Urinary system · circulation control
Kidneys, bladder and blood pressure — a filter plant and a regulator
Two fist-sized kidneys filter about 180 liters of fluid a day, yet usually pass only about a liter and a half into urine. Look inside a nephron, see how the bladder tells you “it's time,” measure blood pressure like a doctor, and find out how the body keeps it in check — within seconds and over days.
This page is educational and is not medical advice. The models are simplified and are not meant for assessing your own health. If you are worried about your blood pressure or other symptoms, talk to a doctor.
Click a kidney on the silhouette, then the highlighted part of the cross-section — you'll see one of the roughly one million nephrons in each kidney. Every element in the picture can be clicked.
Erythropoietin
When the blood carries too little oxygen, the kidneys release the hormone EPO, which stimulates the bone marrow to make red blood cells. In adults, most EPO comes from the kidneys.
Vitamin D
The skin and liver turn vitamin D into a form that is “on standby.” Only in the kidney tubules is its active form made — calcitriol, needed to absorb calcium and keep bones strong.
Acid–base balance
The kidneys excrete excess H⁺ ions and recover bicarbonate, so blood pH stays within a narrow range of about 7.35–7.45. Urine, on the other hand, can be acidic or alkaline — typically pH 5–7.
Kidney stones
When urine is very concentrated, salts (most often calcium oxalate) can crystallize into “pebbles.” Drinking enough fluids lowers this risk.
Dialysis — an artificial kidney
When the kidneys stop working, hemodialysis cleans the blood through a semipermeable membrane in a machine: it removes urea, creatinine, excess potassium and water. It typically takes about 4 hours, about 3 times a week. But it doesn't replace the kidneys' hormonal work (EPO, vitamin D) — those must be supplied with medicines. Another route is a kidney transplant; a person can live healthily with one kidney.
Where does urea come from, and why sugar in urine?
Urea is made in the liver from ammonia released when proteins are broken down; the kidneys excrete about 20–35 g of it a day. Glucose appears in urine when its blood level exceeds the renal threshold (about 180 mg/dL) — this can happen, among other things, in untreated diabetes, which a doctor diagnoses from blood tests.
Chart: bladder volume over a day (from 7:00). Blue dots — drinks, amber ticks — toilet, gray area — sleep.
What is urine made of?
- water≈ 95%
- urea (from the liver)≈ 20–35 g/day
- salts: Na⁺, K⁺, Cl⁻, phosphates…depends on diet
- creatinine (from muscles)≈ 1–2 g/day
- uric acid, urochrome (pigment)traces
The yellow color comes from urochrome — a pigment from the breakdown of heme. The less water, the more of it in every milliliter, and the darker the urine.
Two sphincters
The internal sphincter is smooth muscle at the neck of the bladder — it works involuntarily, controlled by the autonomic nervous system. The external sphincter (in the pelvic floor) is made of striated muscle and obeys your will — it's what lets you “hold it.” The bladder wall is the detrusor muscle, which contracts when you pass urine.
Why can't a toddler hold it?
In a baby, a full bladder triggers a reflex through the spinal cord and the micturition center in the pons — without asking the cerebral cortex. The connections that let the brain hold back this reflex mature gradually: daytime control usually appears between ages 2 and 4, nighttime control often later. More about the nervous system.
Measure blood pressure by auscultation
Inflate the cuff, let the air out slowly and listen to the artery with a stethoscope.
Hold “Pump” (or the P key) to inflate the cuff to about 180 mmHg.
What do the numbers mean?
Blood pressure is written as systolic / diastolic, in millimeters of mercury.
Blood pressure categories (ESH 2023, adults, office measurement)
| Category | systolic | diastolic | |
|---|---|---|---|
| Optimal | < 120 | and | < 80 |
| Normal | 120–129 | and/or | 80–84 |
| High normal | 130–139 | and/or | 85–89 |
| Grade 1 hypertension | 140–159 | and/or | 90–99 |
| Grade 2 hypertension | 160–179 | and/or | 100–109 |
| Grade 3 hypertension | ≥ 180 | and/or | ≥ 110 |
Source: 2023 guidelines of the European Society of Hypertension (ESH). The higher of the two values decides the category; systolic ≥ 140 with diastolic < 90 is isolated systolic hypertension. The 2024 guidelines of the European Society of Cardiology (ESC) are simpler: non-elevated < 120/70, elevated 120–139 / 70–89, hypertension ≥ 140/90. The thresholds apply to adults — for children and younger teens doctors use percentile charts. A diagnosis is made by a doctor from many measurements, never from one.
Check the category and mean pressure
Mean arterial pressure (MAP) is calculated as diastolic + ⅓ of the difference, because the heart spends longer resting (diastole) than contracting. The unit “mmHg” comes from mercury manometers — the same as in a barometer.
