Chapter Text
“Nephron segments. Let’s go.”
Erwin barely has to look. “Bowman’s capsule. Proximal convoluted tubule. Loop of Henle—descending, thin ascending, thick ascending. Distal convoluted tubule. Collecting duct.”
Levi clicks his pen twice, scribbles down the list with practiced speed. “Alright. Function of the proximal convoluted tubule?”
Erwin doesn’t miss a beat. “Reabsorbs about 65% of filtered sodium and water. Also glucose, amino acids, bicarbonate. Secretes hydrogen ions and organic acids. Brush border increases surface area.”
Levi hums, half-impressed. “You forgot the part where it’s the reason coffee doesn’t kill us immediately.”
“I was getting there,” Erwin says mildly.
Levi rolls his eyes. “Whatever. Loop of Henle?”
“Descending limb is water-permeable, solute-impermeable—water gets reabsorbed passively. Ascending limb’s the opposite—actively pumps out sodium, impermeable to water. Sets up the gradient in the medulla.”
Levi taps the side of his head like a gun. “Hot. Nothing gets me going like countercurrent exchange.”
Erwin doesn’t even flinch. “Good to know.”
Levi narrows his eyes. “You better be joking.”
“Would it matter if I wasn’t?”
“It would matter if you wanted to live.”
That earns the tiniest twitch of Erwin’s mouth. Not quite a smile. Just enough.
Levi ignores it and moves on. “Distal convoluted tubule?”
“Sodium reabsorption, fine-tuned by aldosterone. Calcium, under the influence of PTH. Mostly impermeable to water unless acted on by ADH in the collecting duct.”
“And what does ADH do?”
“Makes you clingy.”
Levi stares at him in disbelief.
Erwin raises an eyebrow. “Water reabsorption, obviously.”
Levi groans, face in his hands. “I swear to God. One more joke like that and I’m going to stick a scalpel in your renal artery.”
“That’s hot.”
“Oh my fucking god, are you seriously hitting on me right now?”
Erwin flips the page. “No,” he says smoothly. “I’m studying nephrology. You’re the one making it weird.”
Levi throws a crumpled sticky note at his face. “Fuck you.”
“You say that a lot,” Erwin says, plucking the note off his shoulder and setting it aside like it’s beneath him. “Statistically, it’s losing its impact.”
Levi throws his pen across the table, “Shut the fuck up and quiz me on glomerular filtration rate.”
Erwin doesn’t even flinch as the pen skids across the table, narrowly missing his notes.
“Define glomerular filtration rate.”
Levi, still glaring, answers through gritted teeth. “Volume of fluid filtered from the glomerular capillaries into Bowman’s space per unit time. Normal range is about 90 to 120 milliliters per minute.”
“Factors that influence it?” Erwin asks, eyes scanning his notes, but he’s not really looking at them. He’s watching Levi.
“Hydrostatic pressure in the glomerular capillaries. Oncotic pressure in the plasma. Bowman’s capsule hydrostatic pressure. And, obviously, permeability and surface area of the filtration barrier,” Levi recites, not missing a beat.
That earns a smile from Erwin. “My, my, you’re good.”
“Don’t act like you’re surprised.”
“I am most definitely not.
***
They slip into a habit that even Levi doesn’t want to name. They study every 16:00— 4, 4 fucking PM. And they text. Oh, God. They text.
Erwin: Are you awake?
Levi glares at the screen from where he’s lying flat on his stomach, cheek pressed into a pillow, phone half-slipping from his hand. He debates pretending to be asleep. But his thumb betrays him.
Levi: no
Erwin: Impressive multitasking.
Levi: fuck off
Erwin: I was reviewing acid-base balance. Wanted to test something.
Levi: if you’re about to ask me for Henderson-Hasselbalch at 1AM i’m calling the cops
Erwin: What’s the first thing you’d treat in a patient with pH 7.2, CO2 30, and HCO3 15?
Levi: metabolic acidosis w/ partial respiratory compensation. find the cause, start with fluids. maybe bicarb, depending
Erwin: Beautiful.
Levi: i will end you
Erwin: Romantic.
A pause.
Erwin: Are you alone, Levi?
Levi swallows.
Levi: yea my roommate’s out
A few seconds pass. The dots appear. Then vanish. Then reappear.
Erwin: Good.
Levi sits up. What the fuck does that mean?
Erwin: I’m sending you a diagram. Walk me through the steps of interpreting arterial blood gas.
Levi lets out a breath he didn’t know he was holding. Of course. Of course.
Levi: u dick
Erwin: Sorry, were you expecting something else?
The diagram comes through. A clean, typed out ABG chart with pH, pCO₂, HCO₃⁻, and base excess. Levi rubs his face, annoyed at the flush creeping up his neck.
Levi: step 1: look at pH. step 2: determine primary disorder. step 3: check compensation. step 4: calculate anion gap if metabolic acidosis suspected. step 5: determine if there’s a mixed disorder
Erwin: Immaculate. You’re wasted on this hellhole.
Levi: tell that to the professor who gave me a B on the last exam
Erwin: If I were your professor, I’d give you an A.
Levi pauses.
