In respiration, air is inhaled into the lungs, which provide a large surface are

Question

In respiration, air is inhaled into the lungs, which provide a large surface area for transport of oxygen, carbon dioxide, and water to or from the blood. The transported oxygen is delivered to body cells where it oxidizes glucose and fats from food to produce carbon dioxide, water, and thermal energy. The carbon dioxide and water are transported from the cells to the blood and back to the lungs, from which they are exhaled. The exhaled gas is at body temperature and saturated with water, some produced in the glucose and fat oxidation and some additionally drawn from moist lung tissue. Other water-containing streams— food, water, perspiration, and excreted waste streams—maintain the body’s level of hydration.

Suppose an individual inhales air at 20°C and 20% relative humidity, and 25% of the inhaled oxygen is consumed in the glucose oxidation reaction. (Under certain conditions, it is reasonable to neglect the oxidation of fats.) A flowchart of the respiration–metabolism process is shown below. The water in the exhaled gas equals the inhaled water plus the water produced by glucose oxidation plus additional water drawn from the lungs. Qm represents only the heat transferred to or from the body as a consequence of the phenomena just described; not shown are heat and work transferred due to other bodily processes.

(a) Assume a basis of 1 mol bone-dry air (plus the water that goes with it) inhaled at 20°C. Draw and fully label a flowchart of the metabolic process, considering only breathing and glucose oxidation as components of the process. You don’t need to label the streams between the two units, since this problem will not require determining their masses or compositions. Do a degree-of-freedom analysis of the overall system.

(b) Suppose all of the water and CO2 produced from the glucose oxidation are transported to the lungs and exhaled, and calculate the masses of all components of all labeled streams.

(c) If the individual inhales an average of 500 mL of air per breath and takes 12 breaths per minute, how much water (fluid ounces) must she drink per day to make up for the water she loses by breathing?

(d) Returning to the original basis, estimate the heat (kJ) transferred to or from the body as a consequence of breathing and glucose oxidation, assuming that the glucose is oxidized at 37°C and at that temperature Hc= - 2816 kJ/mol:

Inhaled air H20 H3O (rom lung tissue) Glucose (from food) RESPIRATION (lungs) CO HRO METABOLISM 60, C H 6CO2 6H2O mi I Exhaled gas

Explanation / Answer

a.

Basis: 225 L/h Condensate n4(mol 02/h) n5(mol N2/h) '-----,----' ';6(mol H20 (v)/h) 225 liters H20 (I)/h n3(mol H20 (I)/h) (95% of water in feed) We first do the degree-of-freedom analysis. There are six unknowns on the chart-hi through h6 • We are allowed up to three material balances-Dne for each species. We must therefore find three additional relations to solve for all unknowns. One is the relationship between the volumetric and molar flow rates of the condensate: we can determine h3 from the given volumetric flow rate and the known specific gravity and molecular weight of liquid water. A second is the given fact that 95% of the water is condensed. This specification provides a relationship between h3 and hz (h 3 = 0.95I1z),

b.

When carbon dioxide diffuses into the blood plasma and then into the red blood cells (erythrocytes) in the presence of the catalyst carbonic anhydrase most CO2 reacts with water in the erythrocytes and the following dynamic equilibrium is established

H2O + CO2    H2CO3

Carbonic acid, H2CO3, dissociates to form hydrogen ions and hydrogencarbonate ions. This is also a reversible reaction and undissociated carbonic acid, hydrogen ions and hydrogencarbonate ions exist in dynamic equilibrium with one another

H2CO3    H+ + HCO3-

Inside the erythrocytes negatively charged HCO3- ions diffuse from the cytoplasm to the plasma. This is balanced by diffusion of chloride ions, Cl-, in the opposite direction, maintaining the balance of negative and positive ions either side. This is called the 'chloride shift'.

The dissociation of carbonic acid increases the acidity of the blood (decreases its pH). Hydrogen ions, H+, then react with oxyhaemoglobin to release bound oxygen and reduce the acidity of the blood. This buffering action allows large quantities of carbonic acid to be carried in the blood without major changes in blood pH.

Hb.4O2 + H+    HHb+ + 4O2

(Hb.4O2 is sometimes written HbO8.)

c.5 1/2 liters.

d. qm will be 30kJ

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