Describe the structures of the urinary system and the nephron, explain urine formation by filtration, reabsorption and secretion using osmosis and diffusion, explain hormonal control of water balance, and interpret urinalysis and renal imbalances

Learning goal: Identify the structures of the urinary system (kidneys, ureters, bladder and urethra) and of the kidney (renal cortex, medulla, renal pyramids, renal pelvis and nephrons); describe the processes of filtration, reabsorption and secretion in the nephron and the roles of osmosis and diffusion in urine formation; explain how antidiuretic hormone (ADH) and aldosterone influence the volume and concentration of urine and how caffeine and alcohol act as diuretics; describe the gross and histological structure of the urinary bladder and relate it to function; interpret urinalysis values and expected values in patient scenarios; compare hemodialysis with real kidneys; and describe imbalances including urinary tract infections, kidney stones, nephritis, acute and chronic kidney disease and hyperhydration.

What the system does. The urinary system is a regulatory system that helps maintain homeostasis. Its primary functions are excretion (removing metabolic wastes such as urea from the blood), elimination (storing and expelling urine) and regulation of blood volume and pressure, together with the concentrations of ions such as sodium and potassium and the pH of the blood. The kidneys also work with other organ systems: they depend on the cardiovascular system for a steady flow of blood at adequate pressure, and they release hormones, such as erythropoietin, which stimulates red blood cell production, and an enzyme that starts a hormone pathway raising blood pressure.

Gross structure. The two bean-shaped kidneys lie against the back wall of the abdomen. In a section, the outer renal cortex surrounds the inner medulla, which is divided into cone-shaped renal pyramids. Urine drains from the pyramids into the renal pelvis, a funnel-shaped chamber connected to the ureter. Each ureter carries urine by peristalsis to the urinary bladder, which stores it, and the urethra carries it out of the body.

The nephron, the functional unit. Each kidney contains on the order of a million nephrons. A nephron begins in the cortex with a renal corpuscle: a knot of capillaries, the glomerulus, cupped inside the glomerular (Bowman's) capsule. The capsule leads into a tubule with four parts in order: the proximal convoluted tubule, the nephron loop (loop of Henle), which dips into the medulla and back, the distal convoluted tubule, and a collecting duct shared by many nephrons, which runs through a pyramid to the renal pelvis. A second capillary network surrounds the tubule, so the tubule and blood are side by side along its whole length.

Urine formation, worked fully. Three processes turn blood plasma into urine. Filtration: blood pressure in the glomerulus pushes water and small dissolved substances (glucose, amino acids, salts, urea) through the capillary wall into the capsule. Blood cells and most proteins are too large and stay in the blood. The fluid in the capsule is called filtrate. In a healthy adult, the kidneys form roughly 180 liters of filtrate a day, yet produce only about 1 to 2 liters of urine. If 180 L is filtered and 1.5 L leaves as urine, then 180 - 1.5 = 178.5 L was returned, and 178.5 / 180 = 0.992, so about 99 percent of the filtered water is reabsorbed. Reabsorption is that return: useful substances move from the tubule back into the blood. In the proximal tubule, all the glucose and amino acids (as long as blood glucose is in its normal range, so the transporters are not overloaded) and most sodium are actively transported back, and water follows by osmosis, moving from the more dilute filtrate toward the more concentrated fluid around the tubule. Along the nephron loop, the descending limb lets water out while the ascending limb, which water cannot cross, moves salt out into the medulla, which makes the tissue fluid there very salty; this concentration gradient later lets water leave the collecting duct by osmosis. Secretion moves substances the other way, from the blood in the surrounding capillaries into the tubule: hydrogen ions (to control blood pH), potassium ions and some drugs. Urea is filtered, and some of it diffuses back, but much stays in the tubule and is excreted. Diffusion moves small solutes down their concentration gradients at many points, and osmosis moves water. The final urine is what is left: excess water, urea, excess salts and secreted substances.

Hormonal control of hydration. The body adjusts urine volume and concentration to keep blood volume and solute concentration steady. When you are dehydrated, blood becomes more concentrated; sensors in the hypothalamus detect this and the posterior pituitary releases antidiuretic hormone (ADH). ADH makes the collecting ducts more permeable to water, so more water leaves by osmosis into the salty medulla and returns to the blood; urine becomes small in volume and concentrated. When you drink a lot, ADH falls, the ducts become less permeable, and urine is large in volume and dilute. That is how the body eliminates excess fluid. Aldosterone, released by the adrenal cortex when blood pressure or sodium is low, increases sodium reabsorption (and potassium secretion) in the distal tubule and collecting duct; water follows the sodium where the tubule wall lets it, which in the collecting duct depends on ADH, so blood volume and pressure rise. Diuretics increase urine volume: alcohol suppresses ADH release, so heavy drinking can contribute to dehydration, and caffeine increases urine output briefly, though in regular coffee or tea drinkers at moderate intake the fluid in the drink largely makes up for it. Drinking far more water than the kidneys can remove in a short time, hyperhydration, dilutes the sodium in the blood; water then enters cells by osmosis and they swell, which is dangerous in the brain.

Urine and urinalysis. Urine is normally a clear, yellow solution that, in a healthy bladder, carries few or no microbes that grow on a standard culture (newer methods show a small resident community), though its composition varies slightly between people and with diet and fluid intake. Urinalysis is a diagnostic tool. Normal urine has a specific gravity (a measure of concentration compared with water, which is 1.000) in a range of roughly 1.005 to 1.030, a pH usually around 6 though it can range from about 4.5 to 8, and no glucose and little or no protein, blood cells or bacteria. Glucose in urine suggests blood glucose so high that reabsorption is overwhelmed, as in uncontrolled diabetes; protein suggests damaged glomeruli; white cells and bacteria suggest infection; a high specific gravity suggests dehydration or high ADH.

