Renal system

What the exam asks

Expect a table of plasma and urine concentrations, a clearance figure, or a nephron diagram with a drug acting at a labelled point. The single most common item is a clearance table: you are given values and asked how a substance is handled, and the whole question is comparing each clearance against GFR rather than against zero. A clearance above GFR is the one candidates misread, and it has exactly one explanation, which is that filtration alone cannot deliver that much so the rest was secreted. The other trap is treating ADH as though it moves water itself. It opens the channel; the medullary gradient does the moving, which is why a stem that abolishes the gradient makes ADH irrelevant no matter how much of it is present.

The kidney is responsible for the composition of the blood: its volume, its salt and water content, its pH, and the removal of nitrogenous waste. It does this by a design worth stating once, because every question rests on it. Rather than picking waste out of the plasma, the glomerulus throws almost everything small out of the blood indiscriminately, and the tubule then takes back what is worth keeping. Regulation happens on the way back, not on the way out.

Four processes give you every answer: filtration at the glomerulus, reabsorption from tubule to blood, secretion from blood to tubule, and excretion, which is what is left. Excreted equals filtered plus secreted minus reabsorbed. Most of what looks like a hard renal question is that sentence applied to a table.

The exam supplies the anatomy and the numbers. What it wants is the reasoning: what a clearance value implies about a substance's handling, why the loop of Henle's hairpin makes a concentrating kidney possible at all, why a hormone that opens water channels does nothing without a gradient for water to move down, and where a named diuretic acts given what it blocks.

What to hold

• The filtrate leaving the glomerulus is plasma without its cells and large proteins, which is why protein in the urine points at a damaged filter rather than a busy one.
• Filtration is driven by glomerular hydrostatic pressure and opposed by plasma oncotic pressure and the pressure already in Bowman's capsule.
• Constricting the efferent arteriole raises pressure inside the glomerulus and so raises filtration; constricting the afferent arteriole lowers both.
• Clearance is the volume of plasma cleared of a substance per unit time: C equals urine concentration times urine flow rate, divided by plasma concentration.
• Inulin is freely filtered and neither reabsorbed nor secreted, so its clearance equals the glomerular filtration rate and is the yardstick every other clearance is read against.
• For a freely filtered substance, clearance below GFR means net reabsorption, clearance above GFR means secretion, and clearance of about zero means it is reabsorbed completely.
• Reabsorption by a carrier has a ceiling, so once the filtered load exceeds the transport maximum the excess stays in the tubule and appears in the urine, which is why glucose appears there in uncontrolled diabetes.
• The thick ascending limb pumps salt out and is impermeable to water, which can only produce a modest difference between tubule and interstitium at any one level.
• The countercurrent arrangement is what turns that small difference at each level into a large gradient from cortex to medulla, because the descending limb keeps feeding already concentrated fluid into the bend for the pump to work on again.
• The vasa recta run in hairpins for the same reason in reverse: blood picks solute up going down and drops it off coming back, so the medulla is supplied without the gradient being washed away.
• ADH makes the collecting duct permeable to water, but water only leaves if the medulla is concentrated, so ADH without the gradient does nothing.
• Every diuretic works by blocking sodium reabsorption somewhere along the tubule, and water follows the sodium it failed to follow back.
• Loop diuretics are the most powerful because the segment they block is both handling a large sodium load and building the medullary gradient, so the kidney loses the ability to concentrate urine at all.