gas exchange

Gas exchange is a physiological process involving the uptake of O2 from the environment and the discharge of CO2 into the environment.

Partial Pressure:
- Defined as the pressure exerted by a specific gas within a mixture of gases.
- Also applicable to gases dissolved in liquids, including water.
O2 Solubility:
- Oxygen is significantly less soluble in water compared to air.

Breathing in air is generally easier and does not require high efficiency.
In terms of volume, water contains less O2 than air, necessitating a more efficient method of O2 extraction from water.

Function: Gills are outfoldings of the body creating a large surface area for gas exchange.
Ventilation: Movement of the respiratory medium over the respiratory surface is termed ventilation.
Aquatic Animals:
- They either move through water or move water over their gills for effective ventilation.
Countercurrent Exchange in Fish Gills:
- Blood flows in the opposite direction to water over the gills.
- Blood is always less saturated with O2 than the water it meets
- More than 80% of the O2 dissolved in water is extracted by fish gills as water passes over them.

Insects possess a tracheal system consisting of a network of branching tubes extending throughout their bodies.
- Tracheal tubes deliver O2 directly to body cells
- Larger insects must actively ventilate their tracheal systems to satisfy O2 demands. (Large insects have insufficient ventilation)

Infoldings of the body surface that facilitate gas exchange.
The circulatory system (either open or closed) is responsible for transporting gases between the lungs and rest of the body.
The complexity of a lung system is positively correlated with an animal’s metabolic rate.

System of branching ducts convey air to the lungs
Air enters through the nostrils, is filtered, warmed, humidified, and sampled for odors.
Pharynx directs air to lungs as well as food to the stomach
The airflow path:
- Nostrils → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
- Exhaled air passes over vocal cords in the larynx (which can create sounds)
- Cilia and mucus line the epithelium of air ducts and moves particles up to the pharynx (cleans and purifies)
- ‘Mucus escalator’ cleans respiratory system, allows particles to be swallowed into esophagus
Alveoli:
- Tiny air sacs at the tips of bronchioles where gas exchange takes place.
- O2 diffuses into capillaries from the moist films of the epithelium of alveoli, while CO2 diffuses from the capillaries across the epithelium into the air space.

Process that ventilates lungs is breathing
Amphibian Breathing:
Amphibians like frogs use positive pressure breathing, forcing air down the trachea. (Swallowing of air)
Bird Breathing:
- Birds utilize air sacs as bellows for continuous airflow through lungs during inhalation and exhalation.
- Passage of air through entire system of lungs and air sacs requires 2 cycles of inhalation and exhalation
- Air passes unidirectionally through lung systems, enhancing ventilation efficiency.
Mammalian Breathing:
- Reptiles and mammals use negative pressure breathing, drawing air into the lungs by increasing thoracic volume as rib muscles and diaphragm contract.
- Tidal volume: volume of air inhaled with each breath
- Max tidal volume is the vital capacity
- After exhalation, residual volume of air remains in lungs

Regulation of breathing occurs via involuntary mechanisms located in the medulla oblongata of the brain.
Breathing Control Centers:
- These centers monitor pH changes in cerebrospinal fluid to adjust the rate and depth of breathing as necessary.
- Sensors in the aorta and carotid arteries monitor O2 and CO2 levels and signal the brain accordingly.

During inhalation, fresh air mixes with existing lung air, resulting in a higher O2 pressure than in blood flowing through capillaries.
In alveoli O2 diffuses into the blood while CO2 diffuses out into the air.
By the time blood leaves the lungs, the pressures of O2 and CO2 match values for air in alveoli
I systemic capillaries, gradients of partial pressure favors net diffusion of O2 out of blood and CO2 into blood
Having unloaded O2 and loaded CO2, blood is returned to the heart and pumped to lungs again

Respiratory Pigments:
- Proteins such as hemoglobin with erythrocytes (vertebrates) and hemocyanin with copper (arthropods and molluscs) are critical in transporting O2 effectively.
- Hemoglobin's structure allows it to carry four O2 molecules for each iron containing heme group
- Hemoglobin dissociation curve shows that small change in the partial pressure of O2 can result in a large change. In delivery of O2
- The Bohr Shift:
- CO2 produced during cellular metabolism decreases blood pH, affecting hemoglobin's affinity for O2, thereby promoting O2 release in tissues.
- Hemoglobin plays a minor role in transport of CO2 and assists in buffering the blood
Carbon Dioxide Transport:
- A small percentage (around 7%) of CO2 is transported in plasma or bound to hemoglobin.
- Most CO2 combines with water in erythrocytes to form carbonic acid, which dissociates into bicarbonate ions (HCO3−).

Diving mammals exhibit unique adaptations enabling extended submergence underwater without breathing.
- Examples include Weddell seals and Cuvier’s beaked whales.
- These animals can store O2 in myoglobin proteins within their muscles and use strategies to conserve O2 while diving.
- This includes adjusting buoyancy to glide, routing blood to essential organs, and deriving ATP production in muscles from fermentation once oxygen is depleted