Human Phys Chapter 19 - Gas Exchange & Transport

How do O2 and CO2 move between the alveoli and pulmonary blood?

Gases diffuse from HIGH to LOW partial pressure.

• O2: Alveoli (100) → Blood (40).

• CO2: Blood (46) → Alveoli (40).

After exchange, arterial blood is approximately PO2 = 100 and PCO2 = 40 mmHg.

🧠 Analogy:

At the lungs, a delivery truck picks up oxygen and drops off CO2 waste.

Memory trick:

LUNGS = O2 INTO blood, CO2 OUT of blood.

How do O2 and CO2 move between systemic blood and body cells?

Answer:

• O2: Blood (100) → Cells (≤40).

• CO2: Cells (≥46) → Blood (40).

Cells consume O2 to make ATP and produce CO2 during metabolism.

🧠 Analogy:

The truck delivers oxygen to cells and collects their CO2 trash.

Memory trick:

CELLS = TAKE O2, GIVE CO2.

Lower Alveolar PO2 Decreases Oxygen Uptake

What three main factors determine successful oxygen uptake in the lungs?

1. Enough O2 reaches alveoli (ventilation).

2. O2 diffuses across a large, thin exchange surface.

3. Enough blood passes the alveoli (perfusion).

🧠 Analogy:

A package must arrive, pass through the door, and get picked up by a delivery truck.

Memory trick:

AIR ARRIVES → O2 CROSSES → BLOOD CARRIES.

How do emphysema, fibrosis, pulmonary edema, and asthma reduce oxygen uptake?

• Emphysema = Surface area ↓.

• Fibrosis = Barrier thickness ↑; compliance ↓.

• Edema = Fluid ↑; diffusion distance ↑.

• Asthma = Airway resistance ↑; ventilation ↓.

🧠 Analogy:

Emphysema = Missing doors.

Fibrosis = Thick walls.

Edema = Water blocking the path.

Asthma = Narrow entrance.

Memory trick:

EMPHYSEMA = AREA.

FIBROSIS = SCAR.

EDEMA = FLUID.

ASTHMA = AIRWAYS.

Diffusion Problems Cause Hypoxia

According to Fick's law, what factors affect gas diffusion across the alveolar membrane?

Diffusion increases when:

• Surface area ↑

• Partial-pressure gradient ↑

• Membrane permeability ↑

• Diffusion distance ↓

Disease examples:

• Emphysema → Less surface area.

• Fibrosis/edema → Greater diffusion barrier or distance.

🧠 Analogy:

More open doors, a bigger crowd difference, and a shorter hallway help people move faster.

Memory trick:

BIG AREA + BIG GRADIENT + THIN BARRIER = FAST DIFFUSION.

Gas Solubility Affects Diffusion

Why can O2 and CO2 have the same partial pressure but different dissolved concentrations?

Different gases have different solubilities.

• CO2 is about 20 times more soluble in water than O2.

• At equilibrium, gas partial pressures are equal across the air-water interface, but concentrations don't have to be equal.

• O2's low solubility is why hemoglobin is necessary.

🧠 Analogy:

CO2 loves jumping into water; O2 doesn't dissolve nearly as well.

Memory trick:

SAME PRESSURE ≠ SAME CONCENTRATION.

CO2 LOVES LIQUID.

How do you calculate dissolved gas concentration, and why might pulmonary edema affect O2 more than CO2?

Dissolved gas = α × Partial pressure.

• α for O2 = 0.03

• α for CO2 = 0.7

Pulmonary edema increases diffusion distance.

Because O2 is less soluble, arterial PO2 can fall while PCO2 remains normal.

🧠 Analogy:

CO2 is a stronger swimmer than O2 when crossing the fluid-filled barrier.

Memory trick:

MORE FLUID → O2 STRUGGLES FIRST.

19.2 Gas Transport in the Blood

How is oxygen transported in the blood, and how does hemoglobin release it?

• More than 98% of O2 is bound to hemoglobin.

• Less than 2% is dissolved in plasma.

• At lungs: Hb binds O2.

• At tissues: Hb releases O2, which diffuses into cells.

🧠 Analogy:

Hemoglobin is a delivery truck that picks up oxygen at the lungs and drops it off at cells.

