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MODULE 20:
FLUIDS, ELECTROLYTES, and pHA measure of hydrogen ion concentration in a solution.
PART 5 OF 7
Stina’s later labs show her blood drifting slightly acidic with a low bicarbonate — an early acid-base disturbance this page gives the chemistry to read.
Slide 1 Transcript
PATIENT CHART
STINA
THIS PART
Age 44
PRESENTING WITH
Pitting ankle, periorbital edemaExcess fluid in interstitial spaces.
DIAGNOSIS
Partially compensatedWhen a buffer or system (respiratory or renal) has corrected pH despite ongoing imbalance. renal aciosis
CHART CLUES
COLLECTED AS YOU GO
Edema + Low Albumin + ‘Low’ Calcium
Edema Localized to a Plasma → Interstitium Shift
Hypoalbuminemia → Low Oncotic Pressure
A ‘Low’ Calcium That Is Really Lost Albumin
A Low-Bicarbonate Drift = Metabolic Acidosis
Renal Metabolic Acidosis, Only Partly Compensated
Amyloid-damaged glomerulusA network of capillaries in the nephron where blood filtration occurs.
Put your money down: Two organs command the bicarbonate buffer. If one of them fails, whose job can’t be covered?
Bet before the chemistry. The bicarbonate(HCO₃⁻) – A crucial buffer in blood that helps maintain pH balance; formed when carbon dioxide react reaction runs both ways: the lungs work the CO2 end, the kidneys work the bicarbonate end. Commit now — if Stina’s kidney is the failing organ, which half of her acid-base defense is compromised, and can the lungs make up the difference?
Pick the answer you believe now. We’ll come back to it later!
A Battle to Not Be Acidic
Blood pH is held in an astonishingly narrow band, around 7.35 to 7.45, because the proteinsLarge molecules made of amino acids with various functions in the body. and enzymesProteins that speed up chemical reactions in the body. that run the body only work within it. And here is the catch: you manufacture hydrogen ionsCharged atoms or molecules. all day long — from protein breakdown, from lactic acidA substance that releases hydrogen ions (H⁺) in solution., from ketones — without the matching baseA substance that accepts hydrogen ions (H⁺) or releases hydroxide ions (OH⁻). to cancel them. Your default drift is always toward acid; life is chemically a constant fight not to go acidicA solution with a pH below 7, having a higher concentration of H⁺ ions.. The first line of defense is buffering The process of stabilizing pH by binding or releasing H⁺ ions., and the most important buffer in blood is the bicarbonate system.
A Battle to Not Be Acidic
Why the body fights acid every minute of the day.
Slide 2 Transcript
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Single Choice: Why does the body’s default pH drift toward acid?
The Bicarbonate Sponge
One reversible reaction, soaking up acid.
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Drag the Words: Order the CO2 / carbonic acid(H₂CO₃) – A weak acid formed when carbon dioxide dissolves in water; plays a key role in blood pH re / bicarbonate / H+ reaction.
The Bicarbonate Sponge
Carbon dioxide plus waterThe universal solvent essential for life. makes carbonic acid, which splits into bicarbonate and a hydrogen ion — and the whole reaction runs both directions, freely and constantly. That reversibility is the trick. When acid rises, bicarbonate mops up the extra hydrogen ions and the reaction shifts to make more CO2, which the lungs exhale; when acid falls, it shifts the other way. Bicarbonate is the body’s acid sponge, and CO2 is the volatile acid it can literally breathe off.
Two Organs, Two Ends
Lungs on CO2, kidneys on bicarbonate.
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Multiple Choice: Which organ controlsVariables that remain constant to ensure a fair test. the bicarbonate end of the buffer?
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Two Organs, Two Ends
Two organs command the two ends of this reaction, and that is what makes it powerful. The lungs control CO2 — breathe faster and you blow off CO2, pulling toward less acid (a callback to the respiratory module, where CO2 first appeared as an acid in disguise). The kidneys control bicarbonate and fixed-acid excretion — reclaiming filtered bicarbonate, generating new bicarbonate, and secreting hydrogen ions into the urineThe liquid waste excreted by the kidneys.. A disturbance is named for where it starts: respiratory if the problem is CO2, metabolic if it’s bicarbonate or fixed acid. Stina’s low-bicarbonate drift is a metabolic acidosisLow blood pH due to excess acid or loss of bicarbonate..
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FINDING!
A Low-Bicarbonate Drift = Metabolic Acidosis
Stina’s blood drifts slightly acidic with a low bicarbonate — a metabolic acidosisA condition where blood pH falls below 7.35. (a bicarbonate/fixed-acid problem, not a CO2 problem). Chart entry: name the disturbance correctly and note that its two controllers are the lungs (CO2, fast) and the kidneys (bicarbonate, durable) — one of which is failing.
Confirm or refute your bet: Two organs command the bicarbonate buffer. If one of them fails, whose job can’t be covered?
Now that you know the two ends of the buffer, return to the bet. The kidney works the bicarbonate end — the durable half — and it’s the failing organ, so the lungs can only ever partly cover. Chart entry: metabolic acidosis named, with the failing partner identified.
Slide 7 Transcript
NEXT PART
A buffer buys time; it does not fix the source. The real correction comes from two organs working together — and in Stina, one of them is the very organ that is failing.
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All pages for Module 20 — Fluids, Electrolytes & Acid–Base Balance
20.0 Module Overview
20.1 The Swollen Ankles That Point Back to the Kidney
20.2 The Body’s Fluid Compartments & Water Movement
20.3 Hydrostatic vs Oncotic Pressure
20.4 The Major Electrolytes & the Albumin-Corrected Calcium
20.5 Acid-Base Balance & the Bicarbonate Buffer System
20.6 Metabolic Acidosis from a Failing Kidney
20.7 One Filter, Not Three Problems
20.8 Conclusion and Assessment
20.9 Evidence Behind the Case
All Modules
- Anatomical Language, Membranes & Homeostasis
- Just Enough Chemistry
- The Cell & Its Transport
- Making Cells & Proteins
- The Integumentary System
- The Skeletal System
- The Muscular System
- Nervous Tissue & the Senses
- The Spinal Cord
- The Brain & the Blood–Brain Barrier
- The Autonomic Nervous System
- Special Senses (in developmentThe process of growth and differentiation.)
- The Endocrine System
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- Blood Vessels
- The Digestive System
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- The Urinary System
- Fluids, Electrolytes & Acid–Base Balance
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List of terms
- pH
- edema
- compensated
- glomerulus
- bicarbonate
- proteins
- enzymes
- ions
- acid
- base
- acidic
- buffering
- carbonic acid
- water
- controls
- urine
- metabolic acidosis
- acidosis
- development