Environmental Physiology

High Altitude and Acclimatization

• High altitude → ↓↓ barometric pressure → ↓↓ partial pressure of oxygen (PO2).
• (O2 percentage same, total pressure lower).

• Effect: Hypoxic hypoxia.

• Immediate Response upon ascent:
• Hypoxic hypoxia → Stimulates peripheral chemoreceptors → Immediate ↑ ventilation.
• ↑ Ventilation → CO2 washout → Alkalosis
• “Breaking effect”
• Alkalosis → ⛔ ventilation → Slight ↓ in ventilation → after few hours

Acclimatization Process (Over time):

Renal Compensation: 

• ↑↑ Ventilation → Respiratory alkalosis → ↑↑ HCO₃⁻ + Tetany

• Renal Compensation → HCO₃⁻ Excreted via kidney → ↓↓ HCO₃⁻
• Prevents respiratory alkalosis & tetany.
• ↓↓ blood bicarbonate → Bicarbonate shifts from CSF to blood →
• ↓↓ CSF bicarbonate → ↑↑ Acidity of CSF → Stimulates central chemoreceptors 
• Further ↑↑ ventilation

High Altitude Sicknesses :

1. Acute Mountain Sickness
• Complications
• HAPE
• HACE

2. Chronic Mountain Sickness

1. Aka Acute Mountain Sickness (AMS)
(Insufficient acclimatization)

• Develops: First 24 hours.

• Symptoms: Headache, anorexia, nausea, sleep alteration.

• Complications
• High Altitude Cerebral Edema (HACE):
• AMS symptoms + Ataxia or mental changes.
• Edema in brain white matter
• D/t vasodilatation of cerebral arteries
• High Altitude Pulmonary Edema (HAPE):
• Develops: 2 to 4 days.
• Symptoms: Cough, dyspnea, sputum production.

Treatment:

• Most reliable/effective: 
• Immediate descent to low altitude (1 km) +
• Supplemental or hyperbaric Oxygenation

• AMS & HACE:
• Dexamethasone
• Sumatriptan (5HT-agonist) & Gabapentin also used

• HAPE:
• Nifedipine (calcium channel blocker)
• Phosphodiesterase inhibitors (Sildenafil or tadalafil)
• Theophylline & Aminophylline

• Acetazolamide:
• DOC
• Used to facilitate acclimatization & as prophylaxis. 
• Not primary treatment for acute disorder.

2. Chronic Mountain Sickness / Monge's Disease

• Cause
• ↑ erythrocytosis

• Clinically
• Pulmonary hypertension → Cor pulmonale

• Treatment
• Venesection
• Acetazolamide → facilitate acclimatization & as prophylaxis. 
• Monks → leave in mountains → have PAH

Environmental Physiology

On Level Lands

• Atmospheric pressure: 760 mmHg

• O₂ content in atmosphere: 21%
• PO₂ atmosphere: 21% of 760 mmHg = 160 mmHg

Deep Sea Physiology

Etiology

• Divers

• Military operation

• Caisson workers

• Recreation activities

Pressure:

• At sea level: 1 atm

• For every 10 m deep: Pressure ↑ by 1 atm

• Leads to high barometric pressure (gases compressed).

• 1 km → 100 atm → 76000 mmHg

1. Barotrauma

• Pneumothorax / TM rupture

2. Caisson's Disease / Diver's Disease / Decompression Sickness

MOA: 

• High barometric pressure → quick ascent → low barometric pressure 
• Dissolved N₂ released as bubbles.

Features (Accumulation of N₂ bubbles):

Rx:

• Hyperbaric chamber

• Slow ascent

• Slow decompression (forms less bubbles)

Note: 

• Decompression sickness also affects pilots & mountaineers.

