A&P Lecture 4.2: Gas Exchange

ANATOMY AND PHYSIOLOGY LECTURE 4.2

GAS EXCHANGE

The air we breathe is a gaseous mixture consisting mainly of nitrogen (78.62%) and oxygen (20.84%), with traces of carbon dioxide (0.04%), water vapor (0.05%), helium, and argon. The atmospheric pressure at sea level is about 760 mm Hg. Partial pressure is the pressure exerted by each type of gas in a mixture of gases. The partial pressure of a gas is proportional to the concentration of that gas in the mixture. The total pressure exerted by the gaseous mixture is equal to the sum of the partial pressures.

PARTIAL PRESSURE OF GASES

Based on these facts, the partial pressures of nitrogen and oxygen can be calculated. The partial pressure of nitrogen is 79% of 760 (0.79 × 760), or 600 mm Hg; that of oxygen is 21% of 760 (0.21 × 760), or 160 mm Hg. Chart 21-3 spells out terms and abbreviations related to partial pressure of gases.

Once the air enters the trachea, it becomes fully saturated with water vapor, which displaces some of the gases so that the air pressure within the lung remains equal to the air pressure outside (760 mm Hg). Water vapor exerts a pressure of 47 mm Hg when it fully saturates a mixture of gases at the body temperature of

37°C (98.6°F). Nitrogen and oxygen are responsible for the remaining 713 mm Hg (760 − 47) pressure. Once this mixture enters the alveoli, it is further diluted by carbon dioxide. In the alveoli, the water vapor continues to exert a pressure of 47 mm Hg. The remaining 713 mm Hg pressure is now exerted as follows:

nitrogen, 569 mm Hg (74.9%); oxygen, 104 mm Hg (13.6%); and carbon dioxide, 40 mm Hg (5.3%).

PARTIAL PRESSURE IN GAS EXCHANGE

When a gas is exposed to a liquid, the gas dissolves in the liquid until an equilibrium is reached. The dissolved gas also exerts a partial pressure. At equilibrium, the partial pressure of the gas in the liquid is the same as the partial pressure of the gas in the gaseous mixture. Oxygenation of venous blood in the lung illustrates this point. In the lung, venous blood and alveolar oxygen are separated by a very thin alveolar membrane. Oxygen diffuses across this membrane to dissolve in the blood until the partial pressure of oxygen in the blood is the same as that in the alveoli (104 mm Hg). However, because carbon dioxide is a byproduct

of oxidation in the cells, venous blood contains carbon dioxide at a higher partial pressure than that in the alveolar gas. In the lung, carbon dioxide diffuses out of venous blood into the alveolar gas. At equilibrium, the partial pressure of carbon dioxide in the blood and in alveolar gas is the same.

EFFECTS OF PRESSURE ON OXYGEN TRANSPORT

Oxygen and carbon dioxide are transported simultaneously dissolved in blood or combined with some of the elements of blood. Oxygen is carried in the blood in two forms: first as physically dis-olved oxygen in the plasma, and second in combination with the hemoglobin of the red blood cells. Each 100 mL of normal arterial blood carries 0.3 mL of oxygen physically dissolved in the plasma and 20 mL of oxygen in combination with hemoglobin. Large amounts of oxygen can be transported in the blood because it combines easily with hemoglobin to form oxyhemoglobin: O2 + Hgb ↔ HgbO2

The volume of oxygen physically dissolved in the plasma varies directly with the partial pressure of oxygen in the arteries (PaO2). The higher the PaO2, the greater the amount of oxygen dissolved. For example, at a PaO2 of 10 mm Hg, 0.03 mL of oxygen is dissolved in 100 mL of plasma. At 20 mm Hg, twice this amount is dissolved in plasma, and at 100 mm Hg, 10 times this amount is dissolved. Therefore, the amount of dissolved oxygen is directly proportional to the partial pressure, regardless of how high the oxygen pressure rises.

The amount of oxygen that combines with hemoglobin also depends on PaO2, but only up to a PaO2 of about 150 mm Hg. When the PaO2 is 150 mm Hg, hemoglobin is 100% saturated and will not combine with any additional oxygen. When hemoglobin is 100% saturated, 1 g of hemoglobin will combine with 1.34 mL

of oxygen. Therefore, in a person with 14 g/dL of hemoglobin, each 100 mL of blood will contain about 19 mL of oxygen associated with hemoglobin. If the PaO2 is less than 150 mm Hg, the percentage of hemoglobin saturated with oxygen is lower. For example, at a PaO2 of 100 mm Hg (normal value), saturation is 97%; at a PaO2 of 40 mm Hg, saturation is 70%. 

