Woody flowering plants (trees and shrubs) have an external, impervious bark. Here, gaseous exchange occurs through small, powdery patches in the stem, called lenticels.
The mechanisms, which control the gaseous exchange and water movement, are the guard cells across the stomata. Stomata are found in the epidermis of leaves mainly. The open stomata of a leaf represent an extremely small area of the total leaf. Despite this the epidermis is not a serious barrier to the inward diffusion of carbon dioxide. However, when a deficit of water could threaten the life of the plant by desiccation, the stomata close automatically. Water loss is virtually prevented until the deficit can be made good.
However, gaseous exchange in mammals, including humans, occurs in a pair of lungs. Lungs are enclosed (with the heart) in an airtight compartment, the thorax, delineated by the thorax wall and the diaphragm.
Lungs are delicate, compact and highly elastic organs. Air is drawn into the lungs when pressure within is lower than atmospheric pressure. Air is forced out of the lungs when pressure within is higher than atmospheric pressure. Pressure changes in the lungs are brought about by changes in the volume of the thorax. Airflow in the mammal is tidal; air enters and leaves the lungs along the same route.
The bulk of the lung tissue consists of millions of microscopic air sacs (alveoli in grape-like clusters), each served by a tiny bronchiole. Networks of capillaries surround the alveoli; they are supplied with blood by the pulmonary artery and are ultimately drained by the pulmonary vein. Elastic connective tissue separates the clusters of alveoli. The 700 million alveoli in the lungs of an adult human provide an area for gaseous exchange of 100-150m² of lung surface.
The alveolar walls consist of squamous epithelium. Oxygen dissolves in the film of water on the surface of the wall. Dissolved oxygen diffuses across the epithelial cells and the capillary endothelial cells into the blood plasma. In the plasma, oxygen diffuses into the red cells (erythrocytes) and combines with haemoglobin to form oxyhaemoglobin. Carbon dioxide diffuses from the red cells and from the plasma into the alveoli.
The passage of blood through the lungs is slow. This is because the capillaries are extremely narrow, only just wide enough for red cells to squeeze through. But this slow passage of blood through the lungs facilitates gaseous exchange: there is time for oxygen to combine with haemoglobin and for carbon dioxide to come out of solution.
From these synopses of the mechanisms for gas exchange in flowering plants and mammals, it is apparent that there are significant differences between the structures; one structure relies upon pressure, dissolving of oxygen in moisture and diffusion, to the other structure which controls its intake, where chemical processes will occur providing there are no limiting factors. A further observation is that both mechanisms have opposing products, which they take in and fabricate as waste products.