Environmental Physiology: Cold and Underwater

What are we?

What are we?

Homeotherms, we use behavioural and physiological mechanism to regulate core temperature (Tc)

Resting Tc

36.5-37.5 C

Core

Defined as the temperature of the hypothalamus, which is the site of temperature regulation

Skin temperature

Influenced by the environment, metabolic rate, clothing and hydration state

Mechanisms of Heat Loss: Radiation

The exchange of electromagnetic energy waves emitted from one object and absorbed by another

Mechanisms of Heat Loss: Conduction

Occurs whenever two surfaces with differing temperatures are in direct contact

Insulators

Do not conduct heat readily. Heat conduction in water is about 25 times greater than in air

Mechanisms of Heat Loss: Convection

Requires that one of the media be moving as occurs with a fluid or gaseous medium. Ex: Heat transfer from skin to moving air or water

Wind Chill Index

Gives the equivalent still air temperature for a particular ambient temperature at different wind velocities.

Mechanisms of Heat Loss: Evaporation

The transfer of heat from the body surface through the change of liquid water on the skin to a gaseous water vapour

Two primary physiological responses to defend against cold environment

1. Increase in metabolic rate

2. Increased tissue insulation

Increase in metabolic rate

• Voluntary: exercise - can increase heat production 10-20 times the basal rate

• Involuntary: shivering - can increase basal rate 3-4 times basal rate

Increased tissue insulation

Vasoconstriction of peripheral blood vessels and shunting of blood flow into deeper vessels

Behaviour responses to the cold

Proper clothing, seeking shelter, starting a fire, etc.

Skinfold Thickness

The thicker the fat layer, the greater the insulation

Gender

The average female has more subcutaneous fat than the average male → more insulation. Women have larger surface area to mass ratios than men. Women lose heat more rapidly than men during immersion in cold water

Clothing

The effective insulation afforded by clothing is a function of the air layer next to the skin, the thickness of the clothing, and the air trapped between the layers pf clothing. Multiple layers of clothing are advisable during exercise in cold weather. The outer layer should be water repellent and wind resistant. Middle layers should provide insulation. Inner most layer should wick moisture away from the skin. This reduces evaporative heat loss

Hypothermia

A condition characterized by a body core temperature below 35 C. Shivering will cease when Tc falls below 32-34 C. Death occurs when Tc to 24-28 C

Body core

Refers to brain, heart, lungs, blood, liver, kidneys

Critical areas for heat loss

Head and neck, sides of chest, groin

Factor associated with hypothermia

1. Immersion in cold water or wet clothing

2. Wind

3. Physical exhaustion

4. Inadequate clothing for conditions

5. Low percent body fat

6. Hypoglycaemia

7. Alcohol consumption

Hyperthermia (oxyhemoglobin)

Causes the oxyhemoglobin dissociation curve to shift to the left. Increased hemoglobin affinity for O2 → Less O2 is released to the tissues → Body tissue shift to anaerobic metabolism → Hypoxia of heart and brain plus metabolic acidosis → Depressed brain function and cardiac output → Ventricular fibrillation and death

Frostbite

Freezing of superficial tissues which occurs when skin temperature reaches between -2 C to -6 C.

Cold Exposure and the Respiratory Tract

Inhaled air is conditioned as it enters the upper respiratory passageways - warmed to 37 C and saturated with water vapour. Since cold air is very dry → cells lining the respiratory passageways become dry → possibility of throat irritation

What is decreased when muscle and nerve temperature decrease

• strength and power

• Nerve conduction velocity

• Reaction time

• Manual dexterity

• Flexibility

Effects of Cold on Performance

Optimal marathon race performance occurs at an ambient temperature of about 14 C. This is because more of the circulation can be directed to working muscle, as less is required for heat dissipation

Cold Exposure

Cold-water immersion can result in much more dramatic and rapid heat loss. Water is 25 times more conductive than air

Adopt HELP Await rescue

If individuals choose self-rescue, they should avoid leg-only swimming and should swim at the fastest pace possible given their fitness and the distance to safety. HELP = heat escape lessening position (arms and legs curled up, covering armpits and groin)

Pressure Effects

Pressure of air at sea level = 1 atm or 760 mmHg. The weight of a column of water directly above a diver’s body (hydrostatic pressure) increases directly with increasing depth. Because the tissue of the body are largely water, they are non-compressible

Air cavities

Lungs, respiratory passages, sinus and middle ear spaces - where volume and pressure will change with increases or decreases in diving depth

Boyle’s Law

The volume of any gas varies inversely with the pressure on it if the pressure is doubled, volume is halved

