Early Filtrate Processing
Graphics are used with permission of:
Pearson Education Inc., publishing as Benjamin Cummings (http://www.aw-bc.com)

Page 1. Introduction
• Once the filtrate is formed, the early tubular segments of the nephron reabsorb solutes and water back into the blood to restore its volume and composition.
• They also remove some solutes from the blood and secrete them into the filtrate to fine tune the blood’s composition.

Page 2. Goals
• To examine the passive and active processes of reabsorption and secretion
• To understand how filtrate processing differs in early sections of the tubule.
• To understand the role of the countercurrent multiplier in forming the medullary osmotic gradient

Page 3. Reabsorption: Reclaiming Valued Substances
• The relationship of glomerular filtration and tubular reabsorption is something like a parent who cleans a child's room by arbitrarily throwing much of the room's contents into the trash box. That's analogous to glomerular filtration. But as soon as the parent is gone, the child quickly reclaims his favorite things from the box. This rescue of ‘valued substances’ is like the process of tubular reabsorption of solutes back into the blood. All objects remaining in the trash box will be discarded as waste.

Page 4. Reabsorption Pathways
• To be reabsorbed into the blood, substances in the filtrate must cross the barrier formed by the tubular cells.
• There are two reabsorption pathways:
1. the transcellular pathway
2. the paracellular pathway
• Most solutes that are reabsorbed use the transcellular pathway. They diffuse or are actively transported through the luminal and basolateral membranes of the tubular cells into the interstitial space and then into the peritubular capillaries.
• The second pathway is the paracellular one through the tight junctions into the lateral intercellular space. Certain tight junctions are not as tight as the name implies and will allow this pathway, while others will not.
• Although most substances use the transcellular pathway, water and certain ions use both paths, especially in the proximal convoluted tubule. Both pathways lead into the interstitial space, then through the endothelium of the peritubular capillaries into the blood.
• Label this diagram and using arrows indicate both the transcellular and paracellular pathways:

Page 5. Reabsorption Overview: Diffusion of Water
• The force driving the formation of the filtrate is blood pressure within the glomerulus.
• What drives reabsorption, which reclaims the valued substances?
• How water to diffuses from the lumen through the tight junctions of these generic tubular cells into the interstitial space:
1. Water will move from its higher concentration in the tubule through the tight junctions to its lower concentration in the interstitium.
2. Water will also move through the plasma membranes of the cells that are permeable to water.
** Notice that they use a darker color to indicate a higher osmolarity.
• On this diagram, indicate:
1. Where the water concentration is highest and lowest.
2. Where the osmolarity is higher and lower.

Page 6. Reabsorption Overview: Interstitial Osmolarity
• How can we increase the osmolarity of the interstitium?
• Transporting sodium into the interstitium will raise its osmolarity. Water will then diffuse from the tubule through the tight junctions and permeable plasma membranes to the interstitium, equilibrating the two osmolarities.

Page 7. Reabsorption Overview: Membrane Activity
• How will the reduction in the intracellular sodium ion concentration resulting from the basolateral transport affect the activity at the luminal membrane?
• The transport of sodium ions through the basolateral membrane will cause more ions to be reabsorbed through the luminal membrane.

Page 8. Reabsorption Overview: Summary
• The transport of sodium ions has two important direct results:
1) The interstitial osmolarity increases, causing water