Respiration: Types, Processes, and Gas Exchange

Respiration: Concepts and Types

There are two types of respiration:

• A) Cellular or Internal Respiration: This is the use of oxygen by cells. It involves oxidizing complex organic molecules to obtain energy. This process produces CO2 and water as byproducts. Cellular respiration occurs in the mitochondria of the cell.
• B) External Respiration: This is the intake of oxygen and the output of carbon dioxide from the body. It occurs due to gas exchange across specialized respiratory surfaces. These surfaces must be very thin to allow for gas passage. They must also be wet, as gases dissolve in liquid before crossing. These surfaces are typically coated with a network of blood capillaries to facilitate the entry of oxygen and the exit of carbon dioxide.

Types of External Respiration

Depending on the animal's size, complexity, and habitat, external respiration occurs in the following ways:

• A) Simple Diffusion: Occurs in very simple animals. Gases are exchanged directly through the cell membranes of the cells.
• B) Cutaneous Respiration (Skin): Exchange takes place through the body wall. Typical of animals in aquatic or humid environments. The skin is kept moist to allow for the passage of gases.
• C) Tracheal Respiration: Common in terrestrial arthropods. The body wall invaginates, forming thousands of ramified, wet internal tubes called tracheae. These open to the outside through pores called spiracles. The tracheae are reinforced with chitin rings, except for the tracheoles, which communicate with the cells, and gas exchange occurs there.
• D) Branchial Respiration (Gills): Found in aquatic animals. It uses specialized structures called gills, where gaseous exchange occurs. The surface area for exchange is very large because oxygen is much less concentrated in water than in air. There are two types of gills:
  • External Gills: Lateral expansions of the skin that are highly folded and surrounded by blood vessels. Typical of larvae, annelids, etc.
  • Internal Gills: Endodermal folds surrounded by blood vessels. Found in mollusks and fish.

Pulmonary Respiration (Lungs)

Common in terrestrial animals. It is the most evolved respiratory system. To be effective and adapted to the terrestrial environment, the gas exchange surface is internal to prevent drying out when exposed to air. This led to the formation of lungs. There are two types of lungs:

• Diffusion Lungs: Primitive lungs. They are internal chambers whose walls are highly folded. Found in invertebrates like land spiders and snails.
• Ventilation Lungs: Highly evolved. They are internal chambers that branch into millions of tiny sacs called pulmonary alveoli, surrounded by capillaries. They communicate with the outside through respiratory tracts formed by cartilaginous tubes that progressively become more extreme and branched. Found in vertebrates.

Pulmonary Ventilation

This is the process of gas intake and output from the external environment to the gas exchange surface. In pulmonary respiration, ventilation occurs; in other types, it occurs only through passive diffusion, and there is no forced air movement.

In amphibians, ventilation occurs by a swallowing pump. They move the base of their mouth to fill their lungs.

In reptiles, birds, and mammals, ventilation occurs by suction. Air enters and exits the rib cage due to thoracic movements:

• A) Inspiration: Occurs by active contraction of the intercostal muscles and the diaphragm. The thoracic cage increases in volume, and the air pressure drops, filling the lungs.
• B) Expiration: Passive. Occurs when the rib cage muscles relax, and the rib cage returns to its normal volume, compressing the lungs and expelling the air.

Gas Exchange

Occurs by diffusion. The exchange happens in two places:

• A) In the Pulmonary Alveoli: Oxygen passes through the wall of the alveoli and enters the surrounding blood capillaries. In contrast, carbon dioxide, which is abundant in the blood of the alveoli, does the opposite.
• B) In the Tissues: Oxygen leaves the blood capillaries and spreads to the cells. Carbon dioxide leaves the cells and enters the capillaries.