Respiratory System (All Living Things)
The epidermis of the leaf is perforated with pores called stomata. A single stomata is formed by two specialized cells, the guard cells, whose curved surfaces face each other, forming a slit-like opening through which gases pass in and out of the leaf. Guard cells are responsible for opening and closing of the stomata and therefore control the gas exchange in plants. Beneath the stomata are cavities into which gases diffuse from the surrounding cells and intercellular space and dissolve on the wet surfaces of the cells. It then diffuses into the cells and used for photosynthesis. Water needed for photosynthesis diffuse into the cells from the veins. Oxygen produced diffuses out of the cells and moves out through the stomata.
Turgidity (fullness of the cell that will occur when water moves in the cell) of the guard cells is responsible for the opening and closing of the stomata. When the guard cells are turgid, the stomata opens. But when the guard cells lose water and shrink, the stomata closes. Thus, water is nec essary for gas exchange to take place in plants.
Gas Exchange in Animals
Direct Diffusion. Unicellular organisms like amoeba and Paramecium resort to direct diffusion of oxygen and carbon dioxide through the plasma membrane. Dissolved oxygen in the surrounding water which is in higher concentration than the cells body diffuses across the plasma membrane into the organism's body. While carbon-dioxide in higher concentration in the cell diffuses outward into the surrounding water.
Use of Gills. Large animals use gills for gas exchange. Gills contain blood vessels which carry the oxygen from the surrounding water to the cells and carbon dioxide from the cells to the water. Oxygen diffuses from the water across the moist gill surface and into the blood vessels. Carbon dioxide is transported in the blood to the gills and diffuses outward across the membranes of the gill cells.
Tracheal System. Tracheal system are found in insects. Air enters the body through the opening at the side of the abdomen called spiracles which lead to a system of tubes called tracheae. Tracheae are sub-divided into smaller tube called tracheoles containing a fluid which makes the inner surface of the tube moist that is necessary for diffusion to take place. Air is brought directly to the body cells by the tubes. Circulatory system plays no part in gas transport.
Book Lungs. These are found in spiders. Each "lung" has a number of leaf-like folds. Air enters the book lungs through a pair of slit-like openings on the under surface of the abdomen. Gas exchange takes place across the membranes of the folds. The blood system carries the gases from the lungs to all parts of the body.
Lung System. Terrestial vertebrates use lungs for gas exchange. Air enters through the nostril which usually leads to a nasal cavity which opens to the mouth or pharynx. In amphibians, the air passes through the mouth while in higher vertebrates, the air passes through the pharynx. The air then enters through the glottis (a flaplike structure, which is an opening at the floor of the pharynx). The glottis leads to the larynx (voice-box or adam's apple), to the trachea (windpipe), to the bronchi and into the lungs (lungs) (chief organ for breathing). Blood vessels line the lungs. These absorbed the oxygen in the lungs and release carbon dioxide into the lungs. Lungs are made up of spongy tissues divided into many air sacs called alveoli.
Gas Exchange in the Lungs. Dark red blood (poor in oxygen and rich in carbon dioxide) is carried to the heart from all parts of the body. This is pumped by the heart into the pulmonary arteries to the lungs. The blood ends up in a capillary (network of tiny blood vessels) surrounding the alveoli. Oxygen diffuses from the air space into the blood stream through the thin lining of the alveolus. The blood stream becomes bright red (oxygen rich) which move on through the capillary into the pulmonary veins back to the heart. At the same time that oxygen diffusion is taking place, carbon dioxide is diffusing from the blood into the air space.