2 Limits to snorkel size

1. Pressure effects

2. Increase in pulmonary dead space

Pressure effects

When breathing through a snorkel, the diver must inspire air at atmospheric pressure. At a depth of only 3 ft, the compressive force of water against the chest cavity is so large that the inspiratory muscles are usually unable to overcome external pressure and expand the thoracic cavity

Increase in Pulmonary dead space

Normal anatomical dead space = 150 mL, Dead space of regular snorkel = 150 mL

Alveolar ventilation equation

VA = (VT - VD) x FR or VA = VT - VD

VT = tidal wave, VD = dead space, VA = alveolar ventilation

Breath Hold Diving

As the skin diver descends, the air in the lungs is compressed → lung squeeze. When lung volume is compressed below residual volume → lung damage occurs as blood is sucked from the pulmonary capillaries into the alveoli

Normal maximal breath holding time after maximal inspiration of ambient air

Approximately 50-60 seconds

Paradoxical Drowning

Diver hyperventilates, hold breath, and dives down to a certain depth → gases in lung are compressed and partial pressure are increased → diver holds as long as possible and then starts to ascend → partial pressure of gases in lung decrease on ascent → PaO2 decreases below critical point → diver loses consciousness and drowns

Scuba

Self-contained underwater breathing apparatus

Equipment needed for scuba diving

• Mask and snorkel

• Clothing

• Weight belt

• Buoyancy compensator

• Tank and backpack

• Depth gauge and pressure gauge

• Single hose, two-stage regulator

Ambient water pressure at the diver’s mouth

2500 psi (when tank is full)

Open-circuit scuba

As diver starts to inspire, slight negative pressure causes inspiratory valve on demand regulator to open → air enter diver’s lungs. On exhalation, the exhaled air is discharged into the water

Potential Medical Problems Associated With Scuba Diving

1. Air Embolism

2. Pneumothorax Lung Collapse

3. Nitrogen Narcosis

4. The Bends

5. Oxygen Poisoning

6. Mask Squeeze

7. Middle Ear Squeeze

Embolus

Any material that enters and obstructs a blood vessel

Air Embolism

Diver inflates lungs and begins ascent from depth → diver doesn’t exhale → Pressure decreases as he ascends → air in alveoli expands to the point where alveoli rupture → air bubbles enter blood vessels → block an artery in heart or brain or other area → possibility of death

Pneumothorax: Lung Collaspe

Rupture of Alveoli → air pocket forms outside the lungs between the chest wall and lung tissue → continued expansion of this trapped air during ascent causes the ruptured lung to collapse

Nitrogen Narcosis

At depths over 30m the increased partial pressure and quantity of dissolved nitrogen produces and anesthetic effect on the CNS - effects similar to alcohol intoxication → decreased attentiveness, awareness of cold hallucinations

The Bends

Also called decompression sickness

The Bends Causes

If the diver ascends to the surface too rapidly after a deep, prolonged five, dissolved nitrogen moves out of solution and forms bubbles in body tissues and fluids. Pain usually first felt around joints within 4-6 hours after dive. If bubbles lodge in an artery → permanent damage or death

The Bends Treatment

Recompression in a hyperbaric chamber to force the nitrogen gas back into solution and then slow decompression

The Bends Prevention

Ascend to the surface in stages - decompression sops to allow sufficient time for nitrogen to diffuse from the tissues to the blood without bubbles forming

Oxygen Poisoning

Occurs when the inspired PO2 exceeds 1520 mmHg for longer than 30-60 minutes. Causes irritation of respiratory passages which progresses to pneumonia if exposure continues, muscle twitching, confusion, nausea, convulsions

Mask squeeze

Squeeze can be applied to all troubles that pressure can cause during decent, as a result of pressure differentials between two structures or spaces. If pressure is not equalized, a relative vacuum is created within the mask → blood vessels in and around the eyes rupture as the eyes bulge out of their sockets

Middle Ear Squeeze: Eustachian tube

A small membrane lined passage connecting the middle ear cavity and the back of the throat. Its purpose is to equilibrate the pressure within the ear cavity with the outside by transferring air to or from the lungs

If eustachian tube is either partially or totally blocked what happens?

Increasing pressure against the eardrum during descent is not met by an equal force from the interior, a relative vacuum is created in the middle ear and hemorrhage of tissues in the middle ear and possible rupture of the eardrum.

Middle Ear Squeeze Symptoms

Pain in the ears after descending only a few feet. Pain rapidly becomes severe as the descent continues