Dyspnea – Short Note (4 Marks)

• Heart problems – heart failure, severe high blood pressure, pericardial tamponade (fluid around heart)
• Lung problems – asthma, pneumonia, bronchitis, pneumothorax (collapsed lung), pulmonary embolism (clot in lung)
• Blood problems – severe anemia (low hemoglobin)
• Others – airway blockage, drug overdose (e.g., aspirin poisoning), anxiety, obesity, being unfit (deconditioning)

1. Acute dyspnea – comes on suddenly (e.g., due to heart attack, pneumothorax, clot in lung)
2. Chronic dyspnea – lasts a long time, seen in ongoing lung or heart disease
3. Orthopnea – breathlessness that gets worse on lying down and improves on sitting up (common in heart failure)
4. Paroxysmal Nocturnal Dyspnea (PND) – sudden severe breathlessness at night that wakes the patient from sleep, forcing them to sit up
5. Dyspnea on exertion – breathlessness that appears or worsens with physical activity

• In heart failure: feels like "hunger for air" or urge to breathe
• In airway/lung disease: feels like breathing needs extra effort
• In asthma/bronchoconstriction: feels like chest tightness
• In deconditioning: feels like fast or heavy breathing

Cyanosis – Short Note (4 Marks)

1. Central Cyanosis
   • Affects both skin AND mucous membranes (lips, tongue)
   • Caused by low oxygen level in arterial blood itself
   • Seen in: lung diseases (pneumonia, severe asthma), heart diseases with mixing of blood (congenital heart defects), high altitude, abnormal hemoglobin (methemoglobin)
2. Peripheral Cyanosis
   • Affects only the skin (fingers, toes, ears); mouth/tongue usually normal (spared)
   • Caused by slow blood flow and extra oxygen removal from normal blood, NOT low oxygen in the blood itself
   • Seen in: cold exposure, shock, heart failure, blocked blood vessels (peripheral vascular disease)

• Cyanosis depends on the absolute amount of deoxygenated hemoglobin, not the percentage. So:
  • Patients with severe anemia may NOT show cyanosis even with low oxygen (too little total hemoglobin)
  • Patients with polycythemia (too many red cells) show cyanosis more easily

• Lung disease (pneumonia, asthma, COPD)
• Heart disease (congenital defects, heart failure)
• Cold exposure / shock
• Abnormal hemoglobin (methemoglobinemia, sulfhemoglobinemia)
• High altitude

Symptoms at High Altitude (High Altitude Illness)

1. Acute Mountain Sickness (AMS) - most common

• Headache (main symptom, throbbing, worse at night/on waking)
• Nausea and vomiting
• Loss of appetite
• Tiredness and weakness
• Dizziness
• Trouble sleeping (broken sleep, periodic breathing)

2. High Altitude Cerebral Edema (HACE) - severe/dangerous

• Severe headache not relieved by medicine
• Confusion, poor judgment, and behavior changes
• Ataxia (loss of balance, unsteady walking) - this is a key warning sign
• Drowsiness progressing to coma
• Hallucinations

3. High Altitude Pulmonary Edema (HAPE) - severe/dangerous

• Breathlessness even at rest (not just on exertion)
• Cough (may bring up pink or blood-tinged frothy sputum)
• Chest tightness or congestion
• Extreme fatigue and weakness
• Fast heart rate and fast breathing
• Cyanosis (bluish lips/nails) in severe cases
• Crackling/bubbling sounds in the chest

Other altitude-related problems

• Decreased exercise performance
• Peripheral edema (swelling of hands, face, ankles)
• High-altitude retinopathy (bleeding in the retina of the eye)
• High-altitude bronchitis (dry cough due to cold, dry air)

Key point for exams

Decompression Sickness – Short Note (5 Marks)

• Under high pressure (e.g., deep underwater diving), extra nitrogen gas dissolves into the blood and tissues.
• When the person comes back up (decompresses) too fast, this dissolved nitrogen doesn't have time to leave the body slowly through the lungs.
• Instead, it forms bubbles inside blood vessels and tissues.
• These bubbles block blood flow (like a clot), damage the lining of blood vessels, and trigger inflammation and clotting.

1. Type I – "Pain-only" DCS (mild)
   • Deep pain in joints and limbs (most common symptom)
   • Skin rash/mottling called "cutis marmorata"
   • Lymphatic blockage can cause swelling (lymphedema)
2. Type II – Serious DCS (severe)
   • Affects brain and spinal cord (CNS symptoms) - weakness, numbness, paralysis
   • Vertigo, dizziness, and imbalance - called "the staggers"
   • Breathing difficulty and chest pain - called "the chokes"
3. Type III - occurs when gas bubbles enter the arteries (arterial gas embolism), causing sudden stroke-like symptoms.

