Explain how 70% carbon dioxide is transported from the tissues to lungs.
Give the partial pressure of O2 and CO2 in alveoli

Oxygen and carbon dioxide transported through Blood in human beings.
The blood in mammals contain water. Carbon dioxide is highly soluble in water. Therefore, the carbon dioxide released as a by-product is dissolved in water present in blood. The blood then goes to the lungs, where it is purified (i.e. it is freed of waste products and then oxygenated). This purified blood then goes to all parts of the body cells where it is again deoxygenated. The same process then occurs again. 70% of carbon dioxide transfer occurs in bicarbonate form. 

Haemoglobin present in RBC is the respiratory pigment in human beings which transport the inspired oxygen molecule. Oxygen binds with the red colour pigment known as haemoglobin  in the blood and transported to different parts of the body.  Once used up, the deoxygenated blood contains carbon di oxide and expels it out.

Alveoli are microscopic air sacs. They are the primary sites of exchange of gases. Gas exchange also occurs in lungs and tissues by diffusion. 
The alveolar oxygen partial pressure is lower than the atmospheric O₂ partial pressure. The partial pressure of O₂ in alveoli is 104 mmHg (Torr).
The partial pressure of CO₂ in alveoli is 35 mmHg (Torr).