Glomerular Filtration and Tubular Function

Glomerular Filtration

• Normal GFR is ~125ml/min

• Correlates well with surface area
• 10% lower in women than men
• 180L/day, but only 1 L/day becomes urine, therefore 99% is reabsorbed

• GFR is across a glomerular capillary is determined by:

• Capillary size, permeability and hydrostatic/osmotic pressure gradients across the capillary wall
• GFR = K_f[(P_GC - P_T) - (π_Gc - π_T)

• Where K_f is the glomerular ultrafiltration coefficient is the product of the glomerular capillary wall hydraulic conductivity and the effective filtration surface area
• P_GC is the mean hydrostatic pressure in the glomerular capillaries
• P_T is the mean hydrostatic pressure in the tubule (Bowman's space)
• π_Gc is the oncotic pressure of the plasma in the glomerular capillaries
• π_T is the oncotic pressure of the filtrate in the tubule (Bowman's space)

• Ultrafiltration occurs because the balance of Starling's forces in the glomerular capillaries favours net filtration
• Ultrafiltration - refers to the process whereby all substances except colloids (plasma proteins) are filtered across a semipermeable membrane
• GFR is therefore altered by renal blood flow, systemic/renal blood pressures, changes in K_f, changes in filtration surface area, ureteral obstruction and changes in oncotic factors
• Renal vascular resistance changes due to autoregulation stabilise filtration pressure, but when mean systemic artery pressure drops below range of autoregulation, GFR drops sharply
• Filtration Fraction - the ratio of GFR to RPF, or the amount of renal plasma flow which reaches the tubule. Normally 1/5th or about 0.17 to 0.20.

Permeability

• Glomerular capillaries are 50x more permeable than skeletal muscle capillaries
• Neutral substances <4nm are freely filtered, >8nm diameter filtration approaches zero
• Between 4 and 8nm filtration is inversely proportional to diameter
• Negatively charged particles are pass through the capillaries in half the amount of neutral substances the same size due to negatively charged sialoproteins in the glomerular capillar wall

• This is why negatively charged albumin has a glomerular concentration of only 0.2% of its plasma concentration, despite being 7nm

• Positively charged particles have a filtration rate greater than that of neutral substances
• Amount of protein is normally very small, and comes from shed tubular cells rather than filtrate
• In nephritis, the negative charges in the glomerular wall are dissipated, and albuminuria can occur without an increase in the size of the membrane pores

Capillary Bed Size

• K_f - can be altered by mesangial cells - contraction causes a decrease by reducing the area available for filtration
• Contraction of points where capillary loops bifurcate and distortion of capillary lumen with contraction by decreasing flow to capillary lumen
• Mesangial contraction is caused by: Endothelins, angiotensin II, vasopressin, noradrenaline. Mesangial relaxation is caused by: ANP, dopamine, PGE2 and cAMP
• Angiotensin II is important in regulating mesangial contraction, and there are angiotensin II receptors in the glomeruli
• Mesangial cells make renin

Hydrostatic & Osmotic Pressure

• Glomerular capillary pressures are high pressure because their afferent arterioles are short, and the downstream efferent arterioles have high resistance, which maintains upstream pressure
• Capillary hydrostatic pressure is opposed by hydrostatic pressure in Bowman's capsule, and by the oncotic pressure gradient across glomerular capillaries (π_Gc - π_T)

• (π_T is normally negligible therefore the oncotic pressure gradient is equal to the oncotic pressure of the plasma proteins

• Net filtration pressure is ~15mmHg at the beginning of the capillaries, falling to 0mmHg by the end

Tubular Function

• After being filtered, the tubular cells may:

• Add more of the substance to the filtrate (tubular secretion) or
• Remove some or all of the substance from the filtrate (tubular reabsorption)
• Or it may do both.

• Amount of a substance excreted per unit time equals (amount filtered + net amount transferred by the tubules T_x)
• Clearance = GFR if there is no net tubular secretion/reabsorption (inulin)
• Clearance > GFR if there is net tubular secretion (PAH)
• Clearance < GFR if there is net tubular reabsorption (glucose)

Mechanisms of Tubular Reabsorption & Secretion

• Small proteins and some peptide hormones are reabsorbed in the tubules by endocytosis
• Other substances move in or out of tubules via passive diffusion down chemical/electrical gradients, or active transport against gradients
• Movement occurs via ion channels, exchangers, cotransporters and pumps
• Different set of pumps/transporters occurs in the luminal membrane vs. the basolateral membrane. This polarised distribution makes net movement of solutes across epithelia possible.
• Tm - the transport maximum - the maximal rate at which a solute can be transported. Transport rate of a solute is proportional to amount present up to this level. At higher concentrations it is saturated and no further increase in transport occurs
• Tubular epithelium is leaky epithelium, meaning that tight junctions between cells allow some water/electrolytes to pass. This occurs especially in the proximal tubule.
• Daily solute load is 700mOsm/day, made up of:

• Na⁺ - 100-150mmol/day
• K⁺ - 70-100mmol/day
• Cl^- - 150mmol/day
• Urea - 400mmol/day

Na⁺ Reabsorption

• Na⁺/Cl^- reabsorption is vital in electrolyte/water homeostasis
• Na⁺ transport is coupled to movement of H⁺, glucose, amino acids, organic acids, phosphate and other electrolytes
• In proximal tubules, thick portion of ascending limb of the loop of Henle, distal tubules and collecting ducts, Na⁺ moves by cotransport or exchange from the tubular lumen into the tubular epithelial cells, down its concentration/electrical gradients, and is then pumped into the interstitial space
• Na⁺ is pumped into the interstitium by Na/K ATPase, which extrudes 3 Na⁺ for each 2 K⁺ pumped into the cell
• 60% of filtered Na⁺ is reabsorbed in the proximal tubule, mainly by Na-H exchange
• 30% is then absorbed in the thick ascending loop of Henle via the Na-2Cl-K cotransporter