Space Physiology

Effects of Microgravity

• Loss of Ca²⁺ & PO₄³⁻ from bones → Prone to fractures

• Loss of muscle mass

• ↓ ↓ RBC count

Gravitational Forces

• Positive G: 
• ↓ Cerebral perfusion → Black out
• ↓ Cardiac output
• ↓ Venous return (due to ↑ venous pooling)

• Negative G: 
• ↑ Venous return
• ↑ Cardiac output
• ↑ Cerebral perfusion
• Congestion in eyes (red out)

Exercise

• Relaxation heat occurs only in isotonic contraction

Respiration During Exercise

1. Initial rapid ↑ ventilation (Phase 1) at exercise start:

• Feedforward control system

• Anticipatory Tachypnea
• Due to psychic stimulation & proprioceptor stimulation.

2. Moderate Exercise:

• Exercise → ↑ Muscle metabolism → ↑ CO2 production, ↑ O2 consumption → Need to increase ventilation

• Primary stimuli for ↑ ventilation:
• ↑↑ in
• Potassium ion (released from muscles)
• Body temperature
• Stimulate Peripheral chemoreceptors → Results in ↑ Ventilation
• Isocapnic buffering
• Arterial PO2, PCO2, pH remains relatively normal

Why is not ↓ O2 content, ↑ CO2 content, ↑ H+ ions in Venous blood not triggering hyperventilation during exercise?

• Venous blood changes do not stimulate peripheral chemoreceptors

3. Severe Exercise:

• Significant lactic acid production → ↑ H+ and CO2.

• This acidosis and hypercapnia in arterial blood → additional stimuli for ventilation.

Increased Oxygen Consumption

Oxygen Debt & EPOC (Excess Post-Exercise Oxygen Consumption)

Diffusion of Gases

• Oxygen dissociation curve: Shifts to right

• O₂ released to tissues to meet demand.

Arteriovenous (A-V O₂) Difference

• ↑ during exercise.

• Cause: Tissues extract more O₂.

• Also during stagnant hypoxia

Cardiovascular Changes in Exercise

1. Cardiac Output (CO):
• CO ↑↑
• CO = SV x HR.
• due to ↑ HR.
• Reaches a plateau.
• ↑ 7 fold in athletes
• ↑ 4 fold in non-athletes

2. Anticipatory Tachycardia:
• Even before starting exercise.
• Cause: Joint proprioceptors → Impulse → Brain.

3. Blood Pressure:

Blood Pressure	Isotonic	Isometric
Systolic BP	↑	↑
Diastolic BP	↓	↑
Cause	Local accumulation of metabolic end products → Vasodilation → ↓ TPR → ↓ DBP	Sustained muscle contraction → ↑ Tension → Compresses blood vessels (Arterioles) → ↑ TPR → ↑ DBP

4. Preload:
• Sympathetic nervous system stimulation 
• Releases Norepinephrine 
• Venoconstriction → ↑ mean systemic filling pressure (MSFP) → ↑ preload.

Blood Flow during Rhythmic Muscle Exercise

General Blood Flow:

• Rest: 3 ml/100g/min

• During exercise: 80 ml/100g/min
• (↑ by 20 times).

Blood Flow during Rhythmic Muscle Exercise:

• Fluctuates rhythmically:
• ↓ during contraction.
• ↑ during relaxation
• Causes
1. Local metabolites from exercising muscles
• Most Important
• Vasodilation
2. Increased arterial BP.
3. Sympathetic-mediated vasodilation
• Beta-2 receptors on muscle arterioles

• Exercise Hyperemia:
• ↑ metabolism 
• Local accumulation of H+, K+, lactate (metabolic end products) 
• Vasodilation → ↑ blood flow → Hyperemia

• Organ-Level Redistribution:
• ↓ blood flow in Renal & Splanchnic (GIT) circulation.
• ↑ blood flow to Heart and skeletal muscles.

• Cerebral Circulation: 
• Always maintained constant due to autoregulation.

• Skin Blood Flow:
• Initial ↓
• Later ↑ for heat dissipation.

Effects of Smoking

• ↓ exercise capacity

• ↓ work capacity

• Mechanism:
• Toxic chemicals in smoke → Constriction of airways 
• ↑ air flow resistance.

• Also causes:
• Ciliary paralysis
• Swelling of epithelial cells.
• ↑ fluid secretion into bronchial tree.