71. Phylogeny and development of respiratory system..
Traces how the respiratory system evolved, from gas exchange across the skin and pharyngeal slits in protochordates, through gills in jawless and jawed fishes, to lungs in lobe-finned fishes and tetrapods. It then compares ventilation and lung design in amphibians, reptiles, birds and mammals, and ends with embryonic development from the pharyngeal endoderm, including the role of FGF10 in branching and the effects of its absence. Four mnemonics summarise the evolutionary and developmental sequence, and recap questions close the note.
Inside: 10 sections · 15 flashcards · 11 min
Topic 72 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 10 sections, about 1,517 words (11 min of reading), with 15 flashcards.
In this note
Phylogeny and development of respiratory system.
Overview of the Respiratory System
Phylogeny of the Respiratory System
Evolution of Lungs and Transition to Land
Tetrapods and the Refinement of Lung-Based Respiration
Embryological Development of the Respiratory System
Branching Morphogenesis and Lung Formation
Developmental Disorders
RECAP QUESTIONS
MNEMONICS
Overview of the Respiratory System
The primary function of the respiratory system is to facilitate gas exchange, particularly the uptake of oxygen and release of carbon dioxide.
In vertebrates, respiratory organs develop from the endoderm of the pharynx and take two main forms:
Gills (used by aquatic animals for extracting oxygen from water)
Lungs (used by air-breathing animals for extracting oxygen from air)
Phylogeny of the Respiratory System
Protochordates (e.g., Amphioxus or lancelets)
Do not possess specialized respiratory organs.
Oxygen is absorbed directly through the body surface, which is effective due to their small size and low activity level.
Gas exchange also occurs across the walls of multiple pharyngeal slits, assisted by a steady water flow that increases contact with respiratory surfaces.
Early Vertebrates (Agnathans or jawless fishes)
With larger and more active bodies, early vertebrates could not rely solely on passive diffusion through the skin.
Gills evolved and were located within the walls of pharyngeal slits, converting them into functional branchial slits.
Modern agnathans possess between six and fifteen pairs of gills.
Gnathostome Fishes (Jawed Fishes)
Gills became more structurally complex, consisting of many thin, folded layers called lamellae, richly supplied with blood.
Most species have five pairs of gill slits.
In cartilaginous fishes, these gill slits are visible externally.
In bony fishes, gills are concealed by a bony cover known as the operculum, which is part of the hyoid arch. This forms a single shared gill chamber.
Some larval bony fish, such as African lungfish, have external, feather-like gills which are beneficial in low-oxygen environments.
Fish nostrils are used only for smell and not connected to the respiratory system. There are two pairs of nostrils—anterior and posterior—but no link to the pharynx.
Gills in fish also play a vital role in nitrogen excretion, mainly through the release of ammonia. This is why even air-breathing lungfish retain their gills.
Continue reading free
The whole note, all 10 sections, is free with an AceMed account, with its 15 flashcards. Premium adds flashcard recall, highlights and Ask AI.
