mitral valve.

• To body: left ventricle contracts, oxygenated blood to aorta.

3.2.2 Systemic circulatory system

• Closed-loop network: arteries, arterioles, capillaries, venules, veins.

• Functions: Transports oxygen, hormones, water, and nutrients to tissues; returns waste and CO2 to the heart.

Start of systemic circulation

• Left ventricle pumps blood to aorta.

• Ascending aorta rises and arches left, becoming descending aorta in thorax and abdomen.

Major arterial branches:

• Right and left coronary arteries from ascending aorta -> heart.

• Brachiocephalic, left common carotid, left subclavian from aortic arch -> head, neck, arms.

• Descending aorta branches -> organs like liver, spleen, kidneys.

Microcirculation and return

• Arteries -> arterioles -> capillaries for exchange.

• Blood drains into venules, merges into veins.

Vein names often match arterial counterparts (e.g., renal artery, renal vein).

End of systemic circulation

• Lower veins -> inferior vena cava.

• Upper veins -> superior vena cava.

• Both drain into right atrium.

3.2.3 Cardiac circulation

• Oxygen supply to the heart

• Heart's oxygen not supplied by blood in its chambers.

• Right and left coronary arteries supply oxygen-rich blood.

Origin and timing

• Coronary arteries arise from base of aorta.

• Fill when ventricles are relaxed; closed during ventricular contraction.

Drainage

• Blood from coronary arteries -> cardiac veins.

• Drains into coronary sinus on posterior wall of right atrium.