E. Homeostasis · HomeostasisUrinalysis, Kidney Disorders, Dialysis and Transplants
E3.3 E2.1 E3.1 E2.3 A1.5 A1.8 A1.10 A1.13 A2.1 A2.2About 70 minutes
Learning goals- I can interpret urinalysis results and connect them to nephron function.
- I can describe kidney stones, urinary tract infections and chronic kidney disease, and their causes.
- I can explain how hemodialysis and peritoneal dialysis replace kidney functions, using diffusion and osmosis.
- I can weigh the benefits and challenges of dialysis and kidney transplantation.
- I can name careers and Canadian contributions related to kidney health.
Words to know
urinalysis
A set of tests on a urine sample, such as specific gravity, pH, glucose, protein, ketones and blood.
specific gravity
The density of urine compared with water; a measure of how concentrated it is (water = 1.000).
renal threshold
The blood level above which a substance (such as glucose) can no longer be fully reabsorbed and appears in urine.
chronic kidney disease (CKD)
A long-lasting loss of kidney function, often caused by diabetes or high blood pressure.
dialysis
A treatment that removes wastes and extra water and ions from the blood when the kidneys fail, using diffusion across a membrane.
dialysate
The fluid used in dialysis; its makeup is chosen so wastes diffuse out of the blood while useful substances stay.
immunosuppressant
A drug that reduces the activity of the immune system; transplant recipients take it to prevent rejection.
What your urine says about you
For thousands of years, doctors have examined urine to learn about health. Today, a quick urinalysis with a test strip and a microscope is one of the most common lab tests in Canada. Because urine is made from the blood, it gives clues about the kidneys and about the rest of the body, from diabetes to infections.
In this lesson you will learn to read those clues, then look at what happens when kidneys stop working and how dialysis and transplants take over their job.
Reading a urinalysis
Each result connects directly to something you learned about the nephron:
| Test | Typical healthy result | An unusual result may suggest | Nephron connection |
|---|
| Specific gravity | about 1.005 to 1.030 | very high: dehydration; always low: kidneys can't concentrate | ADH and the collecting duct |
| pH | about 4.5 to 8 (often near 6) | very high or low: diet, infection, acid–base problems | H⁺ secretion |
| Glucose | none | blood glucose above the renal threshold (about 10 mmol/L), e.g., untreated diabetes | PCT reabsorption saturated |
| Protein | none or a trace | damage to the glomerulus (for example from diabetes or high blood pressure); can also appear briefly after hard exercise | the filter is leaking |
| Ketones | none | the body is burning fat for fuel: fasting, or diabetic ketoacidosis | metabolism, not the kidney itself |
| Blood | none | kidney stones, infection, injury | bleeding somewhere in the urinary tract |
| Nitrites, white blood cells | none | urinary tract infection (bacteria) | infection of bladder or kidney |
Worked exampleExample 1: Interpreting three urinalyses
Interpret each simulated result. (a) Patient A: specific gravity 1.030, dark yellow, everything else normal, after a long shift outdoors in summer. (b) Patient B: glucose ++, ketones +, specific gravity 1.025, very thirsty and urinating often. (c) Patient C: protein ++, blood pressure 160/100 mm Hg for several years.
- (a) High specific gravity and dark colour mean concentrated urine. With no other abnormal results, the likely cause is dehydration: lots of ADH, lots of water reabsorbed. Advice: drink water.
- (b) Glucose in urine means blood glucose is above the renal threshold. Ketones mean cells are breaking down fat because they can't use glucose. Thirst and frequent urination fit too: glucose in the tubule holds water by osmosis. This pattern suggests uncontrolled diabetes; the patient needs medical care soon.
- (c) Protein in urine means the glomerulus is leaking. Long-term high blood pressure damages the glomerular capillaries. This suggests kidney damage linked to hypertension; the doctor would test blood creatinine and estimate kidney function.
(a) Dehydration. (b) Likely uncontrolled diabetes. (c) Likely glomerular damage from high blood pressure.
Common kidney and urinary disorders
- Urinary tract infections (UTIs): bacteria enter through the urethra and infect the bladder (or, less often, travel up to the kidneys). They cause burning, urgency and sometimes fever. They are treated with antibiotics.
- Kidney stones: hard crystals, most often made of calcium oxalate, form when urine is very concentrated. A stone moving down the ureter can be very painful. Drinking enough water is the best prevention. Large stones can be broken up with shock waves (lithotripsy) or removed.
- Chronic kidney disease (CKD): a slow, long-term loss of nephrons. The most common causes are diabetes and high blood pressure, both of which damage the glomeruli. Early CKD often has no symptoms, which is why doctors check urine protein and blood creatinine in people at risk. Managing blood glucose and blood pressure slows it down.
- Kidney failure: when kidney function falls to roughly 10 to 15% of normal, wastes (urea, creatinine), K⁺, acid and water build up. All the homeostasis you studied in the last three lessons starts to fail. At this stage, a person needs dialysis or a transplant.
