KIDNEY I - FILTRATION, REABSORPTION AND SECRETION


Understand the nature of the glomerular filter and dynamics of ultrafiltration

There are three layers of glomerular filtration, two of which are cellular: the epithelial cells of the glomerular capillaries (cellular), the glomerular basement membrane (non-cellular), and the slit diaphragm (cellular).

First filtration layer: The epithelial cells of glomerular capillaries are highly fenestrated, with pores up to 50nm (compare with about 1nm in brain capillaries), large enough to allow the free flow of water and dissolved solutes, but small enough to not allow the passage of red and white blood cells and platelets into the filtrate. The capillaries are surrounded by negatively charged proteins - anionic glycoproteins which prevent the flow of plasma proteins into the filtrate. This is why little to no proteins are usually found in the urine filtrate.

Second filtration layer: the glomerular basement membrane, made of collagen IV and proteoglycans, offering further barriers for the passage of proteins. In the kidneys, this basement membrane is more than 5 times thicker than that of other vessels.

Third filtration layer: formed by podocytes; cells shaped like an octopus with interdigitated pedicles (foot processes) which wrap around the glomerular capillaries themselves. Between these pedicles there are slit diaphragms, which also prevent the passage of proteins into the filtrate. Genetic defects in these diaphragms results in massive leakage of proteins into the urine, proteinuria.

Other issues can be haemoglobin in urine (haemoglobinuria) and red cells in urine (haematuria).

Ultrafiltrate = formed under high pressure, hydrostatic pressure of the blood. Two forces; one favouring the flow of plasma into the glomerular capsule because the afferent arteriole is larger than the efferent arteriole, and the hydrostatic pressure of the fluid already in the glomerular capsule. But since the protein concentration of the tubular fluid is low compared to plasma, the greater colloid pressure in the blood promotes the osmotic return of the filtered water. So the net filtration pressure is only about 10mmHg.

Define Clearance and its use in the study of renal physiology

Clearance is the volume of plasma cleared of a substance per unit of time: so the units are ml/min or ml/24h.

C = UV/P

Where U is the urine concentration, V the urine volume and P the plasma concentration.

What could happen is that a substance found in the filtrate can either:

1) be filtered and secreted and not reabsorbed: it is fully removed by by the kidneys in one passing through the glomerulus, eg. PAH (Paraaminohepuric acid)

2) be partially filtered but then some of it reabsorbed: such as water and most electrolytes

3) be partially filtered but then fully reabsorbed: such as glucose and bicarbonate ions.

Explain how glomerular filtration rate and effective renal plasma flow are measured

Glomerular Filtration Rate GFR

Volume of fluid filtered from the glomeruli per minute (ml/min).

- So you want something that is filtered, but not absorbed or secreted: Inulin is used for this measurement, with the same formula C = UV/P.

- It can also use BUN and creatinine (serum) to estimate GFR.

Depends on:

1) Starling forces

2) Surface area of filtration interface

3) Hydraulic permeability of capillaries

Is regulated by both neural and hormonal input.

GFR decreases with age, although serum (creatinine) remains constant because of decreased muscle mass.

Effective Renal Plasma Flow RPF

Firstly, it is called effective because not all plasma/blood going to the kidney ends up being filtered. For example, some of it goes to the fat surrounding the kidney and does not contribute to the production of urine, so there is about a 10% underestimation of the true RPF.

It is about 25% of the cardiac output.

Directly proportional to the pressure difference between the renal artery ad the renal vein.

The measurement is made with PAH (paraaminohippuric acid) because it is filtered and secreted by renal tubules.

RBF = RPF/(1-Hematocrit)

1-hematocrit is the fraction of blood volume occupied by plasma.

Starling Forces: are opposing forces: on on hand the hydrostatic pressures from the difference between afferent-efferent arterioles pushing the plasma out of the blood vessels and on the other hand the colloid osmotic/oncotic pressures pushing the water back in because of the higher concentrations of proteins in the blood. The net filtration pressure ends up being 10-16mmHg.

PGC = hydrostatic pressure

Describe the processes of tubular reabsorption of glucose, amino acids etc.

What do we reabsorb from the filtrate?

Water!, Glucose, amino acids, water soluble vitamins (B complex and C), lactate, acetate, ketones, Krebs cycle intermediates, 100% of polypeptides and 50% of urea, 65% of Na+, 65% of Cl-, 65% of H2O, 55% of K+, 80% of Ca2+, 85% of PO42-, 80% of HCO3-.

