Lesson 1.5 · Unit 1 · HS-LS1-6
Water, pH and the four molecule families a body is built from
This course is not a chemistry course, and it does not need all of one. It needs a specific short list: why water behaves the way it does, what pH measures and how blood defends it, and what the body actually does with each of the four classes of large molecule. Everything on that list returns in a later unit, usually more than once.
- Water is about 60 percent of body mass and it is not inert. Its high heat capacity buffers temperature swings, its high heat of vaporization makes sweating an effective cooling method, its polarity makes it the universal solvent for the body's reactions, and its cohesion gives it the surface tension that matters in the alveoli.
- pH measures hydrogen ion concentration on a logarithmic scale. Each unit is a tenfold change, so a shift from 7.4 to 7.0 is not a small drift; it is a two and a half fold rise in acidity and is fatal.
- Blood pH is held between 7.35 and 7.45. That is a strikingly narrow band, and holding it is the job of three systems working on three timescales: chemical buffers in seconds, the respiratory system in minutes, the kidneys in hours to days.
- The bicarbonate buffer is the main one in blood. Carbon dioxide plus water gives carbonic acid, which gives hydrogen ion plus bicarbonate, and the reaction runs both ways. Added acid is mopped up by bicarbonate; added base is neutralized by carbonic acid. This one reaction reappears in units 8, 9 and 11.
- Carbohydrates are the body's ready fuel. Monosaccharides such as glucose, disaccharides such as sucrose and lactose, polysaccharides such as glycogen, which is how the liver and muscle store glucose.
- Lipids store energy densely, build membranes and act as signals. Triglycerides store more than twice the energy per gram of carbohydrate, phospholipids form every membrane in the body, and steroids include cholesterol and the steroid hormones of unit 7.
- Proteins do nearly everything else. Structure (collagen, keratin), movement (actin, myosin), transport (hemoglobin), catalysis (every enzyme), defense (antibodies) and signaling (many hormones). A protein's function depends on its shape, which is why denaturation by heat or pH destroys function.
- Nucleic acids carry and express information. DNA stores the instructions, RNA carries them to the ribosome. ATP, the energy currency of every chapter of this course, is a nucleotide.
Where students lose marks: treating denaturation as "breaking down" a protein. Denaturation unfolds the shape without breaking the chain into amino acids. The protein is intact and useless, which is a different claim from digested, and it explains why a fever of 41 degrees is dangerous long before anything is destroyed.
The problem. A patient's blood pH is measured at 7.28. Explain what this means quantitatively, identify which direction the body will push, and trace how each of the three lines of defense responds and on what timescale.
Step one: classify the reading. Normal blood pH is 7.35 to 7.45. A value of 7.28 is below that range, so this is acidosis. Note that 7.28 is still alkaline on the chemistry scale, where neutral is 7.0; "acidosis" means acidic relative to the body's normal, not relative to pure water. Students lose marks on exactly this point.
Step two: express the deviation as a concentration. pH is logarithmic, so a drop of 0.12 units from 7.40 corresponds to a rise in hydrogen ion concentration by a factor of 10 raised to the power 0.12, which is about 1.32. The hydrogen ion concentration is roughly 32 percent above normal. That is the honest way to describe it; saying pH "fell slightly" understates it considerably.
Step three: state the direction of correction. Hydrogen ion concentration is too high, so every response will act to remove hydrogen ions or to stop producing them.
Step four: first line, chemical buffers, acting within seconds. Bicarbonate in the plasma binds free hydrogen ions to form carbonic acid, which takes them out of solution as far as pH is concerned. This is immediate but limited, because the supply of bicarbonate is finite and is being consumed.
Step five: second line, the respiratory system, acting within minutes. The carbonic acid formed in step four breaks down to carbon dioxide and water. Rising carbon dioxide and falling pH both stimulate the respiratory centers, so breathing becomes deeper and faster. Blowing off carbon dioxide pulls the whole bicarbonate reaction toward the carbon dioxide side, consuming hydrogen ions as it goes. This is powerful but it cannot eliminate hydrogen ions from the body, only convert and exhale them.
