It takes a considerable deal of time to transform a normal human into a Space Marine. They receive implants which transform their bodies and give them superhuman abilities - making them capable of spitting acidic venom, absorbing the memories of the dead by eating their flesh, darkening their skin to protect it from radiation, and operating for long periods without sleep by switching off parts of their brains at a time.
Recruits are subjected to psycho-indoctrination and conditioning, strengthening their resolve and honing them into dedicated and merciless warriors. Not all recruits survive the brutal training of course, and not all are accepted.
Implants:
Secondary Heart - This is the first and least difficult implant to install. The Secondary Heart increases blood supply and pumping capacity and is capable of taking over entirely should the primary heart fail.
Ossmudula - This implant strengthens the skeleton of a Space Marine by inducing the bones to absorb a ceramic-based substance administered in a Marine's diet. Within two years after the surgery, the Marine's skeleton will be larger and exponentially stronger; and the rib cage will be fused into a solid bone plate.
Biscopea - Implanted into the chest cavity, this implant massively bolsters muscle development and density throughout the Marine's body. This is commonly implanted at the same time as the Ossmodula.
Haemastamen - Implanted into a main blood vessel, the Haemastamen alters the Marine's blood composition to carry oxygen and nutrients more efficiently.
Larraman's Organ - This organ manufactures Larraman Cells. These serve the purpose of platelets, but act faster and more effectively. When a Marine is wounded, Larraman Cells are released, attached to leukocytes. At the site of the injury, they form scar tissue in a matter of seconds, effectively preventing massive blood loss and infection.
Catalepsean Node - Implanted into the back of the brain, this implant allows a marine to not sleep, instead entering an almost comatose trance where they "recharge". It also allows the resting of half the brain while the other remains alert, thus removing the need for total sleep. The longest known record of any space marine going without rest while on active combat duty is 319 hours by a member of the Crimson Fists.
Preomnor - The Preomnor is a decontamination chamber inside the chest cavity. It chemically analyzes ingested materials and neutralizes toxins. The Preomnor enables the Marine to eat normally inedible substances and resist poisons.
Omophagea - Implanted into the spinal cord, this organ is designed to absorb information and DNA related to experience or memory. This enables the Marine to gain information, in a survival or tactical sense, simply by eating an animal indigenous to an alien world.
Multi-lung - The multi-lung is a third lung, able to absorb oxygen from environments poor in oxygen. Breathing is accomplished through a sphincter in the trachea, allowing all three lungs to be used at full capacity. In toxic environments, a similar muscle closes off the normal lungs, thus oxygen is absorbed exclusively by the multi-lung which filters out poisonous elements.
Occulobe - Essentially, this organ enhances a Marine's eyesight, granting him exceptional vision and the ability to see normally in low-light environment.
Lyman's Ear - This implant renders a Marine immune to dizziness and nausea(seeCochlea), and enables a Marine to consciously filter out "white noise".
Sus-an Membrane - This implant allows a Marine to enter a catatonic or "suspended animation" state. It can allow a mortally wounded Space Marine to survive his injuries, and bring the metabolism to a standstill. The longest recorded period of this state was with Brother Silas Err of the Dark Angels for 567 years.
Melanochrome - Linked to pigment cells in skin, this allows the Marine's skin to shield him from dangerous levels of radiation.
Oolitic Kidney - This organ works in conjunction with the Preomnor, filtering blood to remove toxins.
Neuroglottis - This organ allows a Marine to assess a wide variety of things simply by taste. From poisons to chemicals to animals, a Marine can even track his quarry by taste alone.
Mucranoid - Altering sweat glands, this organ causes the secretion an oily substance that coats the skin when necessary, protecting it from extreme temperatures and to some extent, vacuum environments.
Betcher's Gland - Implanted into multiple locations inside a Marine's mouth, these glands transform a Marine's saliva into corrosive, blinding acid when triggered. A Marine trapped behind iron bars would be able to chew his way out given a few hours.
Progenoid Glands - Implanted into both the neck and chest cavity, these serve to collect and cultivate the gene-seed from a Space Marine's body, and to safeguard it for the continuity of a Chapter. The neck gland is removed upon 5 years, and the chest gland upon 10 years; both are then used to create new Space Marines.
The Black Carapace - The last and possibly most important of all implants, this neuroreactive material is implanted directly under the skin in the chest area. Invasive fibre bundles then grow inward and interlink with the Marine's own nervous system. Points pre-cut into the Carapace are effectively connection points, allowing a Marine to directly interface with his Power Armour.