Dialysis: an artificial kidney
Hemodialysis. Blood is taken from the patient (usually through a fistula, a surgically joined artery and vein in the arm that becomes large and strong), pumped through a dialyzer and returned. Inside, the blood flows past a thin, partially permeable membrane. On the other side flows dialysate, a carefully made solution:
- no urea, so urea diffuses out of the blood;
- low K⁺, so extra potassium diffuses out;
- normal glucose and normal Na⁺, so these don't leave the blood;
- bicarbonate, to help correct acidosis.
Blood and dialysate flow in opposite directions (countercurrent), which keeps a concentration gradient all along the membrane, just like the countercurrent exchangers you saw in thermoregulation. Extra water is removed by applying slight pressure. Most people on hemodialysis go to a clinic about three times a week for about four hours each time, though some do it at home.
Peritoneal dialysis. The membrane is the patient's own peritoneum, the lining of the abdominal cavity, which has many capillaries. Dialysate is run into the abdomen through a soft tube (catheter), left for a few hours while wastes diffuse into it, then drained and replaced. It can be done at home, often overnight with a machine called a cycler.
Worked exampleExample 2: Designing a dialysate
A student suggests using pure water as dialysate because "then everything will diffuse out of the blood." Explain why this would be dangerous, and why the real dialysate contains glucose and Na⁺ but no urea.
- With pure water, every small solute would diffuse out: glucose, Na⁺, Ca²⁺, bicarbonate as well as urea. The patient would lose useful substances.
- Water would also move into the blood by osmosis (the blood would be more concentrated than pure water), diluting it dangerously and making cells swell.
- The real dialysate has normal concentrations of glucose and Na⁺, so there is no concentration gradient for them: no net movement. It has no urea, so there is a steep gradient and urea diffuses out.
- So dialysis works like the nephron's reabsorption in reverse: instead of taking useful things back, it never lets them go in the first place.
Pure water would strip useful solutes and dilute the blood. Matching useful solutes to normal blood levels while leaving out wastes lets only the wastes diffuse out.
Kidney transplants
A transplanted kidney can do everything a healthy kidney does, including making hormones such as renin and erythropoietin (EPO), which dialysis cannot replace. Kidneys can come from deceased donors or from living donors, since a person can live a healthy life with one kidney. In 1958, a team at Montreal's Royal Victoria Hospital performed Canada's first kidney transplant, between identical twins.
Challenges:
- Rejection: the immune system recognizes the new kidney as foreign. Donors and recipients are matched for blood type and tissue type, and recipients take immunosuppressant drugs for as long as the kidney works. These drugs raise the risk of infections.
- Supply: there are more people waiting than organs available. In Ontario, people aged 16 and older can register their decision to be an organ and tissue donor online or at a ServiceOntario centre. Talking about the decision with family matters too.
Careers
Kidney care involves many people: nephrologists (doctors specializing in kidneys), urologists (surgeons of the urinary tract), dialysis nurses, renal dietitians who plan low-potassium and low-phosphate diets, medical laboratory technologists who run urinalyses and blood tests, transplant coordinators, and biomedical engineers who design better dialyzers.
Hands-on activityInvestigation: a simulated urinalysis
Grown-up note: this uses only simulated urine made from kitchen ingredients. Never test, taste or handle real urine for this activity. Label every cup clearly and pour everything down the sink afterward.
You need: 4 clear cups labelled A to D, water, yellow food colouring, sugar, salt, white vinegar, baking soda, red cabbage indicator (from the acid–base lesson), a drinking straw, modelling clay or plasticine, a marker, a ruler, and a dark plate. (Optional: urine glucose test strips from a pharmacy.)
Make the samples (300 mL water + 2 drops yellow colouring each):
- A: nothing else (healthy, well hydrated)
- B: add 2 tablespoons sugar (simulated glucose in urine)
- C: add 2 tablespoons salt (very concentrated urine)
- D: add 1 tablespoon vinegar (acidic urine)
Tests:
- Specific gravity: make a hydrometer: push a ball of clay into one end of a straw so it floats upright in water. Mark the water line on the straw in sample A. Float it in each sample and measure how far the line is from the surface (the more concentrated the sample, the higher the straw floats).
- pH: add 1 tablespoon of each sample to 2 tablespoons of cabbage indicator and compare colours.
- Glucose: put a few drops of each on a dark plate and leave overnight. Sugar dries sticky and shiny; salt leaves white crystals. (Or use glucose strips if you have them.)
Analyse: Make a results table. For each sample, suggest what a doctor might investigate. Which tests could tell B and C apart? Which could not? Why do real labs use several tests?
What to look for: B and C both float the hydrometer higher than A (both are more concentrated). Only the glucose test separates them. D turns the indicator pink. Real urinalysis uses many tests because one result can have several causes.
Just for funFunny-but-true: together your two kidneys weigh only about 300 g, less than half of one percent of an adult's body mass, yet they receive about a fifth of all the blood your heart pumps at rest. They are the body's VIP guests at the blood buffet.