• Joint and muscle pain
• Skin rash or itching, mottled skin
• Dizziness, vertigo
• Weakness, numbness, or paralysis
• Breathing trouble, chest pain
• Fatigue and confusion in severe cases

• Diving too deep or too long
• Coming up too fast (not following decompression stops)
• Flying soon after diving
• Dehydration, obesity, old age

• Hyperbaric oxygen (HBO) therapy is the main and definitive treatment. It shrinks the nitrogen bubbles and improves oxygen delivery to tissues.
• 100% oxygen given if HBO is not immediately available
• Keep the patient lying flat, give fluids, transport quickly to a hyperbaric facility (using pressurized aircraft or low-altitude transport if by air)

• Follow standard dive tables / dive computer limits
• Ascend slowly with proper decompression stops
• Avoid flying soon after diving

Exchange of Respiratory Gases (15 Marks)

Introduction

1. External respiration - exchange of gases between alveoli and pulmonary capillary blood
2. Internal respiration - exchange of gases between blood and body tissues/cells

The Respiratory Membrane

1. Layer of fluid containing surfactant (lines the alveolus, reduces surface tension)
2. Alveolar epithelium (thin epithelial cells)
3. Epithelial basement membrane
4. Thin interstitial space
5. Capillary basement membrane (often fused with epithelial basement membrane)
6. Capillary endothelial membrane

• Average thickness of membrane: about 0.6 micrometer (extremely thin)
• Total surface area: about 70 square meters (as big as a 25x30 foot room)
• There are about 300 million alveoli in both lungs
• Because the membrane is so thin and the surface area so large, gas exchange happens very fast

Principle of Gas Diffusion

• A gas always moves from a region of higher partial pressure to lower partial pressure
• The bigger the pressure difference, the faster the diffusion

• Since alveolar PO2 (104) is much higher than venous blood PO2 (40), oxygen moves from alveoli into blood.
• Since venous blood PCO2 (45) is higher than alveolar PCO2 (40), carbon dioxide moves from blood into alveoli to be breathed out.

Exchange of Oxygen (O2)

• Oxygen in alveolar air diffuses across the respiratory membrane into the pulmonary capillary blood.
• It dissolves in plasma first, then most of it (about 97%) binds to hemoglobin in red blood cells to form oxyhemoglobin.
• A small amount (about 3%) stays dissolved in plasma.
• This oxygenated blood then travels to the left side of the heart and is pumped to all body tissues.
• At the tissue level, PO2 in tissue cells is low (because cells constantly use oxygen), so oxygen diffuses from blood into the tissue cells.

Exchange of Carbon Dioxide (CO2)

• CO2 is produced constantly in tissue cells as a waste product of metabolism.
• It diffuses from tissue cells (high PCO2) into the blood (lower PCO2).
• CO2 is carried in blood in three forms:
  1. Dissolved in plasma (small amount, about 7%)
  2. Bound to hemoglobin as carbaminohemoglobin (about 23%)
  3. As bicarbonate ions (HCO3-) after combining with water, helped by the enzyme carbonic anhydrase (about 70%, the major form)
• When blood reaches the lungs, this process reverses: CO2 is released from bicarbonate and hemoglobin and diffuses into the alveoli, from where it is breathed out.

Factors Affecting Rate of Diffusion of Gases

1. Thickness of the respiratory membrane - thicker membrane (e.g., in fibrosis, edema) slows diffusion
2. Surface area available - reduced in diseases like emphysema, slows exchange
3. Diffusion coefficient of the gas - CO2 diffuses about 20 times faster than O2 because it is more soluble in the membrane, even though O2 has a lower molecular weight
4. Partial pressure difference across the membrane - greater difference means faster diffusion

Ventilation-Perfusion Ratio (V/Q Ratio)

• Normal V/Q ratio = about 0.8, giving ideal gas exchange
• High V/Q ratio (more air, less blood) - creates "physiological dead space," ventilation is wasted since there isn't enough blood to pick up the oxygen
• Low V/Q ratio (more blood, less air) - creates "physiological shunt," blood passes without getting properly oxygenated

Summary (Quick Points for Revision)

• Gas exchange = diffusion of O2 and CO2 across a thin respiratory membrane
• Driven by partial pressure differences
• O2 moves: alveoli → blood → tissues
• CO2 moves: tissues → blood → alveoli
• Carried mainly as oxyhemoglobin (O2) and bicarbonate (CO2)
• Efficiency depends on membrane thickness, surface area, and proper matching of ventilation and blood flow (V/Q ratio)

Q13. Respiratory Membrane and Factors Affecting Gas Exchange (10 Marks)

Part A: Respiratory Membrane (5 marks)

