The hilum of the lung is a triangular part which allows the other structures forming the root of the lung to enter and exit. It permits arteries, veins, nerves, bronchi and other structures to enter and exit.

During the act of expiration, the diaphragm potentially relaxes and the elastic recoil of the lungs, chest wall, and abductor muscles compresses the lungs.

It is a conical extension of the right ventricle in the heart of mammals, from which the pulmonary artery originates.

It is the volume of gas remaining in the lungs at the end of a normal tidal expiration. It is the balance point between the inward elastic recoil of the lungs and the outward elastic recoil of the chest wall.

It extends from the base of the skull to the upper surface of the soft palate. It includes the space between the internal nares and the soft palate, and lies above the oral cavity. The adenoids are lymphoid tissue structures located in the posterior wall of the nasopharynx.

The lower respiratory tract or lower airway consists of the trachea, bronchi (primary, secondary and tertiary), bronchioles (including terminal and respiratory) and lungs.

Stomata are the openings that are generally present on the lower surface of the leaves through which the gases and water vapour diffuse in and out easily. The oxygen diffuses in through the stomata and then enters the leaf cells. Similarly, the carbon dioxide produced by the leaf cells diffuses out through the stomata.

Lungs filter gas micro-bubbles occurring in the venous blood stream such as those created during decompression after underwater diving. They also filter out small blood clots formed in the veins.

The cricoid cartilage is the only complete ring of cartilage around the trachea.

Cytochrome a3 helps in the transfer of electrons to oxygen. The oxygen has great affinity to accept the electrons and in the presence of protons, a water molecule is formed.

HMP pathway generates NADPH molecules which are used as reductants in biosynthetic process under conditions when NADPH molecules are not generated by photosynthesis. It is, therefore, important in non- photosynthetic tissues such as in differentiating tissues, generating seeds and during periods of darkness. Production of NADPH is not linked to ATP generation in pentose phosphate pathway.

Acetyl-CoA is an important biochemical molecule in cellular respiration. It is produced in the second step of aerobic respiration after glycolysis and carries the carbon atoms of the acetyl group to the TCA cycle to be oxidised for energy production. It is produced by decarboxylation of pyruvate in the mitochondrial matrix.

During glycolysis, NAD (Nicotinamide adenine dinucleotide) removes electrons from 1, 3- diphosphoglyceric acid using diphosphoglycrealdehyde dehydrogenase. NAD changes to NADH2 and this is either utilised as such in anaerobic respiration or in the presence of oxygen.

Mitochondrion is the organelle which bears various enzymes participating in Krebs cycle. Each mitochondrion is covered by a double membrane. The inner membrane is selectively permeable and forms foldings called cristae. The inner membrane bears oxysomes, enzymes of fatty acids, succinate dehydrogenase (of Krebs cycle) and the electron transport system. All other enzymes of Krebs cycle are present in the mitochondrial matrix.

Chemoautotrophs are organisms that are capable of manufacturing their organic food utilising chemical energy released in the oxidation of some inorganic substances. The process of manufacture of food in such organisms is called chemosynthesis. It includes some acrobic bacteria.

The last step of aerobic respiration is the oxidation of reduced coenzymes, i.e. NADH₂ and FADH₂ by molecular oxygen through FAD, ubiquinone, cyt. f, cyt. c, cyt. c, cyt. a and cyt. ay. By oxidation of 1 molecule of NADH, 3 ATP molecules are produced and by oxidation of 1 molecule of FADH₂, 2 ATP molecules are produced. So, the greatest number of ATP molecules are produced in the electron transport chain.

Sound is generated in the larynx, and that is where the pitch and volume are manipulated. The strength of expiration from the lungs also contributes to loudness.

Respiration is an energy-liberating, enzymatically-controlled multistep catabolic process of stepwise breakdown of organic substances (hexose sugar) inside the living cells. Aerobic respiration includes the 3 major processes - glycolysis, Krebs cycle and electron transport chain. The substrate is completely broken down to form CO₂ and water. A large amount of energy is released stepwise in the form of ATP.

Glycolysis or EMP pathway is the breakdown of glucose to two molecules of pyruvic acid through a series of enzyme mediated reaction releasing energy. Pyruvic acid is a 3-carbon compound. In glycolysis, net gain of 2ATP and 2 NADH2 molecules occurs.

Each lung receives air from a single, large primary bronchus. As the primary bronchi enter the lungs, they branch off into smaller secondary bronchi that carry air to each lobe of the lung.