Difficulty: Intermediate | Prerequisites: Basic cell biology (membrane structure, osmosis, diffusion), introductory biochemistry (active vs. passive transport, ATP), and familiarity with the circulatory system.
Every cell in your body needs a tightly controlled internal environment to function: the right ion concentrations, the right osmotic pressure, the right pH. The kidneys are the organs that make this possible, filtering roughly 180 litres of blood plasma per day and fine-tuning what stays and what goes. This chapter sits at the intersection of cell biology and whole-organism physiology, connecting membrane transport (channels, pumps, cotransporters) to systemic homeostasis (blood pressure, hydration, waste removal). You should already be comfortable with passive and active transport across membranes before diving in.
The kidneys maintain homeostasis by filtering blood at the glomerulus, then selectively reabsorbing useful solutes and water while secreting wastes along the nephron's tubule segments. Each segment of the nephron (proximal tubule, loop of Henle, distal tubule, collecting duct) has distinct permeability and transporter profiles that together produce urine of the right concentration. Hormones such as vasopressin and aldosterone fine-tune the process minute to minute.
Osmoregulation
The active regulation of osmotic pressure in an organism's fluids to maintain homeostasis. Think of it as the body's way of keeping its internal saltiness at exactly the right level.
Nephron
The functional unit of the kidney, consisting of a renal tubule and its associated vasculature. In simple terms, each nephron is one tiny filtration-and-recycling assembly line.
Glomerulus
A ball-shaped cluster of leaky capillaries, situated between the afferent and efferent arterioles, where blood filtration occurs. Think of it as a high-pressure sieve that lets water and small solutes through but keeps blood cells and large proteins back.
Bowman's capsule
The cup-shaped structure surrounding the glomerulus that collects the filtrate. In simple terms, the catcher's mitt for everything the glomerulus pushes out.
Filtration
The first step of urine formation: blood pressure forces water and small solutes from the glomerular capillaries into Bowman's capsule. Blood cells and large molecules are left behind.
Reabsorption
The process by which useful substances (glucose, ions, water) are transported from the filtrate in the tubule lumen back into the blood via peritubular capillaries. Think of it as the kidney retrieving the good stuff it filtered out by accident.
Secretion
The process by which unwanted substances (K+, H+, NH4+, drugs) are moved from the blood into the tubule lumen, the reverse direction of reabsorption. This is the kidney's targeted removal of specific waste.
Excretion
The final elimination of urine from the body, via the ureters, bladder, and urethra.
Proximal tubule
The first segment of the renal tubule after Bowman's capsule, responsible for the bulk of solute and water reabsorption (Na+, glucose, amino acids, H2O, urea). The workhorse segment.
Loop of Henle
The hairpin-shaped section of the tubule that dips into the medulla. Its descending limb reabsorbs water; its ascending limb reabsorbs ions. Together they create the osmotic gradient the kidney depends on.
Distal tubule
The segment after the loop of Henle where fine-tuning of salt, calcium, and water reabsorption occurs, regulated by hormones.
Collecting duct
The final tubular segment that drains multiple nephrons and carries urine to the renal pelvis. Its water permeability is controlled by vasopressin.
Endothelial cells
Cells that line the interior of the glomerular capillaries, forming loose (fenestrated) junctions that allow small molecules through.
Podocytes
Specialised cells with foot processes that wrap around the glomerular capillaries, forming the outer layer of the filtration barrier.
Mesangial cells
Smooth-muscle-like cells within the glomerulus that contract or relax to regulate filtration rate and capillary permeability.
Apical membrane
The side of a tubule epithelial cell facing the lumen (the filtrate side). Carries channels and cotransporters.
Basolateral membrane
The side of a tubule epithelial cell facing the blood (peritubular fluid side). Carries pumps such as the Na+/K+ ATPase.
Na+/K+ ATPase
An active transporter on the basolateral membrane that pumps 3 Na+ out and 2 K+ in, maintaining the sodium gradient that drives most reabsorption. The engine behind nearly every solute recovery in the nephron.
Aquaporins
Water-selective channel proteins. Their number in the collecting duct membrane is regulated by vasopressin.
Equilibrium potential (E_X)
The membrane voltage at which there is no net movement of a particular ion across the membrane, calculated using the Nernst equation.