1. Layer of fluid containing surfactant - lines the alveolus, reduces surface tension so alveoli don't collapse
2. Alveolar epithelium - made of thin epithelial cells
3. Epithelial basement membrane
4. Thin interstitial space - lies between alveolar epithelium and capillary wall
5. Capillary basement membrane - often fused together with the epithelial basement membrane
6. Capillary endothelial membrane - the wall of the capillary itself

• Average thickness: only about 0.6 micrometer (extremely thin, so gas crosses easily)
• Total surface area: about 70 square meters (as large as the floor of a 25x30 foot room)
• Total alveoli in both lungs: about 300 million
• Blood in pulmonary capillaries flows almost like a "sheet," staying in close contact with alveolar air

Part B: Factors Influencing Gas Exchange Across the Respiratory Membrane (5 marks)

1. Thickness of the membrane
   • Thicker membrane = slower diffusion
   • Increases in diseases like pulmonary fibrosis, pulmonary edema (fluid collects and thickens the membrane)
2. Surface area of the membrane
   • Larger area = faster diffusion
   • Decreases in diseases like emphysema (alveoli destroyed), pneumonectomy (lung removal), which reduces gas exchange
3. Diffusion coefficient of the gas
   • Depends on the solubility of the gas in the membrane
   • CO2 diffuses about 20 times faster than O2, mainly because CO2 is much more soluble in the tissue/membrane, even though its molecule is heavier
4. Partial pressure difference of the gas across the membrane
   • Bigger difference in pressure between alveolar air and blood = faster diffusion
   • Example: O2 pressure in alveoli (104 mmHg) is much higher than in venous blood (40 mmHg), so O2 moves quickly into blood
5. Ventilation-perfusion (V/Q) matching
   • Proper matching of air reaching alveoli with blood flow past alveoli is needed for efficient gas exchange
   • Mismatch (too much air with too little blood, or too little air with too much blood) reduces effective gas exchange

• Molecular weight of the gas (lighter gases diffuse a little faster, but solubility matters more)
• Hemoglobin concentration and rate of reaction with the gas (affects overall transfer of O2 from alveoli to blood)

Summary line for exam

Oxygen Dissociation Curve (ODC) - Long Question (15 Marks)

Definition

Shape of the Curve

• X-axis: PO2 (mmHg), from 0 to 100
• Y-axis: % saturation of hemoglobin, from 0 to 100
• The curve rises slowly at first, then steeply in the middle, then flattens out at the top (plateau)

Important Points on the Curve

Amount of Oxygen Carried and Released

• Normal hemoglobin: about 15 g/100 mL of blood
• Each gram of hemoglobin carries about 1.34 mL of O2 when fully saturated
• So 100 mL of blood can carry about 20 mL of O2 (20 volume%) when 100% saturated
• Arterial blood (97% saturated) carries about 19.4 mL O2/100mL
• Venous blood (75% saturated) carries about 14.4 mL O2/100mL
• So about 5 mL of O2 is delivered to tissues by every 100 mL of blood passing through

P50 and Its Importance

• It tells us the affinity (attraction) of hemoglobin for oxygen
• Low P50 = higher affinity, Hb holds onto O2 tightly (curve shifted left)
• High P50 = lower affinity, Hb releases O2 more easily (curve shifted right)
• Used to compare different types of hemoglobin (fetal Hb, abnormal Hb) and to understand how factors affect oxygen delivery to tissues

Factors That Shift the Curve to the RIGHT (decreased affinity, more O2 released to tissues)

• CO2 increase
• Acid (decreased pH) - Bohr effect
• 2,3-DPG (BPG) increase - occurs in chronic hypoxia, high altitude, anemia
• Exercise
• Temperature increase (fever)

Factors That Shift the Curve to the LEFT (increased affinity, Hb holds O2 tighter)

• Decreased CO2
• Decreased H+ (increased pH, alkalosis)
• Decreased 2,3-DPG
• Decreased temperature
• Fetal hemoglobin (HbF) - has naturally higher affinity for O2 than adult Hb, so its curve is shifted left, helping it pull oxygen from the mother's blood
• Carbon monoxide poisoning (CO binds Hb, curve shifts left, and also reduces the amount of O2 that can be carried)

Bohr Effect (special mention)

Clinical/Physiological Significance

1. Explains how oxygen is picked up efficiently in the lungs and released efficiently in the tissues
2. Explains why mild drops in PO2 (e.g., at moderate altitude) don't cause much oxygen problem (flat part of curve)
3. Explains oxygen delivery increase during exercise (right shift due to heat, CO2, acid, DPG)
4. Helps understand fetal oxygen transfer (HbF has left-shifted curve)
5. Used clinically to interpret pulse oximetry and arterial blood gas results
6. Helps understand carbon monoxide poisoning and anemia adaptation

Summary Line for Exam