NSFW- types of climaxes/ biology vampire edition- (Patreon)

Edit: I added one more diagram for the mock-up of organs. It quick and simple- so not extremely anatomical !

The secondary blood sac, was an evolutionary byproduct that became increasingly useful in reproduction and social development. it stimulated arousal, with a visible effect. It was initally made as a smaller secondary body bloodbank, with blood cells in a high iron/protein environment, without urea.  condusive to stimulate regrouth by tapping in reserves already saturated with energy rather than taking the food reserve (prey blood) bank and quickly transfering it into energy, which the ratio of energy cost to energy gain was small) It’s position eventually settled down the bladder, due to the placemeant of tubes from kidneys to bladder and two secondary reserves. This made it easy to separate urea and plasma from blood, and quickly have the -extra- filtered blood into the blood sac. (With hosts own fatty blood cells) this was an emergency blood sac that eventually became redundant do to the expoential growth of the primary blood sac that encapsulate around the gut tract and kidneys. This created a large enough food reserve as well as an increasingly effiecient regrowth, and energy conversion…. That reduced the necessity of that emergency blood sac.

they also developed a method of taking the primary blood banks reserves and converting some of it into fatty tissue with the help of the fatty blood cells. It takes much longer than humans due to the low nutritional density of blood but it happens. Overall purebloods need a looot of blood to survive in an athletic sense, and vampires less so. (with their more mixed human biology, which allows them to seek more food and blood options than purebloods, who essentially specialized into only blood)  

another cool byproduct of blood feeding, they have intense immune systems and hyperefficent gut bacteria that break down the blood and create all that other delicious nutrients.

 Over evolutionary history it was repurposed as a sexual byproduct that became socially attractive early on in vampire evolution.  It displayed a few things. healthiness, as your getting enough food to make more expendible blood, therefore overall more likely to be well. The blood itself can be analyzed to find any negative health complications, due to the processes it takes to be made.  Smell, viscosity, amount, like other products of the reproductive, urinary, digestive, and endocrine systems, can quickly tell good and bad. blood was attractive, and so is a partner that is healthy, sturdy, and capable of thriving, rather than surviving. It intertwined  itself with social modalities, cultures, and behaviors. It quickly nestled in already used senses, like smell, hormonal detection (pheromones), eyesight, and specialized taste receptors

you can essentially identify others by hormones and smell, visual aspects also play in long distances and attractiveness.  so screw someone and they can tell if you did. You can Remove the blood but the hormones stick like poison and they know anyways. Cheating is easily detectable, and polyamory is often used.  you can tell illnesses, horniness, and healthiness.

and it stayed in vampire biology as an essential to sexual social relations.

unrelated but cool:

purebloods used to have lots of color and markings due to identifiability, they strike out more, and also because they were capable of it due to hunting at night. (color aint gonna matter when Everything is black and white) They don’t actually need a lot of sleep just a lot of energy.  They lost those colors due to the lack of genetic diversity, essentially losing to the vampire/human mix who had more food options and a more complex gut.  they retained their existence by advancing in civilization and holding power. Their sprinters not endurance runners, and that applied to physical power as well.  Fast twitch muscles rather than slow twitch muscles.

but their genetic bottle-neck made the coloring fade, and the markings go away over time. They became more dependent on more food, worsened over time, and had lower life expectancies from the worsening health of the closing genetic diversity (like those royals in history dating their cousins cus they think their bloodline is all special, but your just gonna be hemophiliac and you children will NOT be okay) 

so vladd is a struggling athlete with sun sensitivity, chronic illness at his doorstep and a few days away from Starving.
he drinks bottles to withstand his athleticism and energy levels. Most purebloods get sluggish to preserve energy, oftentimes being still like a statue. it’s creepy to humans since it brings alarm bells than anything else.

good ole, evolutionarily suucessful vampires are less privy to those consequence, cus they can eat meat to A degree.