What is the function of the human spleen?

Assertion $A$: In an open circulatory system, the blood volume is high, but the blood pressure is low and irregular.
Reason $R$: In a closed circulatory system, the blood volume is limited, and the blood pressure is high and regular.
Which option is correct for Assertion $A$ and Reason $R$?

Consider the following four statements regarding the human circulatory system:
$(a)$ Arteries have thick walls and have a smaller lumen compared to veins.
$(b)$ Angina is a sudden chest pain.
$(c)$ In the $ABO$ blood grouping system,a person with $AB$ blood group can donate blood to anyone.
$(d)$ Calcium ions play an important role in blood clotting.
Which two of the above statements are correct?

Given below are four statements $(i-iv)$ regarding the human blood circulatory system.
$(i)$ Arteries are thick-walled and have a narrow lumen as compared to veins.
$(ii)$ Angina is acute chest pain when the blood circulation to the heart muscle is reduced.
$(iii)$ Persons with blood group $AB$ can donate blood to any person with any blood group under the $ABO$ system.
$(iv)$ Calcium ions play a very important role in blood clotting.
Which two of the above statements are correct?

Which one of the following is a matching pair of a certain body feature and its value/count in a normal human adult?

Cords of Billroth are blood-filled spaces found in which of the following organs?