Memory trick:

Hb = Hauls O2.

LUNGS = LOAD.

CELLS = UNLOAD.

What is the Fick equation for oxygen consumption?

O2 consumption = Cardiac output × (Arterial O2 content − Venous O2 content).

It calculates how much oxygen tissues remove from passing blood.

🧠 Analogy:

Packages leaving the warehouse minus packages returning = packages delivered.

Memory trick:

IN − OUT = USED.

How does hemoglobin's structure allow it to carry oxygen?

Hemoglobin has 4 globin chains, each with a heme group containing iron.

Each iron binds 1 O2 molecule, so one hemoglobin can carry up to 4 O2 molecules.

Oxygen binds reversibly, allowing hemoglobin to pick it up and release it.

🧠 Analogy:

Hemoglobin is a car with 4 seats. Each seat can carry one oxygen passenger.

Memory trick:

4 CHAINS → 4 HEMES → 4 IRONS → 4 O2.

Oxygen Binding and the Law of Mass Action.

How does the Law of Mass Action control oxygen binding to hemoglobin?

Hb + O2 ⇌ HbO2

• HIGH PO2 → Reaction favors RIGHT → Hb binds O2.

• LOW PO2 → Reaction favors LEFT → Hb releases O2.

• Active tissues consume more O2 → PO2 decreases → More O2 is released.

🧠 Analogy:

Hemoglobin is a bus.

At the lungs, oxygen passengers get ON.

At the tissues, oxygen passengers get OFF.

Memory trick:

HIGH PO2 = LOAD O2.

LOW PO2 = UNLOAD O2.

Hemoglobin Transports Most Oxygen to the Tissues

Why is hemoglobin essential for oxygen delivery, and what happens when PO2 decreases?

Hemoglobin greatly increases blood's oxygen-carrying capacity.

At PO2 ≈ 100 mmHg:

• Without Hb: ~3 mL O2/L blood.

• With Hb: ~200 mL O2/L blood.

When PO2 decreases, Hb saturation decreases, so blood carries less oxygen.

🧠 Analogy:

Hemoglobin is a delivery truck.

Without trucks, very few oxygen packages can be transported.

Lower PO2 means the trucks leave with fewer packages.

Memory trick:

Hb = OXYGEN TRUCK.

PO2 DOWN → Hb SATURATION DOWN → O2 CONTENT DOWN.

What two factors determine how much oxygen is bound to hemoglobin?

1. Plasma PO2 determines the percentage of Hb binding sites filled (saturation).

2. Amount of Hb determines the total number of oxygen-binding sites available.

Low Hb can cause low total O2 content even when PO2 and saturation are normal.

🧠 Analogy:

Hemoglobin is a school bus.

PO2 determines how full the buses are, while Hb amount determines how many buses you have.

Memory trick:

PO2 = HOW FULL.

Hb AMOUNT = HOW MANY.

Oxygen Binding Is Expressed as a Percentage

What does the oxygen–hemoglobin saturation curve show?

It shows how hemoglobin saturation changes with PO2.

• PO2 = 100 → ~98% saturated.

• PO2 = 40 → ~75% saturated.

• PO2 = 20 → ~35% saturated.

HIGH PO2 means more oxygen is bound.

LOW PO2 means more oxygen is released.

🧠 Analogy:

Hemoglobin is a bus.

When oxygen is plentiful, the bus fills up.

When oxygen becomes scarce, passengers get off.

Memory trick:

HIGH PO2 = FULL BUS.

LOW PO2 = EMPTYING BUS.

Why does the hemoglobin saturation curve have a flat and a steep region?

• Flat region (high PO2): Hb stays nearly full even if PO2 decreases somewhat.

• Steep region (low PO2): Small drops in PO2 cause large amounts of O2 to be released.

During exercise, tissue PO2 decreases, allowing Hb to unload more oxygen.

🧠 Analogy:

The bus stays nearly full on the highway (flat), but lots of passengers get off when it reaches a busy stop (steep).

Memory trick:

FLAT = STAYS FULL.

STEEP = RELEASES O2.

What factors shift the oxygen–hemoglobin curve RIGHT or LEFT, and what is the Bohr effect?

