External Respiration

Pulmonary diffusion - exchange of oxygen and carbon dioxide between the lungs and the blood

Replenishes blood's oxygen supply that has been depleted for oxidative energy production

Removes carbon dioxide from returning venous blood

Occurs across the thin respiratory membrane

Pulmonary Circulation

Pulmonary circulation - serves the external respiratory function

Bronchial circulation - supplies the internal respiration needs of the lung tissue itself




Laws of Gases

Dalton's Law: The total pressure of a mixture of gases equals the sum of the partial pressures of the individual gases in the mixture.

Henry's Law: Gases dissolve in liquids in proportion to their partial pressures, depending on their solubility in the specific fluids and depending on the temperature.

Partial Pressures of Air

Standard atmospheric pressure (at sea level) = 760 mmHg

Nitrogen (N2) is 79.04% of air; the partial pressure of nitrogen (PN2) = 600.7 mmHg

Oxygen (O2) is 20.93% of air; PO2 = 159.1 mmHg

Carbon dioxide (CO2) is 0.03%; PCO2 = 0.2 mmHg

Did You Know…?

The solubility of a gas in blood and the temperature of blood are relatively constant. Differences in the partial pressures of gases in the alveoli and in the blood create a pressure gradient across the respiratory membrane. This difference in pressures leads to diffusion of gases across the respiratory membrane. The greater the pressure gradient, the more rapidly oxygen diffuses across it.


Partial Pressures of Respiratory Gases at Sea Level

Key Points:Pulmonary Diffusion

Pulmonary diffusion is the process by which gases are exchanged across the respiratory membrane in the alveoli to the blood and vice versa.

The amount of gas exchange depends on the partial pressure of each gas.

Gases diffuse along a pressure gradient, moving from an area of higher pressure to lower pressure.

Oxygen diffusion capacity increases as you move from rest to exercise.

The pressure gradient for carbon dioxide exchange is less than for oxygen exchange, but carbon dioxide’s membrane solubility is 20 times greater than oxygen, so CO2 crosses the membrane easily.

Oxygen Transport

Hemoglobin concentration largely determines the oxygen-carrying capacity of blood.

Increased H+ (acidity) and temperature of a muscle allows more oxygen to be unloaded there.

Training affects oxygen transport in muscle.

Carbon Dioxide Transport

Dissolved in blood plasma (7% to 10%)

As bicarbonate ions resulting from the dissociation of bicarbonate ions (60% to 70%)

Bound to hemoglobin (carbaminohemoglobin)
(20% to 33%)



Did You Know…?

The increase in a-vO2 diff during strenuous exercise reflects increased oxygen use by muscle cells. This use increases oxygen removal from arterial blood, resulting in a decreased venous oxygen concentration.

Factors of Oxygen Uptake and Delivery

Oxygen content of blood

Amount of blood flow

Local conditions within the muscle


Key Points: External and Internal Respiration

Oxygen is largely transported in the blood bound to hemoglobin and in small amounts by dissolving in blood plasma.

Hemoglobin saturation decreases when PO2 or pH decreases, or if temperature increases. These factors increase oxygen unloading in a tissue that needs it.

Hemoglobin is usually 98% saturated with oxygen which is higher than what our bodies require, so the blood's oxygen-carrying capacity seldom limits performance.

Carbon dioxide is transported in the blood as bicarbonate ion, in blood plasma or bound to hemoglobin.

The a-vO2 diff - difference in the oxygen content of arterial and venous blood - reflects the amount of oxygen taken up by the tissues.

Carbon dioxide exchange at the tissues is similar to oxygen exchange except that it leaves the muscles and enters the blood to be transported to the lungs for clearance.

Regulators of Pulmonary Ventilation at Rest

Higher brain centers

Chemical changes within the body


Muscle mechanoreceptors

Hypothalamic input

Conscious control


Pulmonary Control

Pulmonary Ventilation

Ventilation (VE) is the product of tidal volume (TV) and breathing frequency (f): VE = VT f

Breathing Problems During Exercise

Dyspnea—shortness of breath. During exercise this is most often caused by inability to readjust the blood PCO2 and H+ due to poor conditioning of respiratory muscles.

Hyperventilation—increase in ventilation that exceeds the metabolic need for oxygen. Voluntary hyperventilation reduces the ventilatory drive by increasing blood pH.

Valsalva maneuver—a breathing technique to trap and pressurize air in the lungs; if held for an extending period, it can reduce cardiac output. This technique is often used during heavy lifts and can be dangerous.

Did You Know…?

Ventilation tends to match the rate of energy metabolism during mild steady-state activity. Both vary in proportion to the volume of oxygen consumed (VO2) and the volume of carbon dioxide produced by the body (VE).

Ventilatory Equivalent for Oxygen

The ratio between VE and VO2 in a given time frame indicates breathing economy

At rest - VE/VO2 = 23 to 28 L of air breathed per L VO2 per minute

At max exercise - VE/VO2 = 30 L of air per L VO2 per minute

Generally VE/VO2 remains relatively constant over a wide range of exercise levels

The Ventilatory Breakpoint

The point during intense exercise at which ventilation increases disproportionately to the oxygen consumption.

When work rate exceeds 55% to 70% VO2max, energy must be derived from glycolysis.

Glycolysis increases CO2 levels, which triggers a respiratory response and increased ventilation.


Anaerobic Threshold

Point during intense exercise at which metabolism becomes anaerobic

Reflects the lactate threshold under most conditions, though the relationship is not always exact

Identified by noting an increase in VE/VO2 without an concomitant increase in the ventilatory equivalent for carbon dioxide (VE/VCO2)


Key Points:Pulmonary Ventilation

The respiratory centers in the brain stem set the rate and depth of breathing.

Chemoreceptors respond to increases in CO2 and H+ concentrations or to decreases in blood oxygen levels by increasing respiration.

Ventilation increases upon exercise due to inspiratory stimulation from muscle activity which causes an increase in muscle temperature and chemical changes in the arterial blood (which further increase ventilation).

Breathing problems associated with exercise include dyspnea, hyperventilation, and the Valsalva maneuver.

During mild, steady-state exercise, ventilation parallels oxygen uptake.

The ventilatory breakpoint is the point at which ventilation increases though oxygen consumption does not.

Anaerobic threshold is identified as the point at which VE/VO2 shows a sudden increase, while VE/VCO2 stays stable. It generally reflects lactate threshold.

Respiratory Limitations to Performance

Respiratory muscles may use more than 15% of total oxygen consumed during heavy exercise and seem to be more resistant to fatigue during long-term activity than muscles of the extremities.

Pulmonary ventilation is usually not a limiting factor for performance, even during maximal effort, though it can limit performance in highly trained people.

Airway resistance and gas diffusion usually do not limit performance in normal healthy individuals, but abnormal or obstructive respiratory disorders can limit performance.

Respiratory Regulation of Acid-Base Balance

Excess H+ (decreased pH) impairs muscle contractility and ATP formation

The respiratory system helps regulate acid-base balance by increasing respiration when H+ levels rise. The increase in respiration allows more CO2 to be released in the blood (bound to bicarbonate) to be transported to the lungs for exhalation.

Whenever H+ levels begin to rise, from carbon dioxide or lactate accumulation, bicarbonate ions can buffer the H+ to prevent acidosis.