trigraph. noun. 1  Three letters spelling one consonant, vowel, or diphthong, such as Sch in Schiller or igh in high or thigh. 2  A group of three letters, especially of frequent occurrence in a given language, as the or ing in English or gli in Italian.3  Any combination of three letters of an alphabet. 4  Also called trigram.rigraph. noun. 1  Three letters spelling one consonant, vowel, or diphthong, such as Sch in Schiller or igh in high or thigh. 2  A group of three letters, especially of frequent occurrence in a given language, as the or ing in English or gli in Italian. 3  Any combination of three letters of an alphabet. 4  Also called trigram.
Oxygen-carbon dioxide-The process by which oxygen passes from the air in the LUNGS to the HEMOGLOBIN in the BLOOD, and carbon dioxide from the hemoglobin passes into the air in the lungs.Oxygen-carbon dioxide exchange is fundamental to life and is the primary function of the lungs.Oxygen-carbon dioxide exchange takes place between the alveoli, the tiny bubblelike sacs deep within the lungs, and the capillaries, the tiniest blood vessels of the cardiovascular system.xygen and carbon dioxide molecules (as well as the molecules of other gases such as nitrogen and highly toxic carbon monoxide) can easily pass through the walls of the alveoli and the capillaries, moving in the direction of least resistance. Oxygen molecules move from the alveoli into the capillaries with inhalation.
Words to Know. Alveoli: Tiny air-filled sacs in the lungs where the exchange of oxygen and carbon dioxide occurs between the lungs and the bloodstream. Bronchi: Two main branches of the trachea leading into the lungs.Bronchial tree: Branching, air-conducting subdivisions of the bronchi in the lungs.he exchange of these gases occurs across cell membranes both in the lungs (external respiration) and in the body tissues (internal respiration). Breathing, or pulmonary ventilation, describes the process of inhaling and exhaling air. The human respiratory system consists of the respiratory tract and the lungs.