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Having reviewed the anatomy and microanatomy of the urinary system, now is the time to focus on the physiology. You will discover that different parts of the nephron utilize specific processes to produce urine: filtration, reabsorption, and secretion. You will learn how each of these processes works and where they occur along the nephron and collecting ducts. The physiologic goal is to modify the composition of the plasma and, in doing so, produce the waste product urine.
Failure of the renal anatomy and/or physiology can lead suddenly or gradually to renal failure. In this event, a number of symptoms, signs, or laboratory findings point to the diagnosis ( [link] ).
| Symptoms of Kidney Failure |
|---|
| Weakness |
| Lethargy |
| Shortness of breath |
| Widespread edema |
| Anemia |
| Metabolic acidosis |
| Metabolic alkalosis |
| Heart arrhythmias |
| Uremia (high urea level in the blood) |
| Loss of appetite |
| Fatigue |
| Excessive urination |
| Oliguria (too little urine output) |
The volume of filtrate formed by both kidneys per minute is termed the glomerular filtration rate (GFR) . The heart pumps about 5 L blood per min under resting conditions. Approximately 20 percent or one liter enters the kidneys to be filtered. On average, this liter results in the production of about 125 mL/min filtrate produced in men (range of 90 to 140 mL/min) and 105 mL/min filtrate produced in women (range of 80 to 125 mL/min). This amount equates to a volume of about 180 L/day in men and 150 L/day in women. Ninety-nine percent of this filtrate is returned to the circulation by reabsorption so that only about 1–2 liters of urine are produced per day ( [link] ).
| Calculating Urine Formation per Day | ||
|---|---|---|
| Flow per minute (mL) | Calculation | |
| Renal blood flow | 1050 | Cardiac output is about 5000 mL/minute, of which 21 percent flows through the kidney.
5000*0.21 = 1050 mL blood/min |
| Renal plasma flow | 578 | Renal plasma flow equals the blood flow per minute times the hematocrit. If a person has a hematocrit of 45, then the renal plasma flow is 55 percent.
1050*0.55 = 578 mL plasma/min |
| Glomerular filtration rate | 110 | The GFR is the amount of plasma entering Bowman’s capsule per minute. It is the renal plasma flow times the fraction that enters the renal capsule (19 percent).
578*0.19 = 110 mL filtrate/min |
| Urine | 1296 ml/day | The filtrate not recovered by the kidney is the urine that will be eliminated. It is the GFR times the fraction of the filtrate that is not reabsorbed (0.8 percent).
110*.008 = 0.9 mL urine /min Multiply urine/min times 60 minutes times 24 hours to get daily urine production. 0.9*60*24 = 1296 mL/day urine |
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