The Patient Who Still Passes Some Urine
A 52-year-old man reports tiredness, puffiness around the eyes and ankles, and passes only about 300 mL of urine a day although he drinks nearly 2 L of water. His blood report shows creatinine 6.0 mg/dL (normal up to about 1.2), blood urea 90 mg/dL, and a urine dipstick shows protein 2+. His blood pressure is 160/100 mmHg, and he has gained 3 kg in two weeks. The nephrologist advises haemodialysis three times a week and restricts salt and fluids. The family asks why dialysis is needed when he is still passing some urine, and whether dialysis will cure him so that he can stop treatment after a few months.
1. Using the nephron, explain why protein should not appear in urine and what protein 2+ suggests about the glomeruli. Why is the daily urine volume only 300 mL when he drinks 2 L?
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Each nephron begins with the glomerulus, a knot of capillaries inside Bowman's capsule. Blood pressure forces water, glucose, salts, urea and other small molecules through the filtration membrane into the capsule. Blood cells and large proteins such as albumin are too big to pass, so they stay in the blood. Almost all the glucose and much of the water and salts are then reabsorbed along the tubule, and the remainder leaves as urine.
Protein 2+ on the dipstick means a substantial amount of protein is escaping into the urine. That points to damage to the glomerular filter, which has become leaky, or to very high pressure forcing protein through. Normal urine should contain no detectable protein.
Although he drinks 2 L, damaged glomeruli filter much less blood, so little filtrate forms. The salt and water that are not excreted stay in the body, producing puffiness, a 3 kg weight gain and raised blood pressure, with only 300 mL leaving as urine.
2. Explain the composition of the dialysis fluid (why it contains glucose and salts but no urea) and the direction in which urea moves. Why must dialysis continue three times a week even though some urine is still produced?
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The dialysis fluid is made to resemble normal blood plasma in its useful solutes: glucose at about the same concentration as in blood, and mineral salts at the correct concentrations. It contains no urea. The membrane allows small molecules to diffuse from higher to lower concentration, so urea moves from the blood into the fluid and is carried away. Glucose and salts have the same concentration on both sides, so there is no net movement of these useful substances. If the fluid had no glucose, glucose would diffuse out of the blood; if it were too salty, water would be drawn out of the blood.
Dialysis replaces only filtration. His remaining nephrons still filter a little, which is why 300 mL of urine appears, but that is far too little to clear the urea he produces each day or to balance salt and water. Without three sessions a week, urea rises, fluid accumulates and the blood pressure climbs further.
3. State why dialysis does not cure the kidney disease, and give the reasoning behind restricting salt and fluid in this patient.
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Dialysis is an artificial filter outside the body. It removes wastes that have already entered the blood, but it cannot repair damaged glomeruli or restore the kidney's reabsorption, so it substitutes for one function of the organ rather than treating the cause. It also works intermittently, whereas healthy kidneys work continuously, so waste levels still rise between sessions.
Salt and fluid are restricted because the kidneys cannot excrete excess sodium and water. Every extra gram of salt holds water in the body, raising blood volume, blood pressure and the swelling in the ankles and around the eyes. Too much fluid between sessions means a large volume must be removed in one dialysis session, which can drop blood pressure sharply and cause cramps and vomiting. Restriction keeps the weight gain between sessions small and makes dialysis safer; it is part of the treatment, not a punishment.