A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many arachnids, such as scorpions and spiders. Each of these organs is located inside an open ventral abdominal, air-filled cavity (atrium) and connects with its surroundings through a small opening for the purpose of respiration.
In this spider diagram, the position of the book lungs is labelled 1.
Underside of a female horseshoe crab showing the legs and book gills
The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals, the respiratory surface is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of small air sacs; in mammals and reptiles, these are called alveoli, and in birds, they are known as atria. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood. These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds, the bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.
Fig. 1. Respiratory system
Fig. 2. The lower respiratory tract, or "Respiratory Tree" Trachea Mainstem bronchus Lobar bronchus Segmental bronchus Bronchiole Alveolar duct Alveolus
Fig. 4 The effect of the muscles of inhalation in expanding the rib cage. The particular action illustrated here is called the pump handle movement of the rib cage.
Fig. 7 The muscles of breathing at rest: inhalation on the left, exhalation on the right. Contracting muscles are shown in red; relaxed muscles in blue. Contraction of the diaphragm generally contributes the most to the expansion of the chest cavity (light blue). However, at the same time, the intercostal muscles pull the ribs upwards (their effect is indicated by arrows) also causing the rib cage to expand during inhalation (see diagram on other side of the page). The relaxation of all these muscles during exhalation causes the rib cage and abdomen (light green) to elastically return to their resting positions. Compare with Fig. 6, the MRI video of the chest movements during the breathing cycle.
