Renal 4 Tubular reabsorption

The process of reclaiming water and solutes from the tubular fluid and returning them to the blood of the peritubular capillaries or vasa recta

about 99% of the filtrate is reabsorbed into the

blood

substances reabsorbed include:

organic nutrients

water and

ions

it is a selective transepithelial process

substances follow a transcellular or paracellular

route

Transcellular route

transported substances move through the luminal membrane, the cytosol and the basolateral membrane of the tubule cells and then the endothelium of the peritubular capillaries

Paracellular route

substances move in between the tubule cells

it is limited because of the presence of tight junctions between the cells

leaky nature of tight junctions in the proximal nephron allows absorption by this route

tubular reabsorption occurs via three

mechanisms:

osmosis

diffusion

active Transport

Sodium reabsorption

most abundant cation in the filtrate

Na+ absorption is via the transcellular route

it is almost always active

80% of the energy requirement in the kidneys is used for Na+ transport

it is absorbed throughout the tubule with differing rates

it involves the Na+-K+-ATPase located in the tubular basolateral membrane

as it transports Na+ out of the tubular cells into the lateral space, it keeps the intracellular Na+ concentration low whiles it builds up the lateral space Na+ concentration

active pumping of Na+ by the tubular cells creates a strong electrochemical gradient that favors its passive entry at the luminal face

Na+ transport does not exhibit transport maximum

Reabsorption of glucose and amino acids etc

Secondary active transport is responsible for absorption of glucose, amino acids, vitamins, and most cations,

they are cotransported with Na+ when it moves along its concentration gradient from the lumen into the tubular cells

these solutes diffuse across the basolateral membrane before moving into the peritubular capillaries via non-energy-dependent carriers

the transport systems for these solutes are specific and limited

these transport systems have atransport/tubular maximum (Tm)

this determines renal threshold for reabsorption of substances in tubular fluid

a Tm reflects the number of carriers in the renal tubules available to ferry each substance

when the carriers are saturated, excess of that

substance is excreted in urine

Reabsorption of water, Cl- and urea

These are reabsorbed by passive tubular reabsorption which encompasses osmosis, diffusion and facilitated diffusion

these substances move down their electrochemical gradient

Water

movement of Na+ and solutes establishes a strong osmotic gradient that enhances water movement by osmosis into the peritubular capillaries

this movement is aided by transport proteins called aquaporins

this is obligatory water reabsorption

different types of water channels are present in various parts of the nephron

Cl-

Na+ movement into the peritubular capillaries establishes an electrical gradient that favors passive absorption of anions especially Cl- to restore electrical neutrality in the filtrate and plasma

Urea

A waste product from the breakdown of proteins

the osmotically induced reabsorption of water secondary to active Na+ reabsorption produces a concentration of urea which favors its reabsorption