Cells are exquisitely sensitive to the solute concentration of their surroundings.
Hypertonic solution: extracellular solute concentration exceeds intracellular. Water leaves the cell by osmosis, causing the cell to shrink (crenation in red blood cells).
Hypotonic solution: extracellular solute concentration is lower. Water enters the cell, causing it to swell and potentially lyse.
Isotonic solution: concentrations match. No net water flux, cell volume stays constant.
For the body to work, sodium concentration must be maintained at roughly 140 to 150 mM, and blood pH must sit at about 7.4. These are the conditions under which enzymes and other proteins fold and function correctly. Homeostasis, in this context, means holding those values steady despite whatever you eat, drink, or sweat out.
The hypothalamus produces vasopressin (also called antidiuretic hormone, ADH), which plays a central role in water balance. We will return to vasopressin in the regulation section.
Most animals rely on the kidney to maintain ion and water balance. Multiple cell types combine to produce a tubelike structure that filters, recycles, and excretes.
Six roles of the vertebrate kidney in homeostasis:
Ion balance
Osmotic pressure regulation
Blood pressure regulation
pH balance
Excretion of metabolic wastes and toxins
Hormone production
Blood volume and blood pressure:
Increases in blood volume lead to increases in blood pressure, and vice versa. The kidneys modulate blood volume by excreting or retaining water. If blood pressure is too high, the kidneys excrete more water; if it drops, they retain more.
Gross anatomy:
The mammalian kidney has two layers.
Outer cortex: contains the glomeruli and proximal/distal tubules.
Inner medulla: contains the loops of Henle and collecting ducts; this is where the osmotic gradient is steepest.
The kidney works hand in hand with the circulatory system. The renal artery delivers blood to the kidney; the renal vein carries it away. Blood is filtered and processed within the kidney to form urine. Urine drains first into the minor calyx, then the major calyx, then the ureters, and is stored in the urinary bladder until micturition.
Key orientation shorthand: artery = in, vein = out.
The nephron is the functional unit of the kidney. Each human kidney contains roughly one million nephrons.
Two main components:
Renal tubule: a tube lined with transport epithelium. Different segments are specialised for absorption, reabsorption, and excretion of different solutes.
Vasculature: the blood supply that runs alongside the tubule.
Vasculature in detail:
Glomerulus: a ball of capillaries wedged between two arterioles (afferent in, efferent out). This is where filtration happens. The capillaries are fenestrated (leaky), which is the whole point: water and small solutes pass through easily.
Surrounded by Bowman's capsule, which collects the filtrate.
Capillary walls are made of endothelial cells with loose junctions, plus podocytes with foot processes on the outer surface. Together they form the filtration barrier.
Mesangial cells (smooth-muscle-like) sit among the capillaries and can contract to regulate blood pressure and filtration within the glomerulus.
Peritubular capillaries: capillary beds that surround the renal tubule and facilitate reabsorption and secretion.
Vasa recta: capillaries running alongside the loop of Henle in juxtamedullary nephrons, helping to maintain the medullary osmotic gradient.
Filtrate pathway through the tubule:
Bowman's capsule → proximal tubule → loop of Henle (descending limb, then ascending limb) → distal tubule → collecting duct → renal pelvis.
Urine production involves four sequential processes. The master equation that ties them together:
Amount filtered - Amount reabsorbed + Amount secreted = Amount of solute excreted
1. Filtration
Occurs at the glomerulus.
Blood pressure forces the liquid components of blood (water, ions, glucose, amino acids, urea) through the glomerular wall into Bowman's capsule.
Blood cells and large proteins are NOT filtered; they stay in the blood.
The filtrate at this stage is called primary urine and is isosmotic to blood.
2. Reabsorption
Specific molecules are removed from the filtrate and returned to the blood.
Occurs in all regions of the tubule after Bowman's capsule.
Requires transport proteins and energy.
This is energetically costly but allows precise regulation of what the body retains.
3. Secretion
The reverse of reabsorption: specific molecules are moved from the blood into the filtrate.
Molecules secreted include K+, NH4+, H+, pharmaceuticals (drugs), and water-soluble vitamins.
