Calcium Homeostasis & Respiratory Gas Exchange: Key Concepts for Biology

Hypocalcemia

Low Calcium levels in the blood.

Parathyroid Hormone (PTH)

Hormone released by the parathyroid gland that raises blood calcium levels.

Calcium release from bone

An action of PTH that increases calcium levels in the blood.

Calcium resorption in kidneys

PTH decreases calcium loss in kidneys, increasing blood calcium.

Active Vitamin D (calcitriol)

Form of Vitamin D activated by PTH that increases calcium absorption in the gut.

Calcium reabsorption from kidneys

PTH increases the reabsorption of calcium from urine back into the blood.

Calcitonin

Hormone released by the thyroid gland that lowers blood calcium levels.

Calcium release from bone (Calcitonin)

Calcitonin stops calcium release from bone, lowering blood calcium.

Gut calcium absorption

Calcitonin decreases calcium absorption in the gut.

Renal calcium reabsorption

Calcitonin decreases calcium reabsorption in kidneys, resulting in more calcium excreted in urine.

Bone as a Calcium Reservoir

Bone stores calcium when blood levels are high and releases it when blood levels are low.

Gas exchange

Primary function of the respiratory system to take in O₂ and expel CO₂.

Atmospheric Composition

At sea level, O₂ is 20.95%, N₂ is 78.08%, Ar is 0.93%, CO₂ is 0.04%.

Total partial pressures

Total partial pressures in any environment equal 1 atm.

High altitude effects

Decreased total air pressure leads to decreased partial pressure of O₂.

Underwater pressure effects

Increased pressure underwater causes nitrogen to dissolve more, risking decompression sickness.

Turtle Eggs

Laid on shore, they exchange gases with the environment; low O₂ partial pressure can lead to embryo death.

Water Beetle Example

Lives underwater and creates an air bubble to breathe, maintaining a partial pressure of 0.1 atm O₂.

Gas Flow Principle

Gases diffuse from high to low partial pressure, not based on concentration.

Temperature Effect on Gas Solubility

Increased temperature decreases gas solubility.

Salt Concentration Effect on Gas Solubility

Increased salinity decreases gas solubility.

Effect of ↑ Temperature on Gas Solubility

↓ Solubility

Effect of ↑ Salt on Gas Solubility

↓ Solubility

Unidirectional Flow

Air moves in one direction only. Seen in fish gills, some birds. Efficient — fresh air constantly flows over exchange surfaces.

Tidal Flow

Air moves in and out of same pathway (humans, mammals). Mixes incoming and outgoing air — less efficient but flexible.

Convection

Movement of gas or liquid within the same phase.

Diffusion

Movement across a phase boundary (air ↔ liquid).

Convection Phase

Same phase.

Diffusion Phase

Crosses phase boundary.

Convection Distance

Long distances.

Diffusion Distance

Short distances.

Example of Convection

Airflow in trachea, blood pumped by heart.

Example of Diffusion

O₂ crossing alveolar membrane.

O₂ Transport Pathway

Air → lungs (convection), Alveoli → blood (diffusion), Blood → tissues (convection), Capillaries → cells/mitochondria (diffusion).

CO₂ Transport Pathway

CO₂ generated in cells → diffuses into blood → travels via convection → diffuses into alveoli → exhaled via convection.

Partial Pressure Cascade in Respiration

Each step = drop in partial pressure → ensures continuous O₂ flow.

Ambient air PO₂

0.20 atm; Oxygen in environment.

Alveoli PO₂

0.14 atm; Must be lower than ambient for O₂ to enter lungs.

Arterial blood PO₂

0.13 atm; Slightly lower due to diffusion.

Tissues PO₂

0.05 atm; O₂ diffuses into cells (low pressure area).

Mitochondria PO₂

0.01 atm; Lowest; O₂ used in metabolism.

Diffusion Rate Factors

Depends on partial pressure difference (ΔP), surface area of the membrane, membrane thickness, diffusion constant (D).

Fick's Law of Diffusion

Rate of diffusion = D × A × (P₁ - P₂) / T.

Applications of Diffusion Limitations

Damage to alveoli increases thickness → reduces diffusion efficiency.

Oxygen Cascade Steps

1. Ambient air ~0.21 atm; 2. Alveolar air ~0.14 atm; 3. Arterial blood ~0.13 atm; 4. Capillary/tissue interface ~0.05 atm; 5. Mitochondria ~0.01 atm.

Carbon Dioxide Cascade Steps

1. Mitochondria ~0.06 atm; 2. Tissues ~0.05 atm; 3. Venous blood ~0.053 atm; 4. Alveoli ~0.04 atm; 5. Exhaled air ~0.03 atm.

Key Rule for CO₂

CO₂ travels down its own gradient, separate from O₂.

Alveoli

Thin-walled air sacs surrounded by dense capillary networks.

Surface area

Enormous surface area (~70 m² in humans) for gas exchange.

Efficiency Factors

Surface area, membrane thickness, and blood flow rate determine the efficiency of gas exchange.

Emphysema

Destroys alveolar walls → ↓ surface area.

Pulmonary edema

Fluid buildup → ↑ diffusion distance.

Fibrosis

Thickened membrane → ↓ rate of O₂ transfer.

Dissolved O₂

Only ~1.5% of O₂ is physically dissolved in plasma.

Bound O₂

~98.5% of O₂ bound to hemoglobin (Hb) in red blood cells.

Oxygen-Hemoglobin Dissociation Curve

Sigmoid (S-shaped) relationship between PO₂ and Hb saturation.

PO₂ (mmHg) and Hb Saturation (%)

100 mmHg corresponds to 97% saturation in lungs; 40 mmHg corresponds to 75% saturation in tissues (resting); 20 mmHg corresponds to 35% saturation in active muscles.

Right Shift (Bohr Effect)

Occurs in active tissues, promoting O₂ unloading.

Left Shift

Occurs in lungs, promoting O₂ loading.

CO₂ Transport Mechanisms

CO₂ is transported in three forms: ~7% dissolved in plasma, ~23% bound to hemoglobin, and ~70% as bicarbonate ion (HCO₃⁻).

Carbonic Anhydrase Reaction

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, occurs in red blood cells.

Chloride Shift

To maintain charge balance, Cl⁻ enters RBCs as HCO₃⁻ leaves.

Primary Control Center

Medulla oblongata and pons in the brainstem regulate respiration.

Main Stimuli for Breathing

CO₂ levels (main control), O₂ levels (secondary control), and pH changes.

Feedback Loop

↑ CO₂ → ↑ H⁺ (acidic) → receptors activate → ↑ ventilation → CO₂ expelled → pH normalizes.

High Altitude Adaptation

Chronic exposure → kidneys release erythropoietin (EPO) → stimulates RBC production → ↑ oxygen-carrying capacity.

Diving Mammals

Have large myoglobin stores in muscles for O₂ storage and reduced heart rate during dives.

Hypoventilation

↓ breathing rate leading to ↑ CO₂ (hypercapnia) and acidosis.

Hyperventilation

↑ breathing rate leading to ↓ CO₂ (hypocapnia) and alkalosis.

Diffusion impairment

Thickened alveolar membrane leading to ↓ O₂ transfer efficiency.

Anemia

↓ Hemoglobin leading to ↓ O₂ carrying capacity.

Carbon monoxide poisoning

CO binds Hb with 200× O₂ affinity, blocking O₂ transport.

Gas movement

Follows pressure gradients, not concentrations.

Hemoglobin

Acts as the main oxygen buffer and carrier.

CO₂ control

Primarily controls respiratory rate via pH regulation.