BIOL1019 Chap.12 Tubular Transport, Fluid Balance and Acid-Base Control
Tubular Transport, Fluid Balance and Acid-Base Control
Filtration makes a large volume of filtrate; the tubule decides what the body keeps. This chapter works segment by segment: the PCT reabsorbs water, ions and all organic nutrients, the descending limb lets water out, the ascending limb lets sodium and chloride out but not water, the DCT fine-tunes by secretion and reabsorption, and the collecting duct sets final urine concentration under ADH.
It explains the renal threshold that lets glucose spill into urine, the fluid compartments of a body that is 50 to 60% water, and how ADH and aldosterone respond to sweating. The last part covers acid-base balance, pH 7.35 to 7.45, and the buffers, lungs and kidneys that defend it.
The lectorial's athlete case and the concept checks on drinking water quickly, losing sodium, haemorrhage and hypoventilation all test the same skill: predicting what changes, which receptor notices, which hormone or organ responds and what the urine or blood pH does next.
What this chapter covers
- 01
PCT, nephron loop, DCT and collecting duct
- 02
ADH, aldosterone, angiotensin II and ANP
- 03
Renal threshold and glycosuria
- 04
Intracellular and extracellular fluid
- 05
Water gains, losses and osmosis
- 06
Buffers, lungs and kidneys in pH control
- 07
Respiratory and metabolic acidosis and alkalosis
- 08
Electrolyte roles of sodium, potassium and calcium
Worked example · free
Classifying an acid-base disorder from breathing
- +1Hyperventilation removes carbon dioxide faster than it is produced, so blood carbon dioxide falls.
- +1The reaction of carbon dioxide and water shifts to the left, fewer hydrogen ions remain and pH rises.
- +1Because the primary change is in carbon dioxide, the disorder is respiratory alkalosis.
- +1Over hours to days the kidneys excrete more bicarbonate and reduce net acid excretion.
Key terms
- Collecting duct
- The final tubule segment, where ADH acts and final urine concentration is set.
- Renal threshold
- The plasma level above which a filtered substance starts to show up in urine.
- Intracellular fluid
- The fluid inside cells, high in potassium, phosphate and negatively charged proteins.
- Respiratory acidosis
- A fall in pH caused by carbon dioxide accumulating through inadequate ventilation.
- Aldosterone
- An adrenal cortex hormone that increases sodium reabsorption and potassium secretion in the distal nephron.
Tubular Transport, Fluid Balance and Acid-Base Control FAQ
Why can the kidney not simply pump water?
Cells cannot actively transport water. Water moves only by osmosis, toward whichever compartment holds more effective solute, so the kidney controls water by moving solutes such as sodium and by changing how permeable segments are to water, for example through ADH acting on the collecting duct. Saying the kidney pumps water is a common error.
Why is plain water not always enough after heavy sweating?
Sweat removes electrolytes, mainly sodium and chloride, as well as water. Replacing only the water restores volume but leaves the electrolytes short and can dilute the extracellular fluid. The lectorial uses this question to show that fluid balance depends on both water and solute, with ADH controlling water and aldosterone controlling sodium.
Why do buffers not fix an acid-base problem on their own?
Chemical buffers act immediately, but they only hold hydrogen ions or bicarbonate; they do not remove acid or base from the body. The lungs adjust carbon dioxide within minutes, and the kidneys correct the problem over hours to days by secreting hydrogen ions, reabsorbing and generating bicarbonate and excreting acid with urinary buffers.
What does each part of the nephron tubule do?
The PCT reabsorbs water, ions and all organic nutrients. The descending limb of the nephron loop lets water out, while the ascending limb is relatively impermeable to water and lets sodium and chloride out. The DCT secretes ions, acids, drugs and toxins and fine-tunes reabsorption, and the collecting duct sets the final concentration of urine under ADH.
Exam move
Draw a strip of the nephron segments and write one transport fact and the controlling hormone under each. Learn the four acid-base disorders as a table, then practise with short cases, always naming the primary change, the pH direction, the disorder and the compensating organ in that order. Finish by explaining the sweating case aloud from receptor to hormone to urine.
For fluid balance, practise the sweating chain and a rapid water-drinking case side by side, predicting osmolarity, the hormone response and the urine produced in each.
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