Occurs in all areas after Bowman's capsule (though not significantly in the loop of Henle).
Requires transport proteins and energy.
4. Excretion
The final step: processed urine leaves the body.
Urine exits the kidney via the ureters, is stored temporarily in the urinary bladder, and leaves via the urethra.
Sphincters of smooth muscle control urine flow out of the bladder.
The micturition reflex is controlled by a spinal cord reflex arc but can also be influenced by voluntary control.
Epithelial cell polarity matters:
Renal tubule cells are single-layered epithelial cells with two distinct faces.
Apical membrane (lumen side): has microvilli (made of actin) that increase surface area. Carries channels and cotransporters.
Basolateral membrane (blood side): carries the Na+/K+ ATPase and other transporters.
To reabsorb a solute, it must cross both membranes, which means you need both channels and active transporters working together.
Sodium reabsorption:
Na+ enters the cell from the lumen through sodium channels or cotransporters on the apical membrane, moving down its concentration gradient (passive, or secondary active transport when coupled to another solute).
Na+ is then pumped out of the cell on the basolateral side by the Na+/K+ ATPase, moving up its concentration gradient (primary active transport).
This Na+ gradient is the engine that drives the reabsorption of many other solutes.
Glucose reabsorption (the opposite direction pattern):
Glucose is reabsorbed by secondary active transport using the sodium-glucose cotransporter (SGLT) on the apical membrane.
Here glucose moves UP its concentration gradient (into the cell, coupled to Na+ moving down).
Glucose then exits the cell on the basolateral side through glucose permease (GLUT), moving DOWN its concentration gradient into the blood.
Notice the pattern is reversed compared to sodium: glucose goes up then down, while sodium goes down then up.
Water reabsorption:
Water follows solutes by osmosis. Wherever solutes are reabsorbed, water tends to follow through aquaporins or paracellularly, driven by the osmotic gradient the solute movement creates.
Nernst equation (equilibrium potential for a single ion):
E_X = \frac{61.5 \text{ mV}}{z} \log_{10} \frac{[X_{\text{out}}]}{[X_{\text{in}}]}Where E_X is the equilibrium potential for ion X, z is the ion's charge (valence), [X_out] is the extracellular concentration, and [X_in] is the intracellular concentration.
Reference values for common ions:
Ion | Outside (mM) | Inside (mM) | Calculated E_X (mV) |
|---|---|---|---|
Na+ | 150 | 15 | +61.5 |
K+ | 5 | 100 | -80 |
Ca2+ | 2 | 0.0001 | +132 |
Mg2+ | 1 | 1 | 0 |
Cl- | 150 | 13 | -65.3 |
Solute excretion formula:
Amount filtered - Amount reabsorbed + Amount secreted = Amount excreted
This is the fundamental bookkeeping equation for the nephron. Every ion, every molecule that ends up in your urine is accounted for by these three operations.
Dialysis machines work on essentially the same principle as the nephron: they filter waste from the blood across a semipermeable membrane when the kidneys can no longer do it. Understanding filtration, reabsorption, and the osmotic gradient is how biomedical engineers design those machines.
SGLT2 inhibitors (a class of diabetes drugs, e.g. empagliflozin) work by blocking the sodium-glucose cotransporter in the proximal tubule, preventing glucose reabsorption and allowing excess glucose to be excreted in the urine. This is a direct clinical application of the reabsorption mechanisms covered here.
Students often think filtration is selective. It is not. Filtration is a bulk, pressure-driven process that pushes almost everything small enough through the glomerular wall. Selectivity comes later, during reabsorption and secretion.
Students confuse the direction of secretion and reabsorption. Reabsorption = lumen to blood (taking things back). Secretion = blood to lumen (adding things to the filtrate). They use similar transport machinery but run in opposite directions.
Students sometimes assume urine is just "filtered blood minus blood cells." In reality, most of what is filtered (over 99% of the water and sodium) is reabsorbed. Urine is a highly processed, concentrated product.
Students mix up the descending and ascending limbs of the loop of Henle. Descending = permeable to water, NOT to ions. Ascending = permeable to ions, NOT to water. The asymmetry is the entire basis for the medullary osmotic gradient.
