How does blood donation works?

i heard that within 4 months, blood cells become uselss, so people should donate. so how this blood works on other's body? i just want to know. what actually happens?

well you just go to blood banks to give blood. in worse cases lf a patient needs blood and lf lts the same type of blood then lts there but l think lm pretty sure they can store the blood in a safe place where lt won`t spoil

thats not true at all. the average lifespan of a red blood cell is about 120 days or 4 months like you mentioned, but after this your body gets rid of those old red blood cells and digests them. the heme part or the iron is salvaged for later use and the globin part or protein is degraded to bilirubin. bilirubin is picked up by the liver, which is then secreted as bile into the inestines, where it is metabolized into urobilogen. most of this degraded pigment leaves the body in feces, as a brown pigment called sterocobilin.
dying erythrocytes are engulfed by macrophages which happens in the spleen. Sometimes the spleen gets a little too aggressive and may get rid of RBCs not yet ready for destruction, and you get anemia. anyways the body is replacing all the RBCs just as fast as they are being taken out...at the amazing rate of about 2 million cells per SECOND in a healthy person. in the case of you donating the rate of production would increase substantially, so there really isnt much of a side effect if you donate.

When blood is donated the various useful parts (plasma, red cells, paltelets) are separated and stored.

You are asking about the red cells. They are basically little bags of hemoglobin that your body needs to transport oxygen from your lungs to your tissues. That's all the red cells do. When someone has too few red cells (from bleeding, chemotherapy, malnutrition) they can't get oxygen to their tissues. Transfusing donated red cells into their blood stream helps fix this problem.

The tricky bit is making sure their immune system doesn't notice that they are foreign blood cells, or they will be destroyed. This is why blood typing (A+, O-) is so important.

your thick conservative brain is too dense to understand the process

the useless cells get destroyed in your liver.. in fact, your health will remain perfectly fine if you don't donate

donating encourages your body to regenerate more blood so they ratio of fresh cells to older cells in your body is higher

Maybe you were thinking that the blood that is taken away from us when we donate contains only the dead/useless cells and that is why you are confused as to how can it be of use to the recipient's body. The fact is that the donated blood contains many viable blood components that has their own purposes. Yes it may contain old or dying blood cells but only in very small quantity. As with the 4 months you are talking about, that is the lifespan of red blood cells(RBC), that is why blood products that contain RBC's has only 3-4 months before it expires. Other blood products like fresh frozen plasma has longer storage time before it expires if stored properly.