ultra filtration and re absorption (1)

8/8/2019 Ultra Filtration and Re Absorption (1)

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Objective: Describe and explain the production of urine, with reference to the

processes of ultrafiltration and selective reabsorption.

Starter: Name the parts of the nephron!


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Detail of the glomerulus and Bowmans capsule

fe

nestratedendothelium

primaryprocess

podocytecell bodysecondary

process(pedicel)

basallamina

podocyte


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Blood enters the glomerulus through the afferentarteriole. It is wider than the efferent arteriole thatcarries blood away. Because more blood is enteringthe glomerulus than can leave at any one time,

pressure builds up in glomerulus. This pressure forces water and small molecules out of

the blood, through small pores in the endothelial wallsof the glomerular capillaries. These pores aresometimes called fenestrations.


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The filtrate meets the basement membrane. This ismade of a mesh of collagen fibres and glycoproteins.It acts as a filter and will only let molecules smallerthan 69000 relative molecular mass through.

Big proteins and blood cells are too large to passthrough the basement membrane and so remain in theblood.

As most of the liquid content of the blood has beenlost but it still has large proteins in it, this greatlyreduces the water potential of the blood (makes it

more negative). This will be important later!


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The epithelial cells of the Bowmans capsuleare called podocytes.

Podocytes have large finger-like projections

known as major processes or pedicels. The pedicels ensure that there are gapsbetween cells for the filtrate to pass throughand drain into the lumen of the proximalconvoluted tubule.


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Ultrafiltration removes the

following from the blood:-

WaterA

minoA

cidsGlucoseUreaInorganic ions (sodium, chloride, potassium)

Can you see a problem with the substancesthat have been removed from the blood?


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Ultrafiltrationhappens at theBowmans Capsule,

but many usefulsubstances havebeen removedfrom the blood.

These need to bereabsorbed.

The filtratemoves from theBowmans Capsuleinto the proximalconvoluted

tubule. 85% of thefiltrate isreabsorbed backinto the blood inthis region of thenephron.


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1. Sodium/potassium pumps in the basement membrane of the PCTepithelial cellsactively transport sodium out of the cells into the tissue fluid, reducing the sodiumconcentration in the PCTcell. The numerous mitochondria provide the ATP neededfor the pumps to work.

2. This creates a diffusion gradient for the movement of sodium ions from the filtratethrough co-transporter proteins into the PCTcell. These proteins also co-transport

glucose and amino acids into the PCT. This is an example of facilitated diffusion. Asthe concentration of glucose and amino acids is now high in the PCT cell this reducesthe water potential of the cell and creates a diffusion gradient for movement ofwater from the filtrate into the PCTcell.

3. As the concentration of glucose and amino acids is now high in the PCTcell, thiscreates a diffusion gradient for their movement from the PCTcell into the tissue

fluid then into the blood capillary. As this has now reduced the water potential ofthe blood, water will also move from the PCTcell into the blood.


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Adaptations for Reabsorption

Structure Function

Microvilli

One cell thick

Mitochondria

Proximity of

peritubular capillariesCo-transport proteins

Protein pumps

ultra filtration and re absorption (1)

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