partial pressure

Examples of partial pressure in the following topics:

• Basic Principles of Gas Exchange

  • Partial pressure (Px) is the pressure of a single type of gas in a mixture of gases.
  • For example, in the atmosphere, oxygen exerts a partial pressure, and nitrogen exerts another partial pressure, independent of the partial pressure of oxygen (Figure 1).
  • Total pressure is the sum of all the partial pressures of a gaseous mixture.
  • A gas will move from an area where its partial pressure is higher to an area where its partial pressure is lower.
  • Partial pressure is the force exerted by a gas.

• Gas Pressure and Respiration

  • The pressure for an individual gas in the mixture is the partial pressure of that gas.
  • The partial pressure of any gas can be calculated by: P = (Patm) (percent content in mixture).
  • Patm, the atmospheric pressure, is the sum of all of the partial pressures of the atmospheric gases added together: Patm = PN2 + PO2 + PH2O + PCO2= 760 mm Hg.
  • The pressure of the water vapor in the lung does not change the pressure of the air, but it must be included in the partial pressure equation.
  • At high altitudes, there is a decrease in Patm, causing the partial pressures to decrease as well.

• Gas Exchange across the Alveoli

  • Differences in partial pressures of O2 create a gradient that causes oxygen to move from the alveoli to the capillaries and into tissues.
  • The RQ is a key factor because it is used to calculate the partial pressure of oxygen in the alveolar spaces within the lung: the alveolar PO2 (PALVO2).
  • The lungs never fully deflate with an exhalation; therefore, the inspired air mixes with this residual air, lowering the partial pressure of oxygen within the alveoli.
  • When the RQ is known, the partial pressure of oxygen in the alveoli can be calculated: alveolar PO2 = inspired PO2−((alveolar PO2)/RQ)
  • The partial pressures of oxygen and carbon dioxide change as blood moves through the body.

• Transport of Oxygen in the Blood

  • The binding of oxygen to hemoglobin can be plotted as a function of the partial pressure of oxygen in the blood (x-axis) versus the relative Hb-oxygen saturation (y-axis).
  • As the partial pressure of oxygen increases, the hemoglobin becomes increasingly saturated with oxygen.
  • The oxygen dissociation curve demonstrates that as the partial pressure of oxygen increases, more oxygen binds hemoglobin.

• Water’s Heat of Vaporization

  • This vapor density and the partial pressure it creates are the saturation values.
  • They depend only on the vapor pressure of water.

• Osmosis

  • Semipermeable membranes, also termed selectively permeable membranes or partially permeable membranes, allow certain molecules or ions to pass through by diffusion.
  • This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure.

• Homologous Structures

  • Homology can also be partial: new structures can evolve through the combination of developmental pathways or parts of them.
  • As a result, hybrid or mosaic structures can evolve that exhibit partial homologies.
  • For example, certain compound leaves of flowering plants are partially homologous both to leaves and shoots because they combine some traits of leaves and some of shoots.
  • It is considered that due to lack of the original selective pressure upon one copy of the duplicated gene, this copy is free to mutate and acquire new functions.

• Blood Pressure

  • Blood pressure is the pressure of blood against the blood vessel walls during the cardiac cycle; it is influenced by a variety of factors.
  • Blood pressure is the pressure of the fluid (blood) against the walls of the blood vessels.
  • Fluid will move from areas of high to low hydrostatic pressures.
  • The systolic pressure is defined as the peak pressure in the arteries during the cardiac cycle; the diastolic pressure is the lowest pressure at the resting phase of the cardiac cycle.
  • The blood pressure of the systole phase and the diastole phase gives the two readings for blood pressure .

• Types of Circulatory Systems in Animals

  • The advantage to this arrangement is that high pressure in the vessels pushes blood to the lungs and body.
  • The ventricle is divided more effectively by a partial septum, which results in less mixing of oxygenated and deoxygenated blood.
  • The heart is three chambered, but the ventricles are partially separated so some mixing of oxygenated and deoxygenated blood occurs, except in crocodilians and birds.

• The Mechanics of Human Breathing

  • The relationship between gas pressure and volume helps to explain the mechanics of breathing.
  • As volume decreases, pressure increases and vice versa .
  • Due to this increase in volume, the pressure is decreased, based on the principles of Boyle's Law.
  • This decrease of pressure in the thoracic cavity relative to the environment makes the cavity pressure less than the atmospheric pressure .
  • This increases the pressure within the thoracic cavity relative to the environment.