RIGHT shift = Lower O2 affinity → More O2 unloading.

Caused by:

• pH ↓

• Temperature ↑

• CO2 ↑

• 2,3-BPG ↑

LEFT shift = Higher O2 affinity → Less O2 unloading.

Caused by the opposite changes.

Bohr effect = Lower pH decreases Hb's affinity for O2, promoting oxygen release.

🧠 Analogy:

Hemoglobin holds oxygen in its hand.

A right shift loosens its grip;

a left shift tightens it.

Memory trick:

RIGHT = RELEASE.

LEFT = LOVE.

HOT + ACIDIC + CO2 + BPG = RIGHT.

Why does fetal hemoglobin have a higher oxygen affinity than adult hemoglobin?

Fetal Hb has higher O2 affinity, so its curve shifts LEFT compared with adult Hb.

This allows fetal blood to pick up oxygen from maternal blood across the placenta.

Fetal Hb contains gamma chains instead of adult beta chains.

🧠 Analogy:

Mom hands oxygen to her baby, and the baby has a stronger grip to take it.

Memory trick:

FETAL = FIRM GRIP.

LEFT = LOVES O2.

Carbon Dioxide Is Transported in Three Ways

What are the three ways CO2 travels in the blood?

• 7% dissolved in plasma.

• 23% bound to hemoglobin (carbaminohemoglobin).

• 70% converted to bicarbonate (HCO3−).

Most CO2 travels as bicarbonate.

🧠 Analogy:

CO2 is trash.

Some floats in blood, some rides on hemoglobin trucks, and most gets packaged as bicarbonate.

Memory trick:

7 FLOATS.

23 RIDES.

70 CONVERTS.

How does CO2 become bicarbonate, and what is the chloride shift?

Carbonic anhydrase speeds up:

CO2 + H2O ⇌ H+ + HCO3−

• Hb binds H+ to help buffer pH.

• HCO3− leaves the RBC.

• Cl− enters the RBC to maintain electrical balance.

🧠 Analogy:

Carbonic anhydrase is a recycling machine.

Bicarbonate leaves the RBC bus, and chloride takes its seat.

Memory trick:

CA = CO2 CONVERTER.

HCO3 OUT → Cl IN.

Hb HOLDS H+.

What happens to bicarbonate and CO2 when blood reaches the lungs?

The reactions reverse:

• HCO3− enters the RBC as Cl− exits.

• H+ is released from Hb.

• H+ + HCO3− → CO2 + H2O.

• CO2 detaches from Hb and diffuses into alveoli to be exhaled.

🧠 Analogy:

The CO2 trash packaged at the tissues gets unpacked at the lungs and thrown away.

Memory trick:

TISSUES = PACK CO2.

LUNGS = UNPACK CO2.

Chapter 19.3 - Control of Ventilation

How does the brainstem control breathing, and what information affects it?

The brainstem automatically generates breathing rhythms.

• Somatic motor neurons activate the diaphragm and other respiratory muscles.

• Chemoreceptors monitor CO2, O2, and H+ (pH).

• The brainstem adjusts ventilation based on this feedback.

Breathing can also be controlled voluntarily, but only to a point.

🧠 Analogy:

The brainstem is an automatic breathing controller.

Chemoreceptors are sensors that tell it when breathing needs to change.

Memory trick:

BRAINSTEM = BREATHING BOSS.

CHEMORECEPTORS = CHEMICAL SENSORS.

What are the roles of the medulla, pons, and central pattern generator in breathing?

• Medulla: Controls inspiratory and expiratory muscle activity.

• Pons: Integrates information and helps adjust breathing patterns.

• Central pattern generator (CPG): Brainstem neural network that automatically creates the breathing rhythm.

Chemoreceptors, mechanoreceptors, and higher brain centers can modify breathing.

🧠 Analogy:

The medulla is the driver, the pons helps adjust the driving, and the CPG is the automatic rhythm that keeps everything moving.

Memory trick:

MEDULLA = MAKES IT HAPPEN.

PONS = POLISHES THE PATTERN.

CPG = CREATES THE RHYTHM.

What are the main brainstem regions involved in controlling ventilation?

