What happens to the carbon dioxide which collects in human tissues?
The blood which brings O2 to tissues has lower concentration of carbon dioxide, but tissues have higher concentration of CO2. So, due to concentration difference, CO2 diffuses, from tissues into the blood of capillaries. From tissues blood is carried to lungs where CO2 diffuses into the air present due to difference in concentration. In lungs, air has much less concentration of carbon dioxide than the blood brought from tissues.