After Isaac takes damage from an enemy or donates health, he drops a Red Heart that despawns after 1.5 seconds.
The heart dropped depends on the source: half-hearts of damage taken drop Half Red Hearts, and full hearts of damage taken drop full hearts.
The hearts will get launched out of Isaac with momentum, making them difficult to pick up.
If Isaac is firing tears as he gets hit, the heart will get launched in that direction.
Notes
Damage sources that scale up from half a heart of damage to a full heart of damage starting on Womb/Corpse will still only cause Hypercoagulation to spawn half a heart. However, being hit by a champion enemy will still spawn a full red heart, even though all enemies do a full heart of damage by that stage.
Red hearts are not spawned when Isaac loses heart containers (such as through health down pills, buying Devil Room items, using Guppy's Paw, etc), with the only exception being Potato Peeler.
Red hearts will still drop regardless of the type of heart being subtracted from damage, making the item generally useless for characters who cannot utilize red hearts.
However if Isaac takes damage while inside a super secret room layout with specific heart spawn, he will drop a full heart of that type instead. The full heart will drop even if Isaac takes half a heart damage.
The following damage sources will activate Hypercoagulation:
Blood Rights / Razor Blade: Damage taken to use the item also spawns hearts, reducing the cost of using them. The first usage of these items each room will spawn a full red heart.
Cancer / Percs! pill / The Wafer: Damage sources that spawn full red hearts will still spawn a full red heart despite the cost being reduced to half a heart.
Dull Razor: Can be used to spawn full red hearts for Isaac to pick up.
Equality!: As long as Equality is active, all hearts spawned will be doubled and never disappear. Hearts remain on the ground even after the effect is no longer active.
Cambion Conception: By intentionally taking damage with something like a fire and grabbing the half-red hearts, Isaac can easily generate powerful familiars for free.
Hypercoagulation (otherwise known as Thrombophilia) is a medical condition that causes blood clots to spontaneously form in a person’s blood vessels and prevents them from dissolving properly.
Red Heart that despawns after 1.5 seconds.
Half Red Hearts, and full hearts of damage taken drop full hearts.
Womb/
Corpse will still only cause Hypercoagulation to spawn half a heart. However, being hit by a champion enemy will still spawn a full red heart, even though all enemies do a full heart of damage by that stage.
Devil Room items, using 
Guppy's Paw, etc), with the only exception being 
Potato Peeler.
Fire Places
Blood Donation Machines
Devil Beggars
Confessionals
Hell Games
Razor Blade
Blood Rights
Dull Razor (despite it not taking health)
Glass Cannon
Curse Room doors
Mausoleum
Sacrifice Room spikes
Spiked Chests/
Mimic Chests
Golden Battery
Kamikaze!
Breath of Life
A Pound of Flesh shop spikes
IV Bag
Sharp Plug
Shard of Glass
Blood Bombs
Blood Oath
Diplopia / 
D1 / 
Jera: Converted/duplicated hearts will not disappear.
Cancer / Percs! pill / 
The Wafer: Damage sources that spawn full red hearts will still spawn a full red heart despite the cost being reduced to half a heart.
Equality!: As long as Equality is active, all hearts spawned will be doubled and never disappear. Hearts remain on the ground even after the effect is no longer active.
Humbleing Bundle: 
Double Hearts.
Cambion Conception: By intentionally taking damage with something like a fire and grabbing the half-red hearts, Isaac can easily generate powerful familiars for free.
Candy Heart / 
Immaculate Conception: The hearts will count toward gaining stat boosts/familiars.
Compost: The temporary heart will be converted into a Blue Fly/Spider.
D20 / 
GB Bug: Rerolled hearts will still disappear.
Daemon's Tail: Hearts spawned do not turn into keys but simply do not spawn 80% of the time, and the 20% that do spawn will always be 
Black Hearts.
Keeper / 
Tainted Keeper: Spawns blue flies upon getting hit, essentially granting them the same effect as 
Infestation.
IX - The Hermit?: The heart will not be converted to coins.
Maggy's Bow: Doubles the healing received from the hearts spawned.
Magneto / 
Super Magnet: Will immediately start pulling the dropped hearts to Isaac, breaking their momentum and making collection much easier.
