Explain the mechanism involved and the pathway taken by oxygen in human beings from the atmosphere to the liver cells.

-Internal intercostal muscles relax; while external intercostal muscles contract; this causes the rib cage (ribs) to move upwards and outwards; the diaphragm muscles contract; which causes the diaphragm to flatten from its normal dome shape;
-This increases the volume of thoracic cavity; and decreases the pressure inside it; the atmospheric pressure being higher than the pressure inside the thoracic cavity forces air to rush into the lungs; through the nose and trachea inflating the lungs;
-Oxygen is in higher concentration in the alveolar cavity than blood (in the capillary); it then diffuses into the capillary through the epithelium of alveolus, capillary wall/endothelium and the plasma into the red blood cells where it combines with haemoglobin; to form unstable oxyhaemoglobin;
-Oxyhaemoglobin is transported in blood into the left atrium; through the pulmonary vein; to the left ventricle; through bicuspid valve; through the aorta; to the hepatic artery; and finally into the liver;