Describe in brief how urine is produced in the human body.

Consider the following statements in connection with the functions of the blood vessels marked A and B in the diagram of a human heart as shown. 

(i) Blood vessel A — It carries carbon dioxide rich blood to the lungs.
(ii) Blood vessel B — It carries oxygen rich blood from the lungs.
(iii) Blood vessel B — Left atrium relaxes as it receives blood from this blood vessel. 
(iv) Blood vessel A - Right atrium has thick muscular wall as it has to pump blood to this vessel.
The correct statements are-

The correct statement concerning single-celled organisms is-
(i) Complex substances are not broken down into simple substances.
(ii) Simple diffusion is sufficient to meet the requirement of the exchange of gases.
(iii) Specialised tissues perform different functions in the organism.
(iv) The entire surface of the organism is in contact with the environment for taking in food.

Which of the following statements is correct about the role of A (shown in the given diagram) during a breathing cycle in human beings? 

(i) It helps to decrease the residual volume of air in lungs.
(ii) It flattens as we inhale.
(iii) It gets raised as we inhale.
(iv) It helps the chest cavity to become larger.