The function of the respiratory system is to enable gas exchange between the fish and the water, a process that is necessary for the vital functions to be performed. In addition to respiration, in teleosts fish , the respiratory system has other functions such as osmoregulation, excretion of nitrogenous waste (ammonium), acid-base regulation and detoxification.
The basic structures for gas exchange in the water are the gills, which are located on either side of the pharynx (Fig. 1) and supported by gill bony arches in adults (cartilaginous in early stages of development). There are five gill arches separated by five gill slits (see Fig. 2 Musculoskeletal). The layer of epithelium lining the region of the gill arch is continuous with that of the pharynx. In bony fish, the gill system is bounded externally by an operculum on either side of the head, covering the opercular cavity. The posterior edge of the operculum has a branchiostegal membrane supported by rays, which help close the opercular cavity (Fig. 2).
Teleosts have four holobranches and one hemibranch on each side. Each holobranch is supported by a gill arch and is made of two divergent hemibranches that project from the outer edge of the gill arches (Fig. 3). Each hemibranch consists of a series of overlapping filaments, which alternate with filaments of the other hemibranch of the same arch (see Fig. 3), and contains a complex network of blood vessels (Circulatory system). Each filament is supported by a cartilaginous ray that provides support and flexibility and it has striated adductor and abductor muscles that enable their tips to move back and forth. Both faces of each filament have regularly distributed perpendicular folds called lamellae. They are arranged so that those on the upper face of a filament alternate with those on the lower face of the adjacent filament forming a mesh along the entire hemibranch. The lamella is the functional unit of the respiratory system, as it is the site at which gases dissolved in water and gases carried by red blood cells are exchanged. The number and size of these lamellae define the respiratory surface, which may vary considerably according to the habits of the species.
The blood pumped by the heart enters the gills and flows through a complex network of blood vessels . The gill arches have a system of parallel vessels (see Fig. 3), the afferent branchial arteries, which come from the ventral aorta, and the efferent branchial arteries, which open into the dorsal aorta. The afferent branchial artery leads into each filament, and these branches are called afferent filament arteries. They run along one edge of the filament and branch into a network of capillaries in each lamella. These capillaries run between contractile support cells called “pillar cells”. The flexibility of the erythrocyte membranes enables blood to flow through these narrow capillaries. These capillaries converge in the opposite side of the same filament and open into the efferent filament artery, which runs along the opposite edge and opens into the efferent branchial artery. Thus, the capillary system of the lamellae provides a large surface of exposure to water, enhancing gas exchange. Lamella are flap shaped folds protruding from the filaments and consist of a capillary meshwork, fully enveloped by epithelium. Numerous pillar cells are distributed among the blood capillaries giving certain structure to the lamella, as well as holding together the opposite sides of the epithelial coverage (Fig. 4).
The epithelium of the lamella is made up of a thin double cell layer separated by a space in which migrating inflammatory cells and/or resident macrophages may be seen. The inner layer of epithelial cells sits on the basement membrane which contact, at the opposite side, with the enlarged ends of the pillar cells. The basement membrane traverses the opposing inner faces of the lamella in grooves located within the pillar cells, thereby providing additional tensile support. The inner layer of epithelium is formed by fairly undifferentiated cells, while the outer layer formed of squamous epithelial cells, which make up most of the epithelial surface and have microridges, increasing the respiratory surface and may aid mucus retention (Fig. 4). There are also mucous cells, chloride cells, and to a lesser extent, granular and neuroepitelial cells. The mucous cells secrete a protective layer of mucus, which forms the contact surface between the fish and the water, acting as a physical, chemical and immunological barrier. Chloride cells are responsible for excreting chlorides and the trans-epithelial flow of other ions. Both, mucous and chloride cells are more plentiful at the base of the lamellae and more frequent in marine than in freshwater fish.
The rudimentary gill, opercular or hyoid hemibranches, are located dorsally on the underside of the operculum (Figs. 5 and 6). They may be free or lined with the mucosa of the opercular cavity. Although they have a respiratory function at the beginning of embryonic development, in adults its role is uncertain but they receive oxygenated blood from the aorta (which is why they are also referred as a pseudobranch) and communicate by means of vessels to the choroid rete in the eye. It is believed that it may play a role in blood supply to the retina, as well as in osmoregulation and sensing. The structure of opercular hemibranch is lacking only in a few species of silurids and eels.
During ventilation, water enters through the mouth, pass over oral cavity and exits through the gill slits, passing between the gills and out through the opercular opening (see Fig. 7). During inspiration, the mouth opens and the oral cavity enlarges, creating a vacuum while the opercula remain closed. When the opercula open, water flows in a single direction and exits. The skeletal muscle of the oral and opercular cavities, maintain this pumping action, forcing water through the gills. The adductor and abductor muscles located at the base of the filaments enable their tips to move back and forth. The proximity of filaments of adjacent holobranchs forces all the water that enters the oral cavity and passes between the gills to pass over the respiratory exchange tissue before being expelled. The flow of water is almost continuous because there is greater hydrostatic pressure in the oral cavity than in the opercular cavity during all phases of the breathing process. This breathing mechanism has modifications in fish with different habits, such as species that move continuously and swim with their mouths open so that water constantly enters passively. Ventilation is controlled by receptors that detect variations in the flow of water, CO2 pressure in the gills, and CO2 and oxygen pressure in the arteries, so that the nervous system produces changes in heart rate and ventilation rate.
Gas exchange takes place by means of a process called counter current, in which blood flows through the capillaries in a direction opposite to that of the water flow over the lamellae. This process noticeably optimizes gas exchange, which takes place by simple diffusion from the environment where the concentration is higher to that where the concentration is lower. Blood with high CO2 and low O2 pressure loses carbon dioxide and captures oxygen from the water. The efferent vessels then carry oxygen-rich blood to the dorsal aorta. Because the oxygen concentration in water is low, the respiratory process involves an enormous expenditure of energy. Insufficient oxygen dissolved in the water for metabolic requirements, or lesions of the respiratory epithelia, which make gas exchange inefficient, produces respiratory fatigue. The fragility of gill tissue plus constant exposure to the outside environment make gills vulnerable organs. As their function is so important any lesion, however small, may affect fish health. Some of the most common alterations macroscopically observed in gills are coloration changes (pallor or darkening), heavy mucus secretion, cotton-wool-like growth, nodules, haemorrhage, and fusion of lamellae. In addition to these anatomical alterations, there are clinical signs that help detect problems in the respiratory function such as gasping for air and abnormal opercular movements.