Levi: that’s gross
Erwin: :)
Levi: ur so weird
Levi: i’m going to sleep
Erwin: Levi.
Levi: what
Erwin: I was kidding.
Now, what the fuck does that mean?
Levi: what
Erwin: Earlier. When I said that the reason why I asked if you were alone was the diagram.
Levi: um what do u mean
Erwin: Don’t play coy.
He types. Deletes. Types again. Then finally:
Levi: i’m literally not i have no fucking clue what ur saying
Levi sits up straighter in bed, heart now doing something truly unprofessional. His fingers hover, twitching. His mouth feels dry. He types slower this time.
Erwin: Are you going to make me say it?
Levi: say what exactly? that ur a fucking idiot
Erwin: That I think you’re very pretty, Levi.
Oh, fucking God.
Levi: this better be a fucking joke
Levi: fucking creep
The screen stays still. Empty. No typing dots. No retort. Just Levi, suddenly hyper-aware of the sound of his own breathing and the slight tremble in his fingers.
Then, finally,
Erwin: You’d know if I was joking anyway.
Levi: why would u say that
Levi: why the fuck would u say that u prick
Erwin: You asked.
Levi: ur fucking insane
Erwin: Am I?
Levi: ur unwell. u need help. psychiatric
Erwin: Probably.
Erwin: But imagine if I were there right now. Would you stop me?
Levi: go to fuckin sleep
Erwin: You first.
Levi: good night
Erwin: Sweet dreams. If you get any.
Levi stares at the message, phone dimmed to the lowest setting, heart knocking unevenly against his ribs. He lies back down but doesn’t put the phone away. Doesn’t even blink.
Then he types, slow.
Levi: are u trying to say something or are u just being a fucking tease
A beat. Then…
Erwin: Both.
Levi exhales through his nose. God, what the fuck is this.
Levi: ur playing a dangerous game
Erwin: So I’ve noticed.
Erwin: But I like the way you react.
Levi’s mouth goes dry.
Levi: ur disgusting
Erwin: You haven’t blocked me yet.
Levi: i should
Erwin: You won’t.
Erwin: If I told you where I’d put my hands if I were there right now, would you finally stop beating around the bush?
Levi: jesus fucking christ
Erwin: Language.
Levi: oh fuck off
Levi: u talk a lot for someone who hasn’t even touched me
Erwin doesn’t respond immediately. Levi watches the typing dots appear, disappear, then return—like Erwin’s choosing his words carefully, or worse, enjoying the anticipation. The fucker.
Erwin: Do you want me to?
Levi stares at that one for longer than he means to. His heart’s doing something erratic, arrhythmic, stupid. He swallows, hard.
Levi: don’t ask questions u don’t want the answer to
Levi: goodnight
Erwin: Sweet dreams, Levi.
***
The next day, Levi shows up at the library at 16— 4:01 PM. On purpose.
Erwin’s already there, of course. Spot claimed, notes laid out with surgical precision, highlighters arranged by color like some kind of sadistic rainbow. He glances up when Levi approaches, and there’s something annoyingly smug in the way his gaze lingers—like he knows exactly what Levi’s been thinking about since midnight.
Jesus fuck.
Levi sits down without a word. Slaps his notebook on the table. Doesn’t look at him.
“You’re late.”
“By one fucking minute. You’ll live.”
Erwin smiles, the bastard. “Didn’t get any sleep?”
Levi gives him a tight-lipped glare that could strip varnish. “Because someone decided to sext me with acid-base questions at 1AM.”
“Academic foreplay,” Erwin says without missing a beat. “Very niche.”
Levi clenches his pen so hard it nearly snaps in half. “I’m going to shove this up your ass.”
“Careful,” Erwin murmurs, leaning closer. “You’re making promises now.”
Levi doesn’t rise to it. Doesn’t even blink. Just flips his notebook open and starts scribbling something unintelligible—probably not even related to renal physiology. Probably die in a ditch, Smith over and over again.
“Want me to start with a question, or are you going to keep threatening me with poorly sanitized objects?”
“Question.”
Erwin slides a flashcard across the table, and their fingers brush. Levi ignores it. Pretends not to feel it. Pretends it doesn’t send something low and treacherous curling in his gut.
“Patient with elevated BUN and creatinine, low urine sodium, high specific gravity. What is it?”
Levi doesn’t even blink. “Pre-renal azotemia. Hypoperfusion. Usually volume depletion.”
“Very good,” Erwin murmurs, pleased. His gaze lingers a little too long. “And how would you fix it?”
Oh, Jesus.
“I need caffeine.” Levi says, abruptly. He closes his eyes. He’s being painfully obvious, he knows it. He could not care less. Erwin, though, ever so polite, nods. And leaves. Like he knows the way Levi wants his coffee.
Levi is fucking palpitating.
He stands up before Erwin could come back. And he leaves.
***
1AM. He fuckin’ anticipates it.
Erwin: You left.
Levi stares at the screen for a solid minute, thumb hovering. The message is simple—bare. Not accusing. Not angry. Just there. Like Erwin always is. Like he knows.
Levi: yeah captain obvious
Erwin: Did I push too far?
