Respiration and Gas Transport

When muscles work harder than the oxygen supply can support, they switch to lactic acid fermentation. The accumulation of lactic acid is associated with the burning sensation felt in muscles during intense exercise.

The prefix 'an-' means without, and 'aerobic' refers to oxygen. Thus, anaerobic respiration is the process of generating energy without the use of oxygen.

Blood is red because of hemoglobin, an iron-rich protein in red blood cells that binds to oxygen and reflects red light.

The lungs are highly efficient at removing volatile acids like carbonic acid; they can eliminate the equivalent of approximately 30 liters of carbonic acid per day through the expiration of CO2.

Carbonic acid (H2CO3) is a volatile acid. An excess of H2CO3 in the blood, typically caused by hypoventilation, leads to respiratory acidosis.

The bicarbonate buffer system maintains a normal arterial blood pH of 7.4 by keeping a ratio of 20 parts bicarbonate to 1 part carbonic acid.

Oxygen moves from the alveoli into the blood via simple diffusion, which is a type of passive transport that does not require energy.

The water reabsorption in the distal part of the nephron is in accordance with the body's state of hydration, under the influence of ADH and is termed as 'facultative reabsorption of water'.  ADH is secreted in the hypothalamus of brain and released into blood stream from the pituitary gland. The collecting ducts become more permeable to water due to the effect of ADH, so more water is reabsorbed by osmosis and no expenditure of ATP energy is required. The release of ADH is triggered when osmoreceptors in the hypothalamus detect an increase in osmolarity of the blood above a set point of ${300 mosm L^{-1}}$. In this situation the osmoreceptors cells also promotes thirst. Drinking reduces the osmolarity of the blood, which inhibits the secretion of ADH.

BOD refers to the Biological Oxygen Demand. It refers to the amount of the oxygen required by the aerobic bacteria to decompose the organic compounds present in the water body. If the BOD of a water body is low, it is considered to be less polluted. The high BOD shows a large level of pollution because more organic pollutants are present now and the bacteria will need more oxygen to decompose them.

In the alveoli, the oxygen is continuously absorbed by the blood and CO$ _{2}$ is continuously added by the blood. So, the alveolar air has more CO$ _{2}$ concentration than the expired air.

Inhaled air contains more oxygen used to create energy and less CO$ _2$. It contains 21% O$ _2$, 0.04% CO$ _2$ and 79% Nitrogen. Exhaled air contains 16% O$ _2$, 4% CO$ _2$ and 79% Nitrogen. Thus, option C is correct. 

Inhaled air contains more oxygen used to create energy and less CO$ _2$. It contains 21% O$ _2$, 0.04% CO$ _2$ and 79% Nitrogen. Thus, option A is correct. 

External respiration also known as breathing refers to a process of inhaling oxygen from the air into the lungs and expelling carbon dioxide from the lungs to the air. Exchange of gases both in and out of the blood occurs simultaneously. External respiration is a physical process during which oxygen is taken up by capillaries of lung alveoli and carbon dioxide is released from blood. Internal respiration or tissue respiration/cellular respiration refers to a metabolic process in which oxygen is released to tissues or living cells and carbon dioxide is absorbed by the blood. Once inside the cell the oxygen is used for producing energy in the form of ATP or adenosine triphosphate. Therefore, the correct answer is option D.

Inhaled air contains more oxygen used to create energy and less carbon dioxide. Exhaled air which contains more carbon dioxide produced as a waste product of energy production and less oxygen as it has been used in respiration. Inhaled air contains 21% oxygen, 0.04% CO$ _2$ and 79% N$ _2$. Exhaled air contains 16% oxygen, 4% CO$ _2$ and 79% N$ _2$.

Inhaled air contains more oxygen used to create energy and less carbon dioxide. Exhaled air which contains more carbon dioxide produced as a waste product of energy production and less oxygen as it has been used in respiration. Inhaled air contains 21% oxygen, 0.04% CO$ _2$ and 79% N$ _2$. Exhaled air contains 16% oxygen, 4% CO$ _2$ and 79% N$ _2$.