oxygen is transported in blood through lungs , carbon dioxide is transported in blood through tissues to lungs

The primary function of the respiratory system is to exchange oxygen and carbon dioxide. Inhaled oxygen enters the lungs and reaches the alveoli. The layers of cells lining the alveoli and the surrounding capillaries are each only one cell thick and are in very close contact with each other. This barrier between air and blood averages about 1 micron (1/10,000 of a centimeter) in thickness. Oxygen passes quickly through this air-blood barrier into the blood in the capillaries. Similarly, carbon dioxide passes from the blood into the alveoli and is then exhaled.

Oxygenated blood travels from the lungs through the pulmonary veins and into the left side of the heart, which pumps the blood to the rest of the body (see Biology of the Heart and Blood Vessels: Function of the Heart). Oxygen-deficient, carbon dioxide-rich blood returns to the right side of the heart through two large veins, the superior vena cava and the inferior vena cava. Then the blood is pumped through the pulmonary artery to the lungs, where it picks up oxygen and releases carbon dioxide (see Biology of the Heart and Blood Vessels: Function of the Heart).

To support the exchange of oxygen and carbon dioxide, about 6 to 10 liters of air per minute are brought in and out of the lungs, and about three tenths of a liter of oxygen is transferred from the alveoli to the blood each minute, even when the person is at rest. At the same time, a similar volume of carbon dioxide moves from the blood to the alveoli and is exhaled. During exercise, it is possible to breathe in and out more than 100 liters of air per minute and extract 3 liters of oxygen from this air per minute. The rate at which oxygen is used by the body is one measure of the rate of energy expended by the body. Breathing in and out is accomplished by respiratory muscles (see Biology of the Lungs and Airways: Diaphragm's Role in Breathing).

Three processes are essential for the transfer of oxygen from the outside air to the blood flowing through the lungs: ventilation, diffusion, and perfusion. Ventilation is the process by which air moves in and out of the lungs. Diffusion is the spontaneous movement of gases, without the use of any energy or effort by the body, between the gas in the alveoli and the blood in the capillaries in the lungs. Perfusion is the process by which the cardiovascular system pumps blood throughout the lungs. The body's circulation is an essential link between the atmosphere, which contains oxygen, and the cells of the body, which consume oxygen. For example, the delivery of oxygen to the muscle cells throughout the body depends not only on the lungs but also on the ability of the blood to carry oxygen and on the ability of the circulation to transport blood to muscle.

Oxygen and carbon dioxide transport

A) Transport of Oxygen in the blood

· 97% of oxygen carried with Hb from lungs to tissues

· remaining 3% dissolved in plasma

· oxygen reversibly combines with Hb maximum amount of O2 that can combine with the Hb of blood:

i) 15 gms Hb per 100 ml blood

ii) 1 gm Hb combines with 1.34 ml O2

iii) 100 ml blood combines with 20 ml O2 [100% saturated]

· amount of O2 released from Hb in the tissues:

i) Normal arterial blood: 100 ml blood combines with 19.4 ml O2 [97% sat; PO2 95]

ii) Venous blood: 100 ml blood combines with 14.4 ml O2 [75% sat; PO2 40 mm Hg]

iii) Thus, 5ml of O2 is transported by each 100 ml blood through the tissues per cycle

· transport of O2 during exercise:

i) Exercise —> increased cellular O2 utilization -> decreased interstitual PO2 [15mmHg]

ii) Venous blood: 100 ml blood combines with 4.4 ml O2 [20% sat; PO2 18 mmHg]

iii) Thus, 15ml of O2 is transported by each 100 ml blood through the tissues per cycle

iv) Therefore, increased cellular O2 utilization -> increase rate of O2 release from Hb

· utilization coefficient

i) utilization coefficient = fraction O2 released from blood as passes via tissue capillaries

ii) normally 0.25 [25%]

iii) strenuous exercise:- 0.75 - 0.85

· Hb helps maintain a constant PO2 in tissue fluids (oxygen buffer function of Hb) despite exercise or changes in atmospheric changes in PO2

· Effect of blood flow on metabolic use of oxygen

i) total amount of O2 available each minute for use in any given tissue is determined by:

a) quantity of O2 transported in each 100 ml blood

b) rate of blood flow

ii) if rate of blood flow approaches zero, amount of O2 available also approaches zero

· Transport of Oxygen in dissolved state

i) Normal arterial blood: 100 ml blood has dissolved 0.29 ml O2 [PO2 95 mmHg]

ii) Venous blood: 100 ml blood has dissolved 0.12 ml O2 [PO2 40 mm Hg]

iii) Thus, 0.17ml of O2 is transported by each 100 ml blood through the tissues per cycle in the dissolved state

Bohr Effect: increase in CO2 in blood will cause O2 to be displaced from the Hb thereby promoting O2 release in tissues [ie oxygen dissociation curve shifts to the right]; reverse effect occurs in the lungs

B) Transport of Carbon dioxide in the blood

· Normally 4 ml of CO2 is transported from the tissues to the lungs in each 100 ml blood

· Gaseous CO2 (generally not bicarbonate) diffuses out of the cell

· Chemical forms in which CO2 is transported:

1) Dissolved state [7%]

i) arterial blood PCO2= 40 mmHg; 2.4 ml CO2 in 100 ml blood

ii) venous blood PCO2=45 mm Hg; 2.7 ml CO2 in 100 ml blood

iii) therefore, 1.3 ml is transported as dissolved CO2 by each 100 ml blood

2) Bicarbonate [70 %]

i) reaction of CO2 with water in rbc—> carbonic acid

ii) carbonic anhydrase catalyzes the reaction of CO2 & H2O 5000 X

iii) carbonic acid —> H+ & HCO3-

iv) H+ combines with Hb (Hb is a powerful acid-base buffer)

v) HCO3- diffuse into plasma; Cl- diffuses into rbc [chloride shift]

vi) administration of an carbonic anhydrase inhibitor —> reduced CO2 transport —> elevated tissue PCO2

3) Carbaminohemoglobin [23%]

i) CO2 combines reversibly with Hb (and to a much lesser extent other plasma proteins)

· Haldane effect

i) is the effect of the oxygen-hemoglobin reaction on CO2 transport

ii) binding of O2 with Hb tends to displace CO2 from the blood

iii) Tissues: have increased CO2 uptake due to O2 removal from Hb

iv) Lungs: have increased release of CO2 because of O2 pickup by Hb

v) Due to increased acidity of Hb when combined with O2

vi) approximately doubles the amount of CO2 picked up in the tissues and released in the lungs

· the formation of carbonic acid decreases the pH in venous blood [effect is attenuated by buffers]

oxygen and carbon dioxide transported in blood through in Alveoli in lungs where the exchange of gases take place by the mean of diffusion