1. Hypoxia Cellular deficiency of oxygen ….4 types: [1] Hypoxic hypoxia
low arterial PO2
low atmospheric O2,
Inadequate ventilation of the alveoli
Insufficient diffusion of O2 through the respiratory membrane.
[2] Anaemic hypoxia
Normal arterial PO2
ReducedHb available to carry O2 to the tissues.

2. [3] Ischaemic hypoxia
Normal PO2 and Hb concentration
low blood flow to a tissue due to arterial stenosis.
[4] Histotoxic hypoxia
Toxic agents act on respiratory chain enzyme in the mitochondria and prevent the tissue cells to make use of the O2supplied to them(eg. cyanide poisoning) or deficiency of oxidative enzymes such as vitamin B deficiency (Beriberi).

3. Cyanosis Bluish discoloration of the tissues
Reduced Hb concentration of the blood in the capillaries is more than 5 gm/dl
Easily seen in mucus membrane and thin skin areas like lips, fingers and nail bed.
In polycythaemia, cyanosis is very common because of the large amount of Hb in the blood
In anaemia, cyanosis is rare because it is difficult for there to be enough deoxygenated Hb to produce the cyanotic color. Cyanosis divided into 2 types:

4. [1] Central cyanosis
Hb-undersaturation or abnormal Hb derivative
Both mucous membranes and skin are affected(lips, and tongue)
[2] Peripheral cyanosis
Slow blood flow(vasoconstriction) causes great extraction of O2
Moderate cold exposure, shock, heart failure, and peripheral vascular disease.
In very cold weather cyanosis does not developed because O2 consumption by tissues is reduced so the affinity of Hb to O2 is reduced.

5. Hypercapnia Excess CO2in the body fluids
Hypoventilation OR circulatory deficiency causes poor gas exchange which results in both hypercapnia & hypoxia.
Serious hypercapnia usually does not occur because CO2 diffuses 20 times as rapidly as O2.

6. O2Therapy Is of great value in hypoxic types of hypoxia
and of slight value in other types.
In chronic hypoxia the respiratory center becomes adapted to the high PCO2 level and O2 lack becomes more powerful stimulus to respiration than usual, for this reason, O2therapy in hypoxia is sometimes contraindicated.

7. O2 Toxicity
Administration of 100% O2 has been demonstrated to exert toxic effects.
The toxicity seems to be due to the production of the superoxide anion (O2-) and H2O2.
When % O2 administered for periods of 8 hours or more the respiratory passages become irritated, causing sore throat and coughing.
Exposure for hours causes lung damage as well.
The reason O2 produce the irritation is probably due to inhibition the ability of lung macrophages to kill bacteria, and surfactant production is reduced.