The kidneys keep the internal environment stable by filtering blood, returning needed substances to the body, and removing wastes into urine. A central measure of kidney function is the glomerular filtration rate, or GFR, which tells how much fluid is filtered into the nephron each minute. After filtration, the tubules carefully reabsorb water and useful solutes such as glucose, sodium, and bicarbonate.
They also secrete selected substances such as hydrogen ions, potassium, and many drugs to fine tune body chemistry.
Filtration begins at the renal corpuscle, where pressure across the glomerular capillaries drives fluid into Bowman's space while cells and most proteins stay in the blood. As the filtrate moves through the proximal tubule, loop of Henle, distal tubule, and collecting duct, transport proteins and hormones adjust its composition. Reabsorption reduces the large filtered load to a small final urine volume, while secretion adds extra wastes and ions from blood into the tubular fluid.
These processes are tightly regulated by blood pressure, autoregulation, aldosterone, antidiuretic hormone, and acid base balance.
Understanding Kidney Function
Each nephron faces a difficult physical problem. Blood must be cleaned without losing red blood cells, important proteins, or too much water. The filter works because its wall has three layers.
The capillary lining has pores that allow water and small dissolved particles through. A basement membrane blocks many large proteins because of their size and electrical charge. Special cells called podocytes form narrow gaps around the capillaries.
Damage to any layer can allow protein or blood cells to enter urine. This is why urine protein testing can reveal early kidney injury, sometimes before a person feels unwell.
Pressure is the force that starts filtration, but the pressure must stay within a useful range. If blood pressure falls sharply during severe dehydration or blood loss, less fluid reaches the glomerulus and filtration falls. If pressure is too high for years, delicate glomerular vessels can be damaged.
The kidney partly protects itself by changing the width of small arterioles before and after the glomerulus. This local control is called autoregulation. It helps keep filtration fairly steady during ordinary changes in blood pressure.
Some medicines can affect these vessel responses. This matters in people who are dehydrated, have heart failure, or already have reduced kidney function.
Reabsorption is selective rather than automatic. Cells lining the proximal tubule use energy to move sodium from tubular fluid into the surrounding tissue. Water follows because water moves toward areas with more dissolved particles.
Glucose, amino acids, phosphate, and bicarbonate are linked to these transport processes. Glucose normally disappears from the tubular fluid before urine leaves the body. In diabetes, blood glucose can become so high that the transport proteins reach their limit.
Extra glucose then remains in urine and pulls water with it. This helps explain frequent urination and thirst in uncontrolled diabetes.
The loop of Henle creates a concentration gradient in the kidney medulla. That gradient lets the collecting duct conserve water when the body needs it.
Hormones decide how much of the final adjustment occurs. Antidiuretic hormone makes collecting duct cells more permeable to water. More water returns to blood, producing a smaller volume of concentrated urine.
Low antidiuretic hormone produces more dilute urine. Aldosterone increases sodium reabsorption and potassium secretion in later parts of the nephron. These changes link kidney function to blood volume, blood pressure, muscle and nerve activity.
Kidney cells also help control acidity. They remove hydrogen ions into urine while returning bicarbonate to blood. When learning this topic, track the direction of movement at every step.
Decide whether a substance moves from blood into tubule, from tubule back to blood, or is left in the final urine. That habit prevents confusion between filtration, reabsorption, secretion, and excretion.
Key Facts
- Normal GFR in a healthy young adult is about 90 to 120 mL/min/1.73 m^2.
- Filtration fraction = GFR / renal plasma flow, and is normally about 0.2.
- Net filtration pressure = P_GC - P_BS - pi_GC, where P_GC is glomerular capillary hydrostatic pressure.
- Filtered load = GFR x plasma concentration.
- Excretion rate = filtration + secretion - reabsorption.
- Creatinine clearance approximates GFR: C = (U x V) / P.
Vocabulary
- Glomerular filtration rate
- The volume of fluid filtered from glomerular capillaries into Bowman's space each minute.
- Tubular reabsorption
- The movement of water or solutes from the tubular fluid back into the blood.
- Tubular secretion
- The movement of substances from peritubular capillaries into the tubular fluid for excretion.
- Clearance
- The virtual volume of plasma completely cleared of a substance per unit time.
- Autoregulation
- The kidney's ability to keep renal blood flow and GFR relatively stable despite changes in arterial pressure.
Common Mistakes to Avoid
- Confusing filtration with reabsorption, which is wrong because filtration moves fluid from blood into Bowman's space while reabsorption moves substances from the tubule back into blood.
- Assuming all filtered substances are excreted, which is wrong because many useful molecules such as glucose and much of the filtered water and sodium are normally reabsorbed.
- Using serum creatinine alone as a direct measure of GFR, which is wrong because creatinine depends on muscle mass, production rate, and tubular handling as well as filtration.
- Forgetting the sign in the excretion equation, which is wrong because secretion increases excretion but reabsorption decreases it, so excretion = filtration + secretion - reabsorption.
Practice Questions
- 1 A patient has a GFR of 100 mL/min and a plasma glucose concentration of 0.9 mg/mL. What is the filtered load of glucose in mg/min?
- 2 Urine creatinine concentration is 120 mg/dL, urine flow rate is 1.5 mL/min, and plasma creatinine is 1.2 mg/dL. Calculate creatinine clearance in mL/min.
- 3 A drug is freely filtered at the glomerulus. Its excretion rate is greater than its filtered load. What does this tell you about tubular transport of the drug, and why?