Your turn
1Why does untreated diabetes cause glucose in the urine?
Blood glucose rises above the renal threshold (about 10 mmol/L), so the carriers in the proximal tubule are saturated and can't reabsorb all the filtered glucose.
2What does a high specific gravity tell you about urine?
It is concentrated (more solutes per volume), often because of dehydration and high ADH.
3Name the two most common causes of chronic kidney disease.
Diabetes and high blood pressure.
4Why does dialysate contain no urea?
So that there is a steep concentration gradient: urea diffuses from the blood (high) into the dialysate (none).
5What is the advantage of countercurrent flow in a dialyzer?
Blood always meets dialysate with a lower waste concentration, so a gradient is kept along the whole membrane and more waste is removed.
6Give one advantage of a transplant over dialysis and one disadvantage.
Advantage: any one: replaces all kidney functions including hormones (renin, EPO); more freedom (no dialysis sessions); usually better long-term health. Disadvantage: any one: lifelong immunosuppressants with infection risk; surgery risks; shortage of organs; possible rejection.
More practice: warm-up
A set of tests on a urine sample.
2What are kidney stones most often made of?
3What type of drug prevents transplant rejection?
4Which membrane is used in peritoneal dialysis?
The peritoneum (lining of the abdominal cavity).
5Name one career related to kidney health.
Any one: nephrologist, urologist, dialysis nurse, renal dietitian, medical laboratory technologist, transplant coordinator, biomedical engineer.
More practice: on your own
1Why is protein in the urine a warning sign, but glucose in the urine is usually not caused by kidney damage?
Protein means the glomerular filter is damaged. Glucose usually appears because blood glucose is too high, overwhelming normal reabsorption; the kidney itself may be working normally.
2Why does drinking plenty of water help prevent kidney stones?
More water makes urine less concentrated, so minerals like calcium oxalate are less likely to crystallize.
3Explain why people on hemodialysis must limit potassium in their diet between sessions.
Without working kidneys, K⁺ builds up between sessions; high K⁺ can disturb heart rhythm. Dialysis removes it only during sessions.
4A dialysis patient's blood is acidic before treatment. How can the dialysate help?
The dialysate contains bicarbonate, which diffuses into the blood and buffers the extra H⁺, raising pH toward normal.
5Why can't dialysis fix the anemia (low red blood cell count) that many kidney-failure patients have?
Healthy kidneys make the hormone EPO, which stimulates red blood cell production. Dialysis only moves substances by diffusion; it can't make hormones. Patients often receive EPO injections.
Challenge
1A dialyzer membrane lets particles smaller than about 5 nm through. Albumin is about 7 nm across, urea under 1 nm, and glucose about 1 nm. Predict which would cross and explain why glucose still doesn't leave the blood.
Urea and glucose are small enough to cross; albumin is too big. Glucose doesn't leave because the dialysate contains glucose at the same concentration as normal blood, so there is no net diffusion.
2Evaluate this statement: "Dialysis is just as good as a working kidney."
Not accurate. Dialysis removes wastes, extra water, K⁺ and acid, but only intermittently (wastes build up between sessions), it cannot make hormones (renin, EPO, active vitamin D), and it requires many hours each week with diet and fluid limits. It is life-saving, but a working kidney (or a successful transplant) does more.
3In a school lab, you have dialysis tubing, a starch solution, a glucose solution, iodine solution (turns blue-black with starch) and glucose test strips. Design an experiment to show that small molecules cross the membrane and large ones do not.
Fill a tied dialysis-tubing bag with a mixture of starch and glucose solution and place it in a beaker of water with a few drops of iodine. After about 30 minutes: (1) test the outside water with a glucose strip: positive, so glucose (small) crossed out; (2) the inside of the bag turns blue-black while the outside stays amber, so iodine (small) crossed in, but starch (large) did not cross out. Control: a bag of plain water in the same set-up (no glucose outside, no colour change inside). Keep bag size, volumes, time and temperature the same, and repeat. This mirrors the dialyzer: small solutes cross, large ones stay.
Quiz
1. Glucose appears in urine when:
2. Protein in the urine most likely indicates:
3. Dialysate contains glucose at normal blood levels so that:
4. In peritoneal dialysis, the membrane is:
5. Which kidney function can dialysis NOT replace?
6. Transplant recipients take immunosuppressants to:
Big idea: urinalysis results connect directly to nephron function, and dialysis copies the nephron using diffusion across a membrane, with a dialysate designed so only wastes leave.
How to help: the simulated urinalysis is a good kitchen investigation. Make sure only the homemade samples are used and everything is labelled.
Common difficulties:
- Thinking glucose in urine always means kidney damage. Usually it means blood glucose is too high.
- Not seeing why dialysate isn't plain water. Ask: what would diffuse out if the other side had none of it?
- Forgetting the kidney's hormone jobs. Dialysis cannot make EPO or renin.
Sensitive topic: some families have experience with dialysis, transplants or organ donation decisions. Let your teen lead the conversation, and treat organ donation as a personal and family choice.