Vertebrate Respiration and Digestion: An In-Depth Look
Pulmonary Respiration
Pulmonary respiration: The lungs are densely vascularized outgrowths of respiratory surfaces. They are typical of terrestrial vertebrates, although similar structures appear in some invertebrates, like terrestrial gastropod mollusks. In these mollusks, the mantle cavity becomes vascularized and is transformed into a lung chamber, lacking a ventilation mechanism. Therefore, these respiratory organs are called diffusion lungs.
The swim bladder is an organ that stores oxygen that circulates through your blood via active transport. It has several functions: flotation, accessory breathing, and sound production/amplification. In terrestrial vertebrates, the lungs are thin bags that arise from an evagination of the pharynx. They are placed inside and communicate with the outside via ventilation ducts: the bronchioles and trachea, also known as lung ventilation.
- Amphibians: Lung sacs without internal septa, therefore, they must complement respiration with cutaneous respiration.
- Reptiles: Although the lungs are loculated in chambers, pulmonary respiration is still not very effective.
- Birds: Air sacs are an adaptation to the huge demand for oxygen during flight. These expansions are where the walls of the lung sacs dilate and lodge in various body parts.
- Mammals: The lung alveoli of mammals are very numerous, highly vascular vesicles that increase the respiratory surface. These sockets are the end of the last branches of the bronchioles.
Mechanism of Gas Exchange
The mechanism of gas exchange is performed in two stages:
- Pulmonary ventilation: This is inspiration and expiration. The mechanism of inspiration varies. Amphibians introduce air into the lungs through an act of swallowing performed by the tongue. In birds, the compression of air sacs by wing muscles determines the motion of inhalation and exhalation through the lungs. In mammals, air enters the lungs from the outside when the volume of the ribcage increases, which is produced by the action of certain muscles. When thoracic volume increases, the pressure in the lungs decreases, and air enters. The reverse occurs during expiration.
- Gas exchange in the lungs: Gas exchange occurs in the lungs as a result of the different concentrations of gases between the air inside the lungs and the blood. When it reaches the lungs, blood has a very high concentration of CO2 and is very poor in O2. As the pressure of oxygen in the air is higher than in the blood that arrives, oxygen passes by diffusion through the walls of the alveoli and capillaries until the blood pressure is equal in both places. The CO2 exchange mechanism is in the opposite direction; that is, CO2 passes from the blood into the alveoli.
Oral Cavity
The mouth is surrounded by folds of skin called lips, except in turtles and birds. Inside the mouth are the teeth; the lower and upper teeth are all different shapes. Their function is to cut, grind, and crush food (mechanical digestion). The tongue is also in the mouth, which contains a large number of taste buds, and its function is to mix food and facilitate its transit into the pharynx. In fish, the tongue is not mobile due to a lack of musculature. In some reptiles and amphibians, it is triggered to catch insects. In birds, the tongue is corneal, and in mammals, it is very muscular. The oral cavity receives secretions from the salivary glands, which appear in all vertebrate groups except fish. Saliva is an alkaline liquid formed by 98% water, mineral salts, mucin (to facilitate swallowing), amylase, etc. Saliva has several functions: it destroys some bacteria ingested with food, begins chemical digestion, facilitates swallowing, etc.
Pharynx
The pharynx is a muscular tube involved in the swallowing process. It belongs to both the digestive and respiratory tracts; it is a place of transit for food and water to lower portions of the digestive tract. The pharynx has a fold (epiglottis) to prevent food from entering the respiratory system.
Esophagus
The esophagus is a muscular and straight canal whose contractions propel the bolus into the stomach. The combined action of these contractions produces peristalsis, which recalls the movement of an earthworm.