respiratory system pt 2-ish

Partial pressure

The individual pressure exerted by a single gas in a mixture of gases. It equals the total pressure multiplied by that gas’s fraction of the mixture (e.g., PO₂ in dry air = 0.21 × 760 mmHg ≈ 160 mmHg).

Partial pressure gradient

The difference in partial pressure of a gas between two regions (e.g., alveolar air vs. pulmonary blood). Gases diffuse down this gradient from higher partial pressure to lower partial pressure.

Composition of dry atmospheric air

At sea level: ~79% N₂ → PN₂ ≈ 600 mmHg; ~21% O₂ → PO₂ ≈ 160 mmHg; total atmospheric pressure ≈ 760 mmHg.

Why alveolar PO₂ is lower than atmospheric PO₂

Alveolar PO₂ (~100 mmHg) is lower than atmospheric PO₂ (~160 mmHg) because fresh inspired air mixes with existing alveolar air, O₂ is constantly diffusing into blood, and water vapor/CO₂ in alveoli dilute the O₂ fraction.

Why alveolar partial pressures don’t change much with each breath

Each breath only replaces a small portion of the large alveolar volume, so PO₂ and PCO₂ change very little from breath to breath, even though gas exchange is continuous.

Factors that determine how much gas dissolves in blood

(1) Solubility of the gas in blood (CO₂ is much more soluble than O₂). (2) Partial pressure of the gas in the alveoli; higher partial pressure → more gas dissolves until equilibrium.

Typical PO₂ values in the body

Atmospheric air ~160 mmHg; alveolar air ~100 mmHg; systemic arterial blood/beginning systemic capillaries ~100 mmHg; systemic venous blood/end systemic capillaries ~40 mmHg.

Typical PCO₂ values in the body

Atmospheric air ~0.23 mmHg (very low); alveolar air ~40 mmHg; systemic arterial blood/beginning systemic capillaries ~40 mmHg; systemic venous blood/end systemic capillaries ~46 mmHg.

Two forms of oxygen transport in the blood

(1) O₂ physically dissolved in plasma (small amount). (2) O₂ chemically bound to hemoglobin inside red blood cells (major form).

Hemoglobin – basic structure and location

Hemoglobin is an iron-containing protein in RBCs with four subunits, each holding a heme group that can bind one O₂ molecule, so one Hb can carry up to four O₂ molecules.

Reduced hemoglobin vs. oxyhemoglobin

Reduced hemoglobin (HHb) is hemoglobin not bound to O₂ (deoxyhemoglobin). Oxyhemoglobin (HbO₂) is hemoglobin with O₂ bound; binding is loose and reversible.

Main factor determining % hemoglobin saturation

The PO₂ of the blood is the main determinant of hemoglobin saturation. Higher PO₂ → more O₂ binds to Hb; lower PO₂ → Hb releases O₂.

Oxyhemoglobin dissociation curve – overall shape

The relationship between % Hb saturation and PO₂ is sigmoidal (S-shaped) due to cooperative binding: binding or release of one O₂ changes the affinity for the next O₂.

Plateau region of the Hb–O₂ dissociation curve

At high PO₂ (~60–100 mmHg, in pulmonary capillaries) % Hb saturation stays near 100% even with moderate PO₂ drops. This gives a safety margin so arterial blood still carries lots of O₂ if lung function or atmospheric PO₂ falls.

Steep region of the Hb–O₂ dissociation curve

At lower PO₂ (~10–60 mmHg, in systemic capillaries) small drops in PO₂ cause large drops in % saturation, so Hb can release large amounts of O₂ to tissues when they need it.

Role of hemoglobin as an O₂ buffer or reservoir

Hb binds O₂ as it diffuses into blood in the lungs, holding a large store of O₂ without greatly raising dissolved PO₂. Later, Hb releases O₂ in tissues when PO₂ falls, acting like a buffer or reservoir.

Why hemoglobin loads O₂ in pulmonary capillaries

In the lungs, alveolar PO₂ is high (~100 mmHg). Rising blood PO₂ increases Hb affinity for O₂, so Hb becomes highly saturated as blood passes through the pulmonary capillaries.

Why hemoglobin unloads O₂ in systemic capillaries

In tissues, PO₂ is low because cells are using O₂. This lower PO₂ and local metabolic conditions reduce Hb affinity for O₂, causing O₂ to unbind and diffuse into the tissues.

Factors that promote O₂ unloading from hemoglobin in tissues

Increased metabolism raises CO₂, H⁺ (lower pH), temperature, and 2,3-BPG in tissues. These shift the Hb–O₂ curve to the right, decreasing Hb affinity and enhancing O₂ release where it is most needed.

Main questions about hemoglobin and O₂ transport

(1) Hb–O₂ combination vs separation depends mainly on PO₂ and local pH/temp. (2) Hb binds O₂ in lungs (high PO₂) and releases it in tissues (low PO₂/acidic/warm). (3) Variable O₂ release is controlled by local metabolic changes that alter Hb affinity.