The dialogue of the film reminds us on occasion of Marcus' heart. Blair (Moon Bloodgood, Rowr!) mentions it beating strongly when she seductively snuggles up to him and lays her head on Marcus' chest. Connor tells Marcus he sees his "heart beating a mile a minute." However we never see it directly, only through Terminator-vision at the end of a fight, so we don't know it's a human heart. Marcus' metallic skeleton at his chest is exposed but his heart is never visible to us.
Marcus' naughty parts
Here's a picture of Marcus' skeleton that I got off of Rotten Tomatoes. You can see that Marcus either has an organic heart encased in that metal skeleton, or perhaps he doesn't have a heart at all. Maybe the machines gave him some kind of mechanical pump that only sounds like a heart in case he has to snuggle with a human female.
Everyone mentions his heart. We all get it. Marcus has a strong heart. It's robust, can pump blood through an organism made of a combination of organic and manufactured materials, and is the symbol of his emotions, values, and humanity. Marcus' heart is the key to the final action, his redemption, and John Connor's life (so that there can be another movie -- spoiler alert #4.5!). The way the film works this material is both irritating and entertaining.
MARCUS TO THE RESCUE
In the factory during the final big action scene, Connor defribillates Marcus with industrial cables not because he is a being of value, but because Connor can use him as a weapon against a Terminator. Connor needs a weapon, and Marcus is it. But before the revived Marcus can stop it, the Terminator shoves a pipe through Connor's chest, where his heart would be if he had one(1). That is a great moment! It's shot really well, the welling blood suits their palette, and the action is well timed. Marcus then saves Connor and helps get him back to the base.
YOU KNOW THORAX
Back at base, Connor is dying because "his heart is too weak." Really? That's it? How about a collapsed lung, massive loss of blood, perhaps a sucking chest wound, fractured ribs splintering into everything, and insane amounts of bleeding around the mediastinum?
Below are some pictures of the chest. The mediastinum is essentially the area between the lungs, housing the heart in the "pericardial cavity". The lungs, heart, and other tubes are partitioned off from one another by sheets of connective tissue, sometimes referred to as membranes.
Here we see the heart and its vessels nestled in the mediastinum, between the lungs.
This is similar to an aerial view. The sternum is at the top of the picture, the vertebral column at the bottom, center. We see the lungs (right and left) and many of their blood vessels. We also see the top of the heart and the vessels from the lungs leading into the heart (in blue... yes, blue); these are essentially located within the borders of the mediastinum.
If bleeding happens within a partitioned area, the membranous walls keep blood from going everywhere. If one lung collapses due to blood in the chest (hemothorax), the other lung can function because the blood won't leak into its space unless these partitions are compromised.
After Marcus rescues him, Connor seems to be able to breathe and is not hacking up blood loogies, so the pipe and splintered ribs have obviously not pierced his lungs or their cavity (the plural cavity for you trivia buffs). His breathing would be pretty labored if he had a hemothorax or pneumothorax(3) situation. So what the heck happened? What is the scenario to fit the shot of the pipe emerging from Connor's chest, with all that blood welling up?
The only thing we can assume is there is bleeding (and swelling) around Connor's heart, causing compression to its chambers. This is called cardiac tamponade(4). The pressure from the blood build-up would reduce the amount of blood that could get into the heart, reducing cardiac output. To compensate, the heart would have to beat faster to try and get enough blood around the body. This condition would indeed weaken and exhaust the heart. This is the only condition I could possibly come up with to support the action and the script. I'd be curious to know what the writers were thinking...
Here's an illustration of cardiac tamponade from a World War II surgical manual. It's comforting to know that traumatic injuries suffered in the 20th century will still be around well into the 21st century...
I LEFT MY HEART IN SAN FRANCISCO...OR WAS IT L.A?
So, Marcus tells them to take his heart and transplant it into Connor's body. It is Marcus' chance to redeem his criminal life and sacrifice himself for a higher cause. What does Marcus get from Connor for his heart? A curt nod. No "thank you" or, "I guess you're human after all" or anything. Obviously Connor will never need a penis transplant. He's a big enough one as it is.
Then they perform transplant surgery out of doors under a netted camouflage tent. Grit and dust aren't sterile. There is a lot of bacteria in dirt, you know. As an example, anthrax bacteria can live in dirt for years and years. Wind can blow all sorts of bacteria into a wound. They should also do some blood and tissue typing before wasting Marcus' heart, if he even has one. If Connor doesn't die of infection, he may die of tissue rejection(5), which means... no sequel!
Anthrax Bacteria (the rod-shaped things)
CYBORGS AT LARGE
Thinking about Marcus (only professionally, of course) as the first human-based cyborg to show up in a Terminator film, I started thinking about cyborg films, generally. Expect the next entries to be an exploration of the meat/metal interface.
1. Connor's character is so self-centered! Let's face it, if I grew up with a mother who taught me to use firearms and told me I was destined to be the savior of the world, I'd be pretty stuck up, too.
2. Hemo = blood, thorax = chest. Put it all together, and there you have it.
3. Pneumo = air. I sense a pattern. Pneumothorax might happen if the chest wall and/or lung was punctured. As Connor struggled to breathe, air could get into the space around the lung, collapsing it.
4. Tamponade can refer to both the accumulation of blood around the heart or "the act of using a tampon." Thank you, Taber's Cyclopedic Medical Dictionary.
5. For more on transplantation feel free to peruse the Face/Off! entries from March of 2009.