(In both the proximal tubule and thick ascending loop, passive paracellular Na⁺ movement also contributes to overall Na⁺ reabsorption)

• 7% of filtered Na⁺ is absorbed in the distal convoluted tubule by the Na-Cl cotransporter
• The remaining 3% is absorbed via ENaC channels in the collecting ducts under aldosterone regulation, permitting homeostatic adjustment to Na⁺ balance

Tubuloglomerular Feedback and Glomerulotubular Balance

• Tubuloglomerular feedback - signals from each renal tubule feed back to affect filtration in its glomerulus
• As the rate of flow through the ascending limb of the loop of Henle and first part of the distal tubule increases, glomeruar filtration in that nephron decreases
• The sensor mediating this response is the macula densa, which is an area of closely packed cells in the wall of the distal tubule which lie at the vascular pole of the glomerulus
• Na⁺/Cl^- enter macula densa cells, causing increased Na/K ATPase activity
• Increased ATP hydrolysis causes more adenosine to be formed, which acts on macula densa cells to increase their release of Ca²⁺ to the vascular smooth muscle in afferent arterioles
• This causes afferent vasoconstriction and a resulting decrease in GFR
• Also causes decreased renin secretion by adjacent juxtaglomerular cells in the afferent arteriole
• Glomerulotubular balance - increase in GFR causes increase in solute and water reabsorption in the proximal tubule and thick ascending limb
• Particularly prominent for Na⁺ reabsorption, occurring seconds after a change in filtration
• Mediated partly by increased oncotic pressure in the afferent arterioles and capillary branches, increasing Na⁺ reabsorption from the tubule

Water Transport

• 180L fluid filtered through the glomeruli each day, with a daily average urine volume of ~1L
• Maximal urine volume is about 22 litres per day - up to 12% of GFR

• Urine osmolality reaches its minimal value of 30-60mOsm/kg

• Obligatory water loss - the minimum water loss as urine to excrete daily solute load - about 500mls/day (to secrete a load of 700mOsm at 1400mOsm/kg

• Total obligatory water loss is higher, as the body has other sources of obligatory water loss - insensible/GIT losses

• Reabsorption of filtered water can be varied without effecting total solute excretion:

• When urine is concentrated, water is retained in excess of solute
• When urine is dilute, water is lost from the body in excess of solute

• The key regulator of water output is vasopressin
• Aquaporins - allow rapid diffusion of water across cell membranes
• 70% of filtered water is removed in the proximal tubule - mostly via aquaporin 1

• This is a passive process - water moves down its concentration gradient via osmosis - from an area of low solute concentration to an area of high solute concentration
• The reabsorption of solute causes this water movement - namely Na⁺, transported via Na/K ATPase in the basolateral membrane of proximal tubule cells

• 15% of filtered water is removed in the loop of Henle

• This area has a graded increase in osmolality, reaching up to 1200 mOSM/kg of H₂O in the tips of the papillae
• Descending limb of the loop of Henle is permeable to water due to presence of aquaporin 1

• The ascending limb is impermeable to water, but reabsorption of solutes here is vital in delivering hypotonic fluid to the distal tubule
• Na⁺, K⁺ and Cl^- are cotransported out of the thick segment of the ascending limb

• This means that fluid in the descending limb becomes hypertonic as water moves out of the tubule into the hypertonic interstitium
• By the time fluid reaches the top of the ascending limb, it is hypotonic to plasma

• In the thick ascending limb, a carrier cotransports one Na⁺, one K⁺ and 2 Cl^- from the tubular lumen into the tubular cells

• This is secondary active transport - Na⁺ is actively transported from the cells into the interstitium by Na/K ATPase in the basolateral membranes of the cells, keeping intracellular Na⁺ low
• K⁺ diffuses back into the tubular lumen and back into the interstitium via ROMK and other K⁺ channels

• Distal Tubule - 20% of filtered water reaches here - the first part of this is an extension of the thick segment of the ascending limb

• Relatively impermeable to water, and continued removal of solute in excess of solvent further dilutes the tubular fluid

• Collecting Ducts

• Changes in osmolality and volume in the collecting ducts depend on vasopressin
• ADH from posterior pituitary increases collecting duct permeability to water
• Aquaporin-2 is the transporter responsible, stored in vesicles in the cytoplasm of principal cells
• Vasopressin causes rapid insertion of these vesicles into the apical membrane of cells, mediated via the vasopressin V2 receptor
• With enough vasopressin to cause maximal antidiuresis, water moves out of the hypotonic fluid, entering the cortical collecting ducts into the cortical interstitium, and the tubular fluid becomes isotonic, removing as much as 10% of filtered water
• A further 4.7% more filtrate is reabsorbed into the hypertonic interstitium of the medulla, producing concentrated urine with an inulin TF/P > 300

• Maximal urine concentration is 1400mOsm/kg
• If vasopressin is absent, collecting duct epithelium is largely impermeable to water, and large amounts of hypotonic urine are produced

Metabolic Water Production

• Water produced during oxidation of food in the final reaction of the electron transport chain, catalysed by cytochrome oxidase:

2H + 1/2O2 = H2O

• 5ml/kg/day (350 to 400mls/day)