What do we secrete from plasma into the glomerular filtrate?

100% of organic anions (PAH, urate, bile starts, fatty acids, hyroxybenzoates, acetazolamide, chlorothiazide, penicillin, silicates, slfonamides, 100% of organic cations, acetylcholine, creatinine, dopamine, epinephrine, noreprinephine, histamine, serotonin, atropine, isoproterenol, cimetidine, morphine, H+ and NH+

How is any absorption or secretion done?

Things are hard to reabsorb: we can't afford to lose water, we would be peeing 180L a day if we wouldn't absorb it, but we can't actively pump water itself. We can't afford to lose salts and amino acids so we have a mechanism to drive things in the direction we want them to go. The main star of this whole process is Na+. This is how it works to get things into the cells of the proximal convoluted tubule:

1) We establish a sodium gradient. This is done with Na+/K+ pumps on the basolateral side of the tubule which keep pumping K+ into the cell and Na+ out of the cell. (The increasing K+ can just diffuse back out on the same side through K+ channels located there).

2) We place many Na+ channels on the luminal side, and we couple them with other things we want to get into the cell:

- Na+ and glucose cotransporters - SGLT-Na+ dependent glucose cotransporter (and then GLUT 1 or 2 facilitated transported on the basolateral side) -Na+ and H+ antiports (to maintain the electrochemical gradient, and pH as will be explained later) By coupling and conditioning the movement of something energetically favourable (the movement of Na+ down its concentration gradient we made: from the lumen of the tubule into the cell of the tubule) with something we want to move into the cell, we can now get glucose and amino acids back into our cells. -Na+ and amino acid cotransporters - many different types, but with overlapping amino acid specificity. The little filtered protein is endocytosed and degraded into amino acids: important for the destruction of small hormones such as insulin and growth hormone!

This movement of solutes into the cell also then favours the movement of water into the cells, so a lot of water is already reabsorbed in the proximal convoluted tubules. Cl- then also diffuses into the cell down the electrochemical gradient established: this is done in gap junctions between the cells of the tubule. - Urea, chloride, potassium, calcium, bicarbonate are all passively reabsorbed

So:

1) the movement of Na+ out of the cell is done by active transport with Na+/K+ pumps (gradient for Na+ to flow into the cell established) 2) the movement of aa and glucose is done by secondary active transport through channels paired with Na+

3) the movement of water into the cells is done by osmosis, following the movement of all these ions into the cell - the cells of the proximal tubule are very permeable to water

3) the movement of Cl- is done passively, following its electrochemical gradient

This results in about 65% of the original salts and water already reabsorbed in the proximal tubule!

However, these transporters can become saturated, if you do have too much glucose, they can all be taken up, and therefore, that can lead to excretion of glucose in the urine.

What does this tell us about the properties of the actual cells in the proximal tubule? They have high amounts of mitochondria, because they are doing a lot of active transport and also, their luminal membranes must have a lot of microvilli to ensure a large surface area for these transport processes. This makes the cells of the proximal tubule the most acidophilic cells in the body: this is because they stain the best with an acid dye. The reason is that mitochondria are rare in the fact that they contain basic environments with a pH of about 8, and therefore attract acid dye well.

Describe the processes of tubular secretion of organic acids and bases

These are usually bound to proteins, even if they are small molecules so they cannot be removed from the blood/filtered easily What is secreted?

Organic acids (anions)

- Endogenous molecules: bile salts, fatty acids, prostaglandins

- Drugs: furosemide, penicillin, acetazolamide

- Diagnostic agents: paraaminohippuric acid PAH

How?

1) Sodium coupled co-trasport of dicarboxylate DC-: flows down Na+'s concentration gradient and accumulates in the cell: secondary active transport 2) DC- can then be exchanged for an organic anion OA- through organic anion transporters OAT1 or OAT3

3) OA- then enters the tubule lumen via ATP-dependent transporters

Organic bases (cations) - Endogenous molecules: creatinine, dopamine, choline, guanidine, histamine, serotonin, adrenaline

- Drugs: atropine, cimetidine and morphine

How?

1) They enter the cell via facilitated organic cation transporters OCT2 2) They enter the tubule lumen via multi drug and toxin extrusion proteins MATEs anti porter in exchange for H+ and/or OCTN

#ppp #kidneys