OXYHEMOGLOBIN DISSOCIATION CURVE

The oxyhemoglobin dissociation curve  shows the relationship between the partial pressure of oxygen (PaO2) and the percentage of saturation of oxygen (SaO2). The percentage of saturation can be affected by the following factors: carbon dioxide, hydrogen ion concentration, temperature, and 2,3-diphosphoglycerate.

A rise in these factors shifts the curve to the right so that more oxygen is then released to the tissues at the same PaO2. A reduction in these factors causes the curve to shift to the left, making the bond between oxygen and hemoglobin stronger, so that less oxygen is given up to the tissues at the same PaO2. The unusual shape of the oxyhemoglobin dissociation curve is a distinct advantage to the patient for two reasons:

1. If the arterial PO2 decreases from 100 to 80 mm Hg as a result of lung disease or heart disease, the hemoglobin of the arterial blood remains almost maximally saturated (94%) and the tissues will not suffer from hypoxia.

2. When the arterial blood passes into tissue capillaries and is exposed to the tissue tension of oxygen (about 40 mm Hg), hemoglobin gives up large quantities of oxygen for use by the tissues.

Clinical Significance. 

The normal value of PaO2 is 80 to 100 mm Hg (95% to 98% saturation). With this level of oxygenation,

there is a 15% margin of excess oxygen available to the tissues. With a normal hemoglobin level of 15 mg/dL and a PaO2 level of 40 mm Hg (oxygen saturation 75%), there is adequate oxygen available for the tissues but no reserve for physiologic stresses that increase tissue oxygen demand. When a serious incident occurs

(eg, bronchospasm, aspiration, hypotension, or cardiac dysrhythmias) that reduces the intake of oxygen from the lungs, tissue hypoxia will result.

An important consideration in the transport of oxygen is cardiac output, which determines the amount of oxygen delivered to the body and which affects lung and tissue perfusion. If the cardiac output is normal (5 L/min), the amount of oxygen delivered to the body per minute is normal. If cardiac output falls, the amount ofoxygen delivered to the tissues also falls. Under normal conditions, most of the oxygen delivered to the body is not used. In fact, only 250 mL of oxygen is used per minute. Under normal conditions, this is approximately 25% of available oxygen. The rest of the oxygen returns to the right side of the heart, and the PaO2 of venous blood drops from 80 to 100 mm Hg to about 40 mm Hg.

Carbon Dioxide Transport
At the same time that oxygen diffuses from the blood into the tissues, carbon dioxide diffuses from tissue cells to blood and is transported to the lungs for excretion. The amount of carbon dioxide in transit is one of the major determinants of the acid–base balance of the body. Normally, only 6% of the venous carbon dioxide is removed in the lungs, and  nough remains in the arterial blood to exert a pressure of 40 mm Hg. Most of the carbon dioxide (90%) is carried by red blood cells; the small portion (5%) that remains dissolved in the plasma (partial pressure of carbon dioxide [PCO2]) is the critical factor that determines carbon dioxide movement in or out of the blood. Although the many processes involved in respiratory gas transport seem to occur in intermittent stages, the changes are rapid, simultaneous, and continuous.

 Neurologic Control of Ventilation
Resting respiration is the result of cyclic excitation of the respiratory muscles by the phrenic nerve. The rhythm of breathing is controlled by respiratory centers in the brain. The inspiratory and expiratory centers in the medulla oblongata and pons control the rate and depth of ventilation to meet the body’s metabolic demands.

The apneustic center in the lower pons stimulates the inspiratory medullary center to promote deep, prolonged inspirations. The pneumotaxic center in the upper pons is thought to control the pattern of respirations.

Several groups of receptor sites assist in the brain’s control of respiratory function. The central chemoreceptors, located in the medulla, respond to chemical changes in the cerebrospinal fluid, which result from chemical changes in the blood. These receptors respond to an increase or decrease in the pH and convey a message to the lungs to change the depth and then the rate of ventilation to correct the imbalance. The peripheral chemoreceptors are located in the aortic arch and the carotid arteries and respond first to changes in PaO2, then to partial pressure of carbon dioxide (PaCO2) and pH. The Hering-Breuer reflex is activated by stretch receptors in the alveoli. When the lungs are distended, inspiration is inhibited; as a result, the lungs do not become over distended. In addition, proprioceptors in the muscles and joints respond to body movements, such as exercise, causing an increase in ventilation. Thus, range-of motion exercises in an immobile patient stimulate breathing. Baroreceptors, also located in the aortic and carotid bodies, respond to an increase or decrease in arterial blood pressure and cause reflex hypoventilation or hyperventilation.