Levi exhales, slow. Not quite a sigh. Not quite relief, either.
Levi: no. i don’t know
Levi: fuck, i’m no good at this
Levi: ur fucking with me
Erwin: I’m not trying to fuck with you.
Erwin: I am, however, trying to fuck you.
Levi: ur a menace
Erwin: Perhaps. You’re not telling me to stop.
Levi: if i told u to stop, would u
Erwin: Immediately.
Levi: and if i don’t
Erwin: Then I’ll keep going. I’ll tell you what I thought about while you were answering questions like your life depended on it. I’ll tell you about all the places I’d touch if you’d just let me.
Levi: god
Erwin: I’d start with your hands. They’re always moving—tight around your pen, clenched when you’re irritated, tapping when you’re focused. I’d take one in mine and smooth your fingers out, one by one.
Levi swallows. The room feels too hot. His phone too bright. He tugs the collar of his shirt away from his throat, suddenly aware of how still he’s gone.
Erwin: Then your throat. You tilt your head when you’re trying not to roll your eyes. You press your tongue to your cheek when you’re annoyed. I want to kiss right there. Feel your pulse under my mouth.
Levi’s legs shift under the sheets. Tension crawls up the back of his neck, across his scalp, down between his shoulders like a weight he doesn’t know how to carry. He should say something. He should.
Erwin: And your mouth, Levi.
Erwin: Always saying fuck this, fuck that. I want to know if I can make it say something else.
Levi: ur fuckin sick
Erwin: Perhaps.
Levi: u should stop talking
Erwin: Do you want me to?
A pause.
Levi: no
Erwin: Do you want to know what I’d do if I had you right now, Levi?
Levi: fuck
Erwin: I’d push you down the mattress. Hold you by the nape.
Erwin: I’d kiss you slow. Tongue barely there at first—just enough to drag a sound out of you.
Levi swallows. He hates this. His hands are shaking. Erwin knows it’s working.
Levi: u need to be fucking arrested
Erwin: Maybe. Not before I get my hands on you, though.
Levi: jesus
Levi: if ur gonna leave me like this i’m gonna kill u
Erwin: And how exactly would you do that, Levi?
Levi exhales—one sharp, breathless sound through his nose. He bites the inside of his cheek to stop from grinning like a goddamn idiot.
Levi: slowly, painfully, graphically
Erwin: You’ll have to get close to do it.
Levi: ur insane
Erwin: So I’ve been told.
Levi: so now what
Levi: u gonna leave me hanging
Erwin: You’d like that, wouldn’t you?
Levi: like hell i would
Erwin: You’d stew. Get frustrated. Try to sleep it off. Fail.
Erwin: Maybe you’d even touch yourself.
Levi stares at the screen. Heat pools low in his stomach, dark and slow and dangerous. He hates this. Hates him. Hates himself more.
Levi: u think ur so fucking smart
Erwin: I know you haven’t stopped thinking about last night.
Erwin: Tell me what you’d do if I were there.
Levi: no
Erwin: Coward.
Erwin: Say it.
Levi: i’d pull you by the collar. shove you against the door
Erwin: And then?
Levi: then i’d bite. just hard enough to bruise
Erwin: Good. I want to feel it in the morning.
Levi curses under his breath, flips the pillow, and shoves his face into the cool side of it. This is fucking unreal.
Levi: then i’d let you touch me. anywhere you like
Erwin: Anywhere?
Levi: yeah
Erwin: You’re dangerous when you’re honest.
Levi: u wanted it. now u get it
Erwin: I’d start slow. Just hands. Neck. Chest. Down your sides.
Erwin: I’d want to see how long you could pretend it’s not driving you insane.
Levi: wouldn’t take long
Erwin: No, I don’t think it would.
Erwin: You’d squirm. You’d make those little sounds you try to swallow. Think I wouldn’t notice.
Erwin: I would. I’d want more.
Levi exhales hard. His breath fogs the screen.
Levi: what then
Erwin: I’d pin your wrists. Slide my hand under the waistband. Just barely touch.
Erwin: Just enough to make you beg for it.
Erwin: Would you be a darling and beg for it, Levi?
Levi: god
Levi: i hate u
Levi: fuck
Erwin: Going to sleep now?
Levi: like hell i am
Erwin: Good. Don’t.
Erwin: I want you squirming under your sheets thinking about me.
Erwin: Don’t come unless I tell you to.
Levi: god erwin what the fuck
Erwin: That wasn’t a request.
Levi’s breathing goes uneven. The screen glows against his flushed face, too bright, too real. He digs his heel into the mattress, trying to anchor himself. It doesn’t work.
Levi: u fuckin bastard
Erwin: That’s more like it.
Erwin: Are you touching yourself, Levi?
Levi: yeah
Levi: ur a fucking psychopath
Erwin: I guess.
Erwin: Still with me?
Levi: fuck you
Erwin: Yes. Tell me what you want, Levi. I want to hear it from you.
Typing. Stopping. Typing again.
Levi: i want ur hands
Levi: ur clever fucking mouth
Levi: all over me
Levi: god fuck tyou
Erwin: Come for me, will you?
Erwin: Good night, Levi.