How does the body keep blood pressure in check?
Fast — the baroreceptor reflex (seconds). Slow — the kidneys and hormones (hours and days).
The renin–angiotensin–aldosterone (RAA) system
The bars show the level at the current moment of the scenario (×1 = at rest).
- Blood pressure or volume dropssensed by the kidney and baroreceptors
- ×1.0Reninfrom the kidney's juxtaglomerular apparatus
- ×1.0Angiotensin Irenin cuts angiotensinogen from the liver
- ×1.0Angiotensin IIACE enzyme, mainly in the lung's blood vessels
- ×1.0Blood vessels narrowperipheral resistance rises
- ×1.0Aldosteronefrom the adrenal cortex → the kidney retains Na⁺, and water follows
- ×1.0ADH (vasopressin)from the pituitary → aquaporins, less urine
- ×1.0ANP — the counterweightfrom stretched atria of the heart → more Na⁺ and water in urine
Many blood pressure medicines act at exactly these steps: ACE inhibitors block the ACE enzyme, sartans (ARBs) block the angiotensin II receptor, and diuretics increase the excretion of sodium and water. Treatment is always chosen by a doctor.
How does it work?
Each kidney is made of about a million nephrons (from about 200,000 to over 2 million in different people). A nephron works in four steps: filtration in the glomerulus (water and small molecules pass through a “sieve”), reabsorption in the tubules (the body takes back what is valuable), secretion (the tubules add things like H⁺ and K⁺ ions to the urine) and excretion — what remains flows into the renal pelvis and the ureter.
The trick for concentrating urine is the countercurrent multiplier. The ascending limb of the loop of Henle pumps out salt but doesn't let water through, so the kidney medulla gets saltier and saltier toward its depths — in humans up to about 1,200 mOsm/kg (plasma is about 285–295). The collecting duct passes through this salty medulla. When ADH is present, aquaporins appear in its wall and water “escapes” by osmosis (more about how molecules move in Molecules) — the urine thickens. Without ADH the urine stays dilute, as low as about 50 mOsm/kg.
Blood pressure is the force with which blood pushes against the walls of the arteries. Systolic is the peak at the moment the heart ejects blood; diastolic is the lowest value just before the next beat. It is regulated quickly by the baroreceptor reflex (carotid sinus and aortic arch → medulla oblongata → heart and blood vessels). In the long run the kidneys decide: by how much sodium and water they excrete, they set the blood volume.
Formulas
Simplifications: the kidney model calculates a steady state for a whole day (the balance of water and dissolved substances), and the recovery fractions in the nephron segments are typical textbook values. The droplets in the animation show direction and proportions, not the real number of molecules; we leave out, among other things, urea recycling and K⁺ secretion. The blood pressure model is a few coupled reservoirs (water, sodium, blood volume, hormones) with simple feedback loops — it shows directions and time scales and does not predict results for any particular person. The patients in the cuff exercise are random.
Keys
- Everywhere
- Q W E tabs
- Esc mute sound
- Kidney and nephron
- 1 2 3 body / kidney / nephron
- 4–8 scenarios
- Space pause animation
- Bladder
- Space start / pause the day
- 1 glass, 2 bottle
- 3 to the toilet
- 4 adult / child
- Blood pressure
- P (hold) pump
- O valve, X release all
- S / D mark a sound
- N new patient, M sound
- 1–4 regulation scenarios
- B baroreceptors, R restart
See also
Did you know…?
- You have only about 3 liters of blood plasma, yet your kidneys filter about 180 liters a day — so all your plasma passes through their “sieve” roughly 60 times a day.
- The first blood pressure measurement was described in 1733 by Stephen Hales: he inserted a glass tube into a horse's artery, and the blood rose in it to about 2.5 m. Mercury is about 13.6 times denser than water, which is why a handy mercury manometer is so short.
- The blood pressure cuff was invented by Scipione Riva-Rocci in 1896, and in 1905 the Russian surgeon Nikolai Korotkov described the sounds heard under the cuff in a very short report. The auscultatory method is still in use today.
Challenges
- Concentrate urine to the maximumIn the “Kidney and nephron” tab, set heat and little drinking until the urine reaches about 1,200 mOsm/kg — the limit of what a human kidney can do.
- Exceed the renal threshold for glucoseRaise blood glucose above about 180 mg/dL and watch for the pink molecules that make it all the way to the collecting duct.
- Measure blood pressure from Korotkoff soundsMark the patient's systolic (first sound) and diastolic (sounds disappear) pressure to within 5 mmHg.
- Stand up fast — and let your baroreceptors save youWith baroreceptors on, watch the “Get out of bed quickly” scenario until the pressure drops and returns almost to normal.
- Trigger the RAA system after blood lossIn the blood loss scenario, wait for the moment when renin rises at least one and a half times.