Encyclopaedia Britannica has the following to say about LEECHING
The medicinal leech has proved useful in medicine because of its peculiar mouthparts and the pharmacologically active substances present in its saliva. Hirudo medicinalis has three jaws with approximately 100 sharp teeth on each outer rim. The leech feeds by first attaching its sucker onto the skin. The mouth, located in the middle of the sucker, opens to expose the teeth, which cut into the patient’s skin. The saliva of the leech contains substances that anesthetize the wound area (rendering the bite virtually painless) and dilate blood vessels to increase blood flow to the site of the bite. Leech saliva also contains an enzyme that promotes a quick dissipation of substances in the leech saliva away from the bite site. One of these substances is hirudin, a naturally occurring polypeptide that inhibits the actions of thrombin, one of the enzymes that facilitates blood clotting. This powerful anticoagulant, first identified in 1884 but not isolated in purified form until the 1950s, is primarily responsible for the extensive bleeding that results from a leech bite, though other factors are also involved.
In inter-personal relationships we often encounter such LEECHES, who firstly ANESTHETIZE us and then BLEED us. This process of LEECHING is interesting, as one lives of the other without the knowledge of the other. But the problem is that the LEECH after sucking the blood for long and bloating beyond its own recognition, FORGETS TO ANESTHETIZE the part from which it is still feeding. That is when the person gets to feel the pain and realizes that the leech had been not only STICKING TO HIM, but has been BLEEDING him too.
So, the man starts feeling pain with flux of time, as the anesthetizing antics of the blood-sucker have been forgotten by the LEECH and openly starts FEEDING. The LEECH is then plucked and squashed! The blood that oozes out of the place where the LEECH had bitten is, as excerpted above, is because of the anti-coagulant substance in the leech’s saliva which ensures that the blood when it comes in contact with the air doesn’t coagulate! The RAWNESS of the wound remains.
The primary idea is that, a LEECH cannot suck forever, as the original bite which anesthetized wears off with time. The man, sooner or later feels the pain and takes reparative measure. I only wonder, why the LEECH doesn’t drop off when its stomach is full and live to practice its LEECHING some other day?