The Nernst equation and the equilibrium potentials for Na+, K+, Ca2+, and Cl- are commonly tested. Know how to plug values in and interpret the sign.
The four processes of urine production (filtration, reabsorption, secretion, excretion) and where each occurs along the nephron are a staple question type.
Expect questions on what happens to a cell in hypertonic vs. hypotonic solution.
The excretion formula (Filtered - Reabsorbed + Secreted = Excreted) often appears in calculation problems.
Mesangial cells in the glomerulus are specifically called out in lecture notes: know that they are smooth-muscle-like and regulate filtration.
Glucose reabsorption via the sodium-glucose cotransporter is a favourite topic: know the direction of transport at each membrane (up the gradient at apical, down at basolateral via permease).
True or false: Blood cells pass through the glomerular filtration barrier into Bowman's capsule. (False. Blood cells and large proteins are retained.)
Fill in the blank: The functional unit of the kidney is the ________. (Nephron.)
True or false: The descending limb of the loop of Henle is permeable to ions but not water. (False. It is permeable to water but not ions.)
Fill in the blank: Glucose is reabsorbed in the proximal tubule via the ________ cotransporter. (Sodium-glucose cotransporter, SGLT.)
True or false: Secretion moves substances from the tubule lumen into the blood. (False. Secretion moves substances from the blood into the lumen.)
Q: A patient's blood potassium concentration is dangerously high. Using the Nernst equation, explain what would happen to K+'s equilibrium potential and why this is harmful to the nervous system.
A: Elevated extracellular K+ reduces the K+ concentration gradient across the membrane, making E_K less negative (closer to zero). Since the resting membrane potential of neurons depends heavily on K+ permeability, this depolarises the membrane, potentially causing uncontrolled nerve firing and cardiac arrhythmias.
Q: Explain why the kidney filters nearly everything and then reabsorbs most of it, rather than selectively excreting only waste. Is this not wasteful?
A: Bulk filtration followed by selective reabsorption allows the kidney to remove any small solute from the blood, including novel toxins and drugs the body has never encountered before. Selective excretion would require a specific transporter for every possible waste molecule. The "filter everything, reclaim what you need" strategy is metabolically costly but far more flexible.
Q: Describe the path a glucose molecule takes from the tubule lumen back into the blood. Name the membrane, the transporter, and the direction relative to the concentration gradient at each step.
A: Step 1, apical membrane: glucose is cotransported with Na+ via the sodium-glucose cotransporter (SGLT), moving UP its concentration gradient (secondary active transport). Step 2, basolateral membrane: glucose exits the cell via glucose permease (GLUT), moving DOWN its concentration gradient (facilitated diffusion).
Q: What is the role of mesangial cells in the glomerulus?
A: Mesangial cells are smooth-muscle-like cells that can contract or relax, altering the surface area and permeability of the glomerular capillaries. This allows them to regulate the glomerular filtration rate locally.
Q: A cell is placed in a hypertonic solution. What happens to the cell and why?
A: The cell shrinks. Because the extracellular solute concentration is higher than the intracellular concentration, water moves out of the cell by osmosis, reducing cell volume.
This material connects directly to membrane transport (Chapter on cell membranes): every channel, pump, and cotransporter discussed here is an application of the passive and active transport principles from earlier in the course. The cardiovascular system is tightly linked, since the kidneys regulate blood volume and pressure, feeding back to cardiac output via the Frank-Starling mechanism. The endocrine system appears through vasopressin, aldosterone, and ANP, which are covered more deeply in the hormonal regulation notes (Part 2 of this study set).
Osmoregulation, ion balance, water balance, kidney physiology, nephron anatomy, glomerulus, Bowman's capsule, filtration, reabsorption, secretion, excretion, proximal tubule, loop of Henle, distal tubule, collecting duct, Nernst equation, equilibrium potential, Na+/K+ ATPase, sodium-glucose cotransporter, SGLT, aquaporins, podocytes, endothelial cells, mesangial cells, apical membrane, basolateral membrane, microvilli, renal cortex, renal medulla, renal artery, renal vein, ureter, urinary bladder, urethra, primary urine, micturition reflex, tonicity, hypertonic, hypotonic, isotonic, BIO Ch. 13, University of Michigan Biology Exam 2