passive diffusion

Examples of passive diffusion in the following topics:

• Dalton's Law of Partial Pressure

  • In the lungs, the relative concentration of gasses determines the rate at which each gas will diffuse across the alveolar membranes.
  • These pressure differences explain why oxygen flows into the alveoli and why carbon dioxide flows out of the alveoli through passive diffusion (just as a similar process explains alveolar and arterial gas exchange).
  • While inhaled air is similar to atmospheric air due to Dalton's law, exhaled air will have relative concentrations that are in between atmospheric and alveolar air due to the passive diffusion of gasses during gas exchange.

• Tubular Secretion

  • This secretion is caused mainly by active transport and passive diffusion.
  • Passive diffusion—the movement of molecules from the peritubular capillaries to the intersitial fluid within the nephron.

• Pressure Changes During Pulmonary Ventilation

  • Differences in partial pressures of gasses between the alveolar air and the blood stream are the reason that gas exchange occurs by passive diffusion.
  • Recall that gasses travel from areas of high pressure to areas of low pressure, so the greater pressure of oxygen in the alveoli compared to that of the deoxygenated blood explains why oxygen can passively diffuse into the bloodstream during gas exchange.
  • The partial pressure, and thus concentration of carbon dioxide, is greater in the in the capillaries of the alveoli compared to the alveolar air, so carbon dioxide will passively diffuse from the bloodstream into the alveoli during gas exchange.

• Absorption in the Small Intestine

  • Absorption of nutrients occurs partially by diffusion through the wall of the small intestine.
  • Digested nutrients pass into the blood vessels in the wall of the intestine through a process of diffusion.
  • Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions: iron is absorbed in the duodenum; vitamin B12 and bile salts are absorbed in the terminal ileum; water and lipids are absorbed by passive diffusion throughout the small intestine; sodium bicarbonate is absorbed by active transport and glucose and amino acid co-transport; and fructose is absorbed by facilitated diffusion.

• Bacterial Flora

  • For example, undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by the bacteria in the large intestine, and then are absorbed by passive diffusion.

• Lungs

  • During normal breathing, expiration is passive— no muscles are contracted—and the diaphragm relaxes from its contracted state.
  • Oxygen dissolves through the extracellular matrix of the alveoli that lets the gas diffuse into the capillaries based on the relative partial pressures of the gasses (gasses flow from areas of high pressure to low pressure).
  • Oxygen passively diffuses into the deoxygenated blood of the capillaries while carbon dioxide passively diffuses out of the deoxygenated blood and into the airways.

• The Reason for Breathing

  • In the alveoli tissue of the lungs, the exchange of oxygen and carbon dioxide molecules between the air and the bloodstream occurs by passive transport, so that oxygen is taken in and carbon dioxide and water are removed.
  • Passive diffusion (also called bulk flow) is the term for the movement of these gases between the air and bloodstream based on their relative concentration, with the gas with the greater concentration moving across to the area with the lower concentration.

• Tubular Reabsorption

  • Reabsorption in the nephron may be either a passive or active process, and the specific permeability of the each part of the nephron varies considerably in terms of the amount and type of substance reabsorbed.
  • Passive diffusion—passing through plasma membranes of the kidney epithelial cells by concentration gradients.

• Blood Flow in the Lungs

  • Oxygen passively flows from the air inside the alveoli into the blood in the alveolar capillaries, while carbon dioxide passively flows in the opposite direction.
  • The air, along with the diffused carbon dioxide, is then exhaled.

• Bulk Flow: Filtration and Reabsorption

  • Capillary fluid movement occurs as a result of diffusion (colloid osmotic pressure), transcytosis, and filtration.
  • Bulk flow is one of three mechanisms that facilitate capillary exchange, along with diffusion and transcytosis.
  • Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium, and then the transport of these substances from the interstitium into the bloodstream