Fundamentals of Nursing · Urinary Elimination
Functions of the Urinary System
On this page 9 sections
In 30 seconds
Topic 1 introduced the parts of the urinary system — kidneys, ureters, bladder, and urethra. This topic answers the next question: what does the system actually do? The answer is much bigger than "it makes urine." The urinary system is the body's main waste-disposal route, but it is also a set of finely tuned regulators: it balances water and electrolytes, keeps blood pH stable, helps control blood pressure, and produces hormones that affect red blood cells and bones.
Think of the kidneys as doing three jobs at once:
- A filter — removing metabolic waste and foreign substances from the blood.
- A regulator — adjusting water, electrolytes, and acid-base balance so the internal environment stays stable.
- A gland — releasing hormones such as renin, erythropoietin, and Calcitriol The active form of vitamin D, made by the kidneys Full entry → that act far beyond the kidneys themselves.
The ureters, bladder, and urethra handle the transport job: storing and eliminating what the kidneys produce.
Why this matters
- Urine output is a window on perfusion and hydration. So much blood flows through the kidneys that a change in urine output is often an early clue to dehydration, low blood pressure, heart failure, or kidney injury. That is why nurses track intake and output so carefully.
- Kidney disease is rarely "just" a kidney problem. When filtration drops, waste accumulates; when regulation fails, fluid builds up, electrolytes drift, and blood pressure climbs; when hormone output falls, the person develops anemia and bone problems. One failing organ shows up as symptoms all over the body.
- Medication safety. The kidneys excrete many medications and their byproducts, so declining kidney function can mean drug accumulation. Prescribers adjust doses using current references; nurses watch for signs of toxicity.
The college version
Core Concepts
Excretion: removing metabolic waste
Every cell produces waste as it works: protein breakdown yields Urea Waste from protein breakdown Full entry →, muscle activity yields Creatinine Waste from normal muscle activity Full entry →, and nucleic-acid turnover yields uric acid. The nephrons filter these byproducts — and foreign chemicals such as drug metabolites — out of the blood. What the tubules do not reabsorb leaves the body in urine.
Regulation of fluid and electrolytes
The kidneys decide, minute by minute, how much water and which ions the body keeps. Two hormones do much of the directing:
- Antidiuretic hormone (ADH) Hormone that makes the kidney tubules reabsorb water Full entry → makes the tubules reabsorb water, so urine becomes concentrated when the body is short on fluid.
- Aldosterone Hormone that makes the tubules reabsorb sodium and excrete potassium Full entry → makes the tubules reabsorb sodium and, in exchange, excrete potassium.
Sodium is the main ion that "holds" water in the bloodstream, so the two are tied together: retain sodium, retain water, and blood volume rises. Potassium, calcium, and phosphate are fine-tuned along the tubules too — which is why kidney problems often show up as electrolyte disturbances.
Regulation of acid-base balance
The lungs adjust pH in minutes by changing breathing. The kidneys are the slower but powerful long-term regulator, working over hours to days: they excrete hydrogen ions (acid) in the urine and reabsorb and generate Bicarbonate The body's main pH buffer, reabsorbed and generated by the kidneys Full entry →, the body's main buffer, back into the blood. When kidney function fails, acid can accumulate — one reason kidney disease and metabolic acidosis often travel together. (This guide gives no pH or lab values; reference ranges vary by laboratory and institution.)
Regulation of blood pressure
The kidneys influence blood pressure through volume (how much sodium and water the body keeps) and through the renin-angiotensin-aldosterone system (RAAS Renin-angiotensin-aldosterone system — the kidney's blood-pressure control cascade Full entry →). When kidney cells sense a drop in blood flow or pressure, they release renin, which starts a cascade producing angiotensin II — a powerful vasoconstrictor that also triggers aldosterone release. The system protects blood pressure but can overshoot in disease; many blood-pressure medications act on parts of it.
Hormone production: erythropoietin and calcitriol
- Erythropoietin (EPO) Kidney hormone that tells the bone marrow to make red blood cells Full entry →: when the kidneys detect low oxygen in the blood, they release EPO, which signals the bone marrow to make more red blood cells. Declining kidney function → less EPO → the anemia commonly seen with kidney disease (fatigue, pallor, reduced exercise tolerance).
- Calcitriol (active vitamin D): the kidneys convert vitamin D from diet and sunlight into its active form, which the gut needs to absorb calcium. With significant kidney disease, less is made, calcium absorption weakens, and bone problems develop over time.
