Active and Passive Transport

The epithelial cells that compose the wall of the proximal tubule are joined together by tight junctions only on their apical sides—

Ultrafiltration Nutrients Glomerulus
Glomerulus

■ Figure 17.12 Filtration and reabsorption. Plasma water and its dissolved solutes (except proteins) enter the glomerular ultrafiltrate by filtration, but most of these filtered molecules are reabsorbed. The term reabsorption refers to the transport of molecules out of the tubular filtrate back into the blood.

that is, the sides of each cell that are closest to the lumen of the tubule (fig. 17.13). Each cell therefore has four exposed surfaces: the apical side facing the lumen, which contains microvilli; the basal side facing the peritubular capillaries; and the lateral sides facing the narrow clefts between adjacent epithelial cells.

The concentration of Na+ in the glomerular ultrafiltrate— and thus in the fluid entering the proximal tubule—is the same as that in plasma. The epithelial cells of the tubule, however, have a much lower Na+ concentration. This lower Na+ concentration is partially due to the low permeability of the plasma membrane to Na+ and partially due to the active transport of Na+ out of the cell by Na+/K+ pumps, as described in chapter 6. In the cells of the proximal tubule, the Na+/K+ pumps are located in the basal and lateral sides of the plasma membrane but not in the apical membrane. As a result of the action of these active transport pumps, a concentration gradient is created that favors the diffusion of Na+ from the tubular fluid across the apical plasma membranes and into the epithelial cells of the proximal tubule. The Na+ is then extruded into the surrounding tissue fluid by the Na+/K+ pumps.

The transport of Na+ from the tubular fluid to the interstitial (tissue) fluid surrounding the proximal tubule creates a potential difference across the wall of the tubule, with the lumen as the negative pole. This electrical gradient favors the passive transport of Cl- toward the higher Na+ concentration in the interstitial fluid. Chloride ions, therefore, passively follow sodium ions out of the filtrate into the interstitial fluid. As a result of the accumulation of NaCl, the osmolality and osmotic pressure of the interstitial fluid surrounding the epithelial cells are increased above those of the tubular fluid. This is particularly true of the interstitial fluid between the lateral membranes of adjacent epithelial cells, where

Reabsorption

Filtration

Lumen of kidney tubule

O Glucose

Apical membrane

Basolateral membrane

Proximal tubule cell

Lumen of kidney tubule

O Glucose

Apical membrane

Proximal tubule cell

Capillary

Apical Basolateral Proximal Tubule

Basolateral membrane

■ Figure 17.13 The mechanism of reabsorption in the proximal tubule. The appearance of proximal tubule cells in the electron microscope is illustrated. Molecules that are reabsorbed pass through the tubule cells from the apical membrane (facing the filtrate) to the basolateral membrane (facing the blood). There is coupled transport (a type of active transport) of glucose and Na+ into the cytoplasm, and primary active transport of Na+ across the basolateral membrane by the Na+/K+ pump.

Primary active transport

Capillary

■ Figure 17.13 The mechanism of reabsorption in the proximal tubule. The appearance of proximal tubule cells in the electron microscope is illustrated. Molecules that are reabsorbed pass through the tubule cells from the apical membrane (facing the filtrate) to the basolateral membrane (facing the blood). There is coupled transport (a type of active transport) of glucose and Na+ into the cytoplasm, and primary active transport of Na+ across the basolateral membrane by the Na+/K+ pump.

the narrow spaces permit the accumulated NaCl to achieve a higher concentration.

An osmotic gradient is thus created between the tubular fluid and the interstitial fluid surrounding the proximal tubule. Since the cells of the proximal tubule are permeable to water, water moves by osmosis from the tubular fluid into the epithelial cells and then across the basal and lateral sides of the epithelial

transport (passive)

Na+ H2O follows transport salt by (active) osmosis

Glomerular (Bowman's) capsule

Reabsorption

Filtration

transport (passive)

Na+ H2O follows transport salt by (active) osmosis

Glomerular (Bowman's) capsule

Active Transport Passive Transport
Fluid reduced to V3 original volume, but still isosmotic

■ Figure 17.14 Salt and water reabsorption in the proximal tubule.

Sodium is actively transported out of the filtrate (see fig. 17.13) and chloride follows passively by electrical attraction. Water follows the salt out of the tubular filtrate into the peritubular capillaries by osmosis.

cells into the interstitial fluid. The salt and water that were reabsorbed from the tubular fluid can then move passively into the surrounding peritubular capillaries, and in this way be returned to the blood (fig. 17.14).

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Essentials of Human Physiology

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Responses

  • BANDOBRAS
    Is the lumen side active or passive transport?
    5 years ago
  • annika herlevi
    What is active and passive transport in physiology?
    5 years ago
  • joseph
    Is the epithelium active or passive transport?
    5 years ago
  • Lisa Koenig
    Is glomerular transport active or passive?
    5 years ago
  • richard
    Why passive and active transport are needed for reabsorption?
    5 years ago
  • anssi
    Is filtration a type of active transport?
    5 years ago
  • sami
    How are salt and water reabsorption coupled/?
    5 years ago
  • kaija
    Is transport across the apical membrane active or passive?
    5 years ago
  • danyl white
    Is collecting duct an active or passive transport?
    4 years ago
  • JENS
    Why passive and active transport are need for filtration?
    4 years ago
  • Lucrezia
    Is glomerular filtrate active or passive?
    3 years ago
  • Valentin
    Which kind transport Na in apical membrane and basolateral membrane quizlet?
    2 years ago
  • bilba
    Is glomerulus active or passive?
    2 years ago

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