The bladder: structure for storage. The bladder wall is lined with transitional epithelium (urothelium), whose cells change from rounded and many-layered when the bladder is empty to flattened and stretched when it is full, so the bladder can hold much more without tearing and without leaking urine into its wall. Beneath is a thick layer of smooth muscle, the detrusor, which contracts to empty the bladder. At the outlet, an internal sphincter of smooth muscle (involuntary) and an external sphincter of skeletal muscle (voluntary) keep urine in until release is chosen.

Evolution and comparison. Kangaroo rats in the desert produce very concentrated urine. You would predict, correctly, that relative to kidney size their nephron loops are longer and their medulla thicker than a human's, because a longer loop builds a steeper salt gradient in the medulla, which lets more water leave the collecting duct by osmosis.

Homeostatic imbalances. A urinary tract infection is a bacterial infection, usually of the bladder and urethra. Kidney stones are crystals of minerals that form in the kidney and can block the ureter. Nephritis is inflammation of the kidney. Acute kidney injury is a sudden loss of function, and chronic kidney disease a gradual, long-term loss, often linked to diabetes or high blood pressure. In kidney failure, wastes, potassium and fluid build up. Hemodialysis pumps blood past a semi-permeable membrane bathed in dialysis fluid; wastes and excess ions diffuse out down their gradients and excess water is removed. Unlike real kidneys, it runs intermittently rather than continuously (conventional in-center hemodialysis is commonly about three sessions a week; home schedules can be more frequent), and it does not make erythropoietin or perform the kidneys' other hormone functions, which is why diet and fluid limits matter for patients.

Mock patient scenarios. A classroom case study matches urine values to a mock patient and then asks for a treatment plan in general, physiological terms, never for a real person. For dehydration, the plan restores fluid and salts so ADH can fall; for a urinary tract infection, the plan is diagnosis and treatment by a clinician, typically with an antibiotic because the cause is bacterial; for excess coffee or a prescribed diuretic, the plan accounts for the extra urine and fluid loss; for high ADH, the plan first asks why it is high: after dehydration or blood loss it is an appropriate response and fluid is what is needed, while a clinician would consider limiting water only when ADH is high inappropriately and blood sodium is being diluted. Each plan must name the physiological variable it corrects.

Investigation: a dialysis-tubing model. Tie off a length of dialysis tubing, fill it with a solution of starch and glucose, rinse the outside, and place it in a cup of water. After 20 minutes, test the water with a glucose test strip and with a few drops of iodine solution (wear goggles; iodine stains). Glucose in the water and no blue-black color show that the small molecule diffused out while the large one stayed in, modeling a membrane that sorts by size. In the glomerulus the same size sorting happens, but the fluid is pushed through by blood pressure (filtration), whereas in this model it moves only by diffusion, as in a dialysis machine. The model cannot show active reabsorption, secretion, hormones or blood pressure.

Worked example 1 (fraction reabsorbed): A hypothetical person filters 175 L of plasma a day and produces 1.4 L of urine. Water reabsorbed = 175 - 1.4 = 173.6 L. Fraction = 173.6 / 175 = 0.992, so 99.2 percent is reabsorbed. If reabsorption fell to 98 percent, urine would be 2 percent of 175 = 3.5 L a day, two and a half times as much, showing how small changes in reabsorption produce large changes in urine.

Worked example 2 (patient scenario, dehydration): A hypothetical athlete trains for two hours in heat without drinking. Expected urine: small volume, dark yellow, high specific gravity near the top of the normal range. Reason: blood becomes concentrated, the hypothalamus triggers ADH release, collecting ducts become more permeable to water, and more water is reabsorbed by osmosis into the salty medulla.

Worked example 3 (patient scenario, infection): A hypothetical patient has burning when urinating. Urinalysis shows white blood cells and bacteria, while glucose is absent and specific gravity is normal. White cells together with large numbers of bacteria, in a person with symptoms, point to a urinary tract infection (small numbers of bacteria alone can come from contamination or resident microbes); the absence of glucose shows the problem is not failed glucose reabsorption.

Worked example 4 (dialysis model data): In the tubing model, after 20 minutes the water outside turns a glucose strip positive but shows no color change with iodine. Glucose, a small molecule, diffused out down its concentration gradient; starch, a large molecule, could not pass the membrane. This models the size selection of the glomerulus, which keeps large molecules such as proteins in the blood, but not its driving force: the glomerulus uses blood pressure (filtration), while the tubing works by diffusion, as dialysis does.

Representation task: Draw a longitudinal section of a kidney (cortex, medulla, pyramids, pelvis, ureter) and one enlarged nephron with its capillaries. Use three colored arrows to show filtration (glomerulus to capsule), reabsorption (tubule to blood: glucose, sodium, water) and secretion (blood to tubule: hydrogen and potassium ions), and mark where osmosis moves water and where ADH acts.

Independent performance: Carry out the dialysis-tubing model and use it to write a comparison of hemodialysis with real kidneys. Success criteria: (a) the test results for glucose and starch are recorded and explained by diffusion and membrane pore size; (b) filtration, reabsorption and secretion are each defined with an example substance and direction; (c) the role of osmosis in reabsorbing water and the effect of ADH and aldosterone on urine are explained; (d) expected urinalysis results are predicted for dehydration and for a urinary tract infection; (e) at least two functions of real kidneys that hemodialysis does not replace are named; (f) the model's limitations are listed.