• NTS/DRG: Receives sensory information via CN IX and X.

• VRC: Helps coordinate respiratory muscle activity.

• Pre-Bötzinger complex: Helps generate the breathing rhythm.

• PRG (pons): Helps smooth and coordinate the rhythm.

Phrenic nerves activate the diaphragm; intercostal nerves activate intercostal muscles.

🧠 Analogy:

The NTS listens,

the pre-Bötzinger complex keeps the beat,

the VRC conducts,

and the PRG helps the music stay smooth.

Memory trick:

NTS = LISTENS.

Pre-Böt = BEAT.

VRC = COORDINATES.

PRG = POLISHES.

How does neural activity change during quiet inspiration and expiration?

During inspiration, more inspiratory neurons gradually become active (ramping), increasing respiratory muscle stimulation.

At the end of inspiration, neuronal activity stops abruptly.

The muscles relax, allowing passive expiration through elastic recoil.

During forced breathing, additional muscles are activated.

🧠 Analogy:

Inspiration is like gradually turning up a speaker's volume.

At the end, the speaker switches off and the lungs naturally recoil.

Memory trick:

RAMP UP → SHUT OFF → RECOIL.

Smth Smth CO2, O2, and pH with Chemoreceptors

What is the difference between central and peripheral chemoreceptors?

• Central: Located in medulla; respond to changes in CSF pH caused by CO2.

• Peripheral: Located in carotid and aortic bodies; detect arterial PO2, PCO2, and pH.

CO2 is the primary chemical controller of ventilation.

Low O2 strongly stimulates ventilation when arterial PO2 falls below ~60 mmHg.

🧠 Analogy:

Peripheral receptors are guards outside checking multiple chemicals.

Central receptors are alarms inside the brain responding to CO2-related acidity.

Memory trick:

CENTRAL = CO2 through CSF.

PERIPHERAL = O2, CO2, pH.

How does increased CO2 stimulate central chemoreceptors?

1. Blood CO2 increases.

2. CO2 crosses the blood-brain barrier into CSF.

3. CO2 produces H+ and HCO3−.

4. H+ stimulates central chemoreceptors by lowering CSF pH.

5. Ventilation increases, removing CO2.

Blood H+ itself crosses the blood-brain barrier poorly.

🧠 Analogy:

CO2 enters a locked building and sets off an alarm by producing H+ inside.

Memory trick:

CO2 CROSSES → H+ TRIGGERS → BREATHING INCREASES.

How do peripheral glomus cells respond to low oxygen?

Low PO2 → K+ channels close → Cell depolarizes → Ca2+ channels open → Ca2+ enters → Neurotransmitter released → Brainstem increases ventilation.

🧠 Analogy:

Low oxygen activates a fire alarm.

Closing K+ channels starts the electrical signal,

and Ca2+ allows the emergency message to be sent.

Memory trick:

LOW O2 → K CLOSES → Ca ENTERS → SIGNAL SENT.

How do protective airway reflexes cause bronchoconstriction, and what causes bronchodilation?

Irritants activate airway receptors, which signal the brainstem.

• Parasympathetic vagus nerve releases ACh.

• ACh binds muscarinic receptors → Bronchoconstriction.

• Epinephrine binds β2 receptors → Bronchodilation.

Coughing, sneezing, and mucus help remove irritants.

🧠 Analogy:

Your airways are doors.

Parasympathetic signals close them to limit irritants,

while β2 activation opens them to let more air through.

Memory trick:

ACh + MUSCARINIC = CONSTRICT.

EPI + β2 = DILATE.

How do higher brain centers control breathing, and why can't you hold your breath indefinitely?

Higher brain centers allow voluntary breathing changes, while the limbic system influences breathing during emotions.

The brainstem still controls automatic breathing.

When you hold your breath, CO2 increases → CSF H+ increases → Chemoreceptors stimulate the brainstem → Strong urge to breathe.

🧠 Analogy:

The brainstem is automatic cruise control.

Higher brain centers are the driver, but the automatic system can take over when necessary.

Memory trick:

BRAINSTEM = AUTO. HIGHER BRAIN = MANUAL.

HOLD BREATH → CO2 BUILDS → BRAIN DEMANDS AIR.