Step six: third line, the kidneys, acting over hours to days. The tubules secrete hydrogen ions into the filtrate for excretion in urine and simultaneously reabsorb or generate new bicarbonate to replenish what step four consumed. This is the slowest response and the only one that actually removes acid from the body and restores the buffer supply.
Step seven: state the result and the diagnostic consequence. Together the three lines return pH toward 7.4. Because the respiratory response is fast and the renal response is slow, measuring which one is doing the compensating tells a clinician which system caused the problem: a patient with a respiratory cause shows renal compensation, and a patient with a metabolic cause shows respiratory compensation. That logic is developed fully in lesson 11.5.
Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.
- State the normal pH range of arterial blood.
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7.35 to 7.45
- Name the four classes of organic molecule found in the body.
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Carbohydrates, lipids, proteins and nucleic acids
- Give the storage polysaccharide of the human body and the two organs that store it.
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Glycogen, stored in the liver and in skeletal muscle
- Write the bicarbonate buffer reaction.
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CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
- What determines a protein's function?
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Its three-dimensional shape, which arises from its amino acid sequence
- Explain why a fever above about 41 degrees Celsius is dangerous, in molecular terms.
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Protein function depends on shape, and shape is held by relatively weak bonds that are disrupted by heat. Above roughly 41 degrees those bonds begin to fail in large numbers and proteins denature, losing their shape and therefore their function. Since every enzyme in the body is a protein, widespread denaturation halts metabolism. The proteins are not broken into amino acids, which is why the change can sometimes be reversed if it is caught early, but an unfolded enzyme does not work. Proteins denature, losing the shape their function depends on
- Explain why water's high heat of vaporization makes sweating effective, and state one
condition under which sweating fails.
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Evaporating water requires a large amount of energy per gram, and that energy comes from the skin and the blood beneath it, so a small volume of sweat removes a large amount of heat. The mechanism depends entirely on the evaporation, not on the sweat itself. In high humidity the air is already near saturation, so sweat cannot evaporate, sits on the skin and removes almost no heat. This is why humid heat is far more dangerous than dry heat at the same temperature. Evaporation carries away a great deal of heat per gram; it fails in high humidity, when the sweat cannot evaporate
- A patient hyperventilates during a panic attack. Predict the effect on blood pH using
the bicarbonate equation.
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Hyperventilation exhales carbon dioxide faster than it is produced, so blood carbon dioxide falls. Removing carbon dioxide pulls the reaction to the left, consuming carbonic acid, which in turn consumes hydrogen ions and bicarbonate to replace it. Falling hydrogen ion concentration means rising pH. Blood pH rises, producing respiratory alkalosis
- Fat contains more than twice the energy per gram of carbohydrate, yet the body stores
only about a day's worth of glycogen and far more fat. Explain why it stores any glycogen
at all.
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Energy density is not the only criterion; speed of access matters too. Glycogen can be broken down to glucose rapidly and glucose can be used without oxygen, so glycogen supports sudden or anaerobic demand. Fat yields more energy but is mobilized slowly and requires oxygen to be used at all, so it cannot meet a sudden demand. The body keeps a small, fast store and a large, slow one, which is the same design logic as keeping cash as well as a savings account. Glycogen is mobilized far faster and can be used anaerobically, so it meets sudden demand that fat cannot
- Explain why the bicarbonate buffer system is particularly well suited to a body that
is continuously producing carbon dioxide.
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The buffer's components are carbon dioxide and bicarbonate, and the body already produces carbon dioxide continuously as a waste product of cellular respiration, so the raw material is free and never runs short. More importantly, the body has independent control over both sides of the reaction: the lungs adjust carbon dioxide within minutes and the kidneys adjust bicarbonate over hours. A buffer whose two components can each be regulated by a separate organ system is far more powerful than a closed chemical buffer, because it can be reset rather than merely consumed. Its components are continuously supplied and are separately regulated by the lungs and the kidneys, so the buffer can be replenished rather than exhausted