Structure and Function of the Renal Tubule - study from book Flashcards Preview

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What are several techniques to investigate tubular function?

1) clearance studies
2) micropuncture and isolated perfused tubule
3) electrophysiological analysis (potential measurement, patch clamping)

The first one applies to humans, while the second and third apply to lab animals.


What are the two types of nephrons?

We have the cortical nephrons, which make up about 85% of the nephrons. They have a short LoH.

Then, we have juxta-medullary nephrons, which make up 15% of the nephrons. They have a long LoH.


RECAP: describe the proximal convoluted tubule.

It is directly adjacent to the Bowman's Capsule.

It has a high capacity for reabsorption - with special cellular characteristics:
- highly metabolic, numerous mitochondria for active transport
- extensive brush border on luminal side, which provides a large surface area for rapid exchange


RECAP: what are the functions of the PCT (proximal convoluted tubule)?

It is a major site for reabsorption. Around 65-70% of the filtered load is reabsorbed here.


What syndrome is associated with a defective PCT?

Fanconi's Syndrome is when all the PCT reabsorptive mechanisms are defective.


Describe the Loop of Henle (three segments).

The Loop of Henle consists of 3 functionally distinct segments:
- Thin Descending
- Thin Ascending: thin epithelial cells, no brush border, few mitochondria and low metabolic activity
- Thick Ascending: thick epithelial cells, extensive lateral intercellular folding, few microvilli, many mitochondria for high metabolic activity.


RECAP: what are the functions of the Loop of Henle?

The LoH has a critical role in concentrating/ diluting urine. It does this by adjusting the rate of water secretion/ absorption.


Describe the Medullary Osmotic Gradient.

1) The LoH creates an osmolality gradient in the medullary interstitium.
2) The collecting duct transverses the medulla: urine is concentrated as water moves out by osmosis.


What maintains the medullary osmotic gradient (describe the structure)?

The vasa recta are capillaries that flow in parallel to the Loops of Henle. The blood flow to the medulla is set up so as to maintain the osmotic gradient.

The vasa recta delivers O2 and nutrients to the cells of the Loop of Henle. The vasa recta, like ther capillaries, is permeable to both H2O and salts, and could disrupt the salt gradient established by the LoH. To avoid this, the vasa recta acts as a counter-current delivery multiplier system as well.

As the vasa recta descends into the renal medulla, water diffuses out into the surrounding fluids, and salts diffuse in. When the vasa recta ascends, the reverse occurs. As a result, the concentration of salts in the vasa recta is always about the same, and the salt gradient established by the Loop of Henle remains in place.

Water is removed by the vasa recta to so it doesn't dilute the longitudinal osmotic gradient. The medullary blood flow in the vasa recta is slow, which is sufficient to supply the metabolic needs of the tissue, but minimise solute loss from the medullary interstitium.

The reabsorbed Na+ in the descending vasa recta is carried to the inner medulla, equilibrating with the ISF, and increasing regional osmolarity. Na+ in the ascending vasa recta returns to the systemic circulation. The amount of solute in the ascending vasa recta is the product of flow rate and concentration (eg. if the blood flow in the vasa recta increases, then the solutes are washed out of the medulla and its interstitial osmolality is decreased, and vice versa.


Describe the distal convolutes tubule and the connecting/ collecting tubule.

- FIRST PART (macula densa): linked to the juxtaglomerular complex. It provides feedback control of the GFR and tubular fluid flow in the same nephron.
- SECOND PART: very convoluted.

It connects the end of the DCT to the collecting duct - mainly in the outer cortex.
There are overlaps in functional characteristics with the second part of the DCT.


RECAP: describe the functions of the DCT.

Solute reabsorption continues without H2O reabsorption (it has very low H2O permeability). There is high Na+/ K+ - ATPase activity in the basolateral membrane.

There is the further dilution fo the tubular fluid. The ADH can exert its actions. It has a role to play in acid-base balance via the secretion of NH3.


Describe the collecting duct.

Collecting ducts are formed by the joining of collecting tubules (cuboidal epithelia, with very few mitochondria).

There are 2 types of cells:
- intercalated cells: involved in acidification of urine and acid-base balance
- principal cells: have a role to play in sodium balance and ECF volume regulation

This is the final site for processing urine. It is made very permeable to H2O by ADH. It is also permable to urea.

Those last three points mean that it contributes to the counter-current system.


List some major factors contributing to the build-up of solute concentration in the renal medulla.

1) The active transport of Na+ and the co-transport of K+ and Cl- out of the thick ascending limb into the medullary interstitium.
2) The active transport of ions from the collecting ducts into the medullary interstitium.
3) The facilitated diffusion of large amounts of urea from the collecting ducts into the medullary interstitium.
4) The (very little) diffusion of water from the ascending limbs of tubules into the medullary interstitium.


What is Polycystic Kidney Disease (PKD)?

It is a genetic disorder characterised by the growth of numerous cysts in the kidney.


Describe a disease of the glomerulus.

It is usually called glomerulonephritis (GN).
It is the inflammation of the glomeruli of some or all of the million nephrons in the kidney. It can be primary or secondary to systemic diseases like diabetes mellitus.

There are also inherited diseases of the glomerular basement membrane.


Describe diseases of the tubules.

- it could occur from obstruction (reducing glomerular filtrations)
- it could also occur from impairment of transport functions (reducing water and solute reabsorption), eg. Fanconi's Syndrome.


List some acquired kidney disease.

The kidneys regulate ECF volume and hence influence blood pressure. Compensatory mechanisms in response to a high BP can leads to chronic kidney damage.

The fall in cardiac output leads to renal hypoperfusion, which is registered as hypovolaemia, and the compensation results in pulmonary oedema.

As a consequence of diabetes, the filtering system of the kidnes gets destroyed over time.

This results in acquired nephrogenic diabetes insipidus, due to the reduction of AQP2 expression.

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