Unit 3: Respiratory and cardiovascular systems
Human Anatomy and Physiology-I notes · PTU syllabus (BMLS203-18)
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Unit summary
The lungs exchange gases and the heart pumps blood around the body. This unit covers the structure and function of the respiratory system, the mechanism of breathing, lung volumes and capacities, the anatomy and physiology of the heart, circulation, arteries and veins, the conducting system, the cardiac cycle and an introduction to the ECG.
After this unit you can
- Describe the respiratory organs and mechanism of breathing
- Define lung volumes and capacities
- Describe the structure of the heart and circulation
- Explain the conducting system, cardiac cycle and ECG
PTU syllabus topics
- Structure
- function and mechanism of breathing and respiration
- vital capacity
- total volume
- reserve volume
- total lung capacity
- anatomy and physiology of the heart
- blood circulation
- arteries and veins
- conductive system of the heart
- cardiac cycle
- introduction to ECG
- 1
Body (vena cavae)
- 2
Right atrium
- 3
Right ventricle
- 4
Lungs
Pulmonary artery: pick up oxygen
- 5
Left atrium
Pulmonary veins
- 6
Left ventricle
- 7
Body (aorta)
Topic 1
The respiratory system
- 1Nose and nasal cavity
Warm, filter and moisten air
- 2Pharynx and larynx
Voice box with vocal cords; epiglottis guards the airway
- 3Trachea
C-shaped cartilage rings; ciliated epithelium
- 4Bronchi and bronchioles
Branching airways
- 5Alveoli
About 300 million thin sacs surrounded by capillaries — gas exchange
- Lungs: right lung has 3 lobes, left lung 2; covered by pleura.
Topic 2
Mechanism of breathing and respiration
Muscles
Diaphragm contracts and flattens; external intercostals lift ribs
Muscles relax (passive at rest)
Thoracic volume
Increases
Decreases
Pressure
Falls below atmospheric — air flows in
Rises above atmospheric — air flows out
- External respiration: O₂ diffuses from alveoli to blood and CO₂ the reverse. Internal respiration: exchange between blood and tissues. O₂ is carried mainly as oxyhaemoglobin; CO₂ mostly as bicarbonate. Normal rate: 12–20 breaths per minute.
Topic 3
Lung volumes and capacities
Tidal volume (TV)
Air in or out in quiet breathing
About 500 mL
Inspiratory reserve volume
Extra air inhaled forcibly
About 3,000 mL
Expiratory reserve volume
Extra air exhaled forcibly
About 1,100 mL
Residual volume
Air left after maximum expiration
About 1,200 mL
Vital capacity
TV + IRV + ERV
About 4,600 mL
Total lung capacity
Vital capacity + residual volume
About 5,800 mL
- Measured by spirometry; peak expiratory flow rate checks airway obstruction (asthma).
Topic 4
Anatomy of the heart
- A muscular four-chambered organ in the mediastinum, enclosed in the pericardium; wall layers: epicardium, myocardium, endocardium.
- Right atrium
- Receives deoxygenated blood from venae cavae
- Right ventricle
- Pumps to lungs through the pulmonary artery
- Left atrium
- Receives oxygenated blood from pulmonary veins
- Left ventricle
- Thickest wall; pumps to the body through the aorta
- Atrioventricular valves
- Tricuspid (right), mitral or bicuspid (left)
- Semilunar valves
- Pulmonary and aortic
Topic 5
Blood circulation, arteries and veins
- 1Body → venae cavae → right atrium → right ventricle
- 2Pulmonary artery → lungs (oxygenation)
- 3Pulmonary veins → left atrium → left ventricle
- 4Aorta → arteries → capillaries in body tissues
- 5Veins → back to the heart
Direction
Away from the heart
Towards the heart
Blood
Oxygenated (except pulmonary artery)
Deoxygenated (except pulmonary veins)
Wall
Thick, muscular, elastic
Thin, less muscle
Valves
None
Present (limbs)
Pressure
High; pulsatile
Low
- Capillaries: one-cell-thick vessels where exchange occurs. Normal blood pressure about 120/80 mmHg.
Topic 6
Conducting system of the heart
- 1
Sinoatrial (SA) node in the right atrium — pacemaker, 60–100 beats per minute
- 2
Impulse spreads through the atria
- 3
Atrioventricular (AV) node — short delay
- 4
Bundle of His
- 5
Right and left bundle branches
- 6
Purkinje fibres — ventricles contract
Topic 7
Cardiac cycle and introduction to the ECG
- Cardiac cycle (about 0.8 s at 75 beats per minute): atrial systole (0.1 s), ventricular systole (0.3 s), and diastole (0.4 s). Heart sounds: "lub" (closure of AV valves) and "dub" (closure of semilunar valves). Cardiac output = stroke volume × heart rate (70 mL × 72 ≈ 5 L/min).
P wave
Atrial depolarisation
Up to 0.11 s
PR interval
Conduction from atria to ventricles
0.12–0.20 s
QRS complex
Ventricular depolarisation
Up to 0.10 s
T wave
Ventricular repolarisation
—
QT interval
Ventricular depolarisation and repolarisation
About 0.36–0.44 s
- The ECG records the heart's electrical activity with 12 leads; used to diagnose arrhythmias, heart attacks and conduction blocks.
Key terms
- Alveoli
- Air sacs where gas exchange occurs
- Vital capacity
- Maximum air exhaled after maximum inhalation
- SA node
- Natural pacemaker of the heart
- Cardiac output
- Volume of blood pumped per minute
- ECG
- Recording of the heart's electrical activity
Quick revision
- Respiratory tract; lungs and pleura; mechanism of breathing; gas transport.
- TV, IRV, ERV, RV, VC, TLC; PEFR.
- Heart chambers, valves, layers; double circulation; arteries vs veins.
- SA node → AV node → bundle of His → Purkinje fibres.
- Cardiac cycle, heart sounds, cardiac output; P, QRS, T waves.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Which muscle is main in quiet inspiration?
- Q2.Define tidal volume and vital capacity.
- Q3.Name the valves of the heart.
- Q4.Why is the left ventricle wall thickest?
- Q5.What is the pacemaker of the heart?
- Q6.What does the QRS complex represent?
Long-answer questions
- Q1.Describe the respiratory system and the mechanism of breathing.
- Q2.Explain lung volumes and capacities.
- Q3.Describe the structure of the heart and blood circulation.
- Q4.Explain the conducting system, cardiac cycle and ECG.
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