Two smaller jobs
During prolonged fasting, the kidneys can also synthesize small amounts of glucose, though far less than the liver. And the system must move what it makes: the ureters carry urine to the bladder by peristalsis, the bladder stores it, and the urethra and sphincters release it on a socially appropriate schedule — the timed, controlled elimination the rest of this chapter builds on.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| The kidneys just make urine | Kidneys that also regulate and secrete hormones | Filtration is one job; fluid/electrolyte/pH balance, blood pressure control, and hormone output are equally important |
| ADH and aldosterone do the same thing | Two hormones with different targets | ADH retains water; aldosterone retains sodium (and loses potassium) — they work together but are not the same |
| Low urine output always means kidney failure | A signal with several possible causes | Output can drop from dehydration, low blood flow, or obstruction too — the cause must be assessed, not assumed |
| The lungs handle all acid-base balance | A two-organ system | Lungs adjust pH in minutes; kidneys are the durable long-term regulator (hours to days) |
| Anyone on the care team can change drug doses for kidney problems | A prescriber's decision using current references | Nurses monitor and report signs of toxicity, but dose adjustments are made by the prescriber per policy |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your kidneys are like the recycling-and-trash plant for your blood. They take out the trash your cells make, and they also decide how much water and salt to keep. They help keep your blood from getting too sour, and they send messages that tell your body to make more blood cells or to use vitamin D. When the trash plant gets sick, lots of other things in the city go wrong at the same time — not just the trash pickup.
Worked example
Scenario — one failing function explains many symptoms. Mr. Delgado, a 64-year-old with chronic kidney disease, comes to the clinic with several complaints. The nurse notices swelling around his ankles and that his blood pressure has crept upward since the last visit. Mr. Delgado says he has felt unusually tired and pale lately, and his wife mentions he has been complaining of aching bones.
The nurse connects each observation to a function from this topic:
- Ankle swelling and rising blood pressure → the kidneys are retaining more sodium and water than they should (volume regulation), and the RAAS is working overtime to keep blood pressure up.
- Tiredness and pallor → lower EPO output means fewer new red blood cells, producing anemia.
- Aching bones → less calcitriol means less calcium absorbed from food, weakening bone over time.
The nurse documents the observations, reports them to the provider, and — within scope of practice and institutional policy — implements the care plan: daily weights, intake and output monitoring, and the medications that are ordered. Diagnosis and dose adjustments rest with the prescriber and care team, not the nurse alone. Every one of Mr. Delgado's complaints traces back to a specific urinary system function — which is why knowing these functions helps nurses anticipate problems early.
Key takeaways
- The urinary system does far more than make urine: it filters waste, regulates fluid, electrolytes, and pH, controls blood pressure, and makes hormones.
- Key waste products: urea (protein), creatinine (muscle), uric acid (nucleic acids).
- ADH = water retention; aldosterone = sodium retention (with potassium loss). Different jobs, same team.
- The kidneys are the long-term acid-base regulator (hydrogen-ion excretion, bicarbonate reabsorption); the lungs act fast, the kidneys act thoroughly.
- RAAS chain: low kidney perfusion → renin → angiotensin II (vasoconstriction) + aldosterone (sodium/water retention) → blood pressure rises.
- EPO → red blood cell production; calcitriol → calcium absorption. Declining kidney function → anemia and bone problems.
- Declining kidney function can mean drug accumulation — dose decisions belong to prescribers using current references.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Name four functions of the kidneys besides "making urine."
Show answer
Regulating fluid and electrolytes, acid-base balance, and blood pressure; producing erythropoietin; activating vitamin D; and (to a small degree) gluconeogenesis.
A person is dehydrated and producing small amounts of dark, concentrated urine. Which hormone is primarily responsible, and what does it do?
Show answer
Antidiuretic hormone (ADH). It makes the kidney tubules reabsorb water, so urine becomes concentrated and the body holds onto fluid.
Why does chronic kidney disease often come with anemia?
Show answer
The failing kidneys produce less erythropoietin (EPO), so the bone marrow is not stimulated to make as many red blood cells.
Put the RAAS steps in order: aldosterone release, angiotensin II formation, renin release, blood pressure rises.
Show answer
Renin release → angiotensin II formation → aldosterone release → blood pressure rises.
Why can declining kidney function make some medications more dangerous?
Show answer
The kidneys clear many drugs. When filtration declines, drugs build up to harmful levels, so prescribers adjust doses and nurses watch for toxicity.
The lungs regulate pH quickly. How do the kidneys differ, and what do they do?
Show answer
The kidneys are the slow but durable long-term regulator: they excrete hydrogen ions (acid) and reabsorb/generate bicarbonate over hours to days.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Urea
- Waste from protein breakdown
- Creatinine
- Waste from normal muscle activity
- Antidiuretic hormone (ADH)
- Hormone that makes the kidney tubules reabsorb water
- Aldosterone
- Hormone that makes the tubules reabsorb sodium and excrete potassium
- Bicarbonate
- The body's main pH buffer, reabsorbed and generated by the kidneys
- Erythropoietin (EPO)
- Kidney hormone that tells the bone marrow to make red blood cells
- Calcitriol
- The active form of vitamin D, made by the kidneys
- RAAS
- Renin-angiotensin-aldosterone system — the kidney's blood-pressure control cascade
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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