BIOL1030 Chap.12 Digestion, Gas Exchange and Salt Balance
Digestion, Gas Exchange and Salt Balance
Digestion, Gas Exchange and Salt Balance covers the Week 8 lectures on how tissues and organs cooperate to fuel metabolism in changing environments. Digestion comes first: microbial fermentation of plant fibre, transit time, salt and water recovery in the colon, and the secretin and CCK signals that pace the duodenum.
Gas exchange follows, with open and closed circulations, diffusion across gills and the kidney and EPO response to high altitude. The final topic is water and salt: osmoconformers and osmoregulators, the energy cost of pumping ions across fish gills, and the countercurrent loop of Henle that lets mammals make concentrated urine under the control of ADH.
What this chapter covers
- 01
Microbial fermentation and gut transit time
- 02
Secretin, CCK and duodenal pH
- 03
Colonic sodium and water absorption
- 04
Open and closed circulatory systems
- 05
Altitude, EPO and oxygen-carrying capacity
- 06
Osmoconformers, osmoregulators and the cost of ion pumping
- 07
The loop of Henle, ADH and aquaporins
Worked example · free
Predict urine when the thick ascending limb leaks water
- 1Normally the thick ascending limb pumps NaCl into the medulla while holding water back, which builds a rising osmolarity towards the inner medulla.
- 1If water now follows the salt out of the limb, the medullary fluid is diluted, so the gradient becomes much shallower.
- 1ADH still puts aquaporins in the collecting duct, but a weak gradient draws little water out, so the urine stays relatively dilute and plentiful.
- 1Compare medullary and urine osmolarity with normal animals at the same ADH level; low values in both support the explanation.
Key terms
- Secretin
- A duodenal hormone, released in response to acidic chyme, that makes the pancreas secrete bicarbonate.
- Erythropoietin
- A kidney hormone, released when blood oxygen is low, that stimulates red marrow to make erythrocytes.
- Osmoconformer
- An animal whose body-fluid osmolarity changes to match its surroundings.
- Countercurrent multiplication
- The loop of Henle mechanism that builds a high medullary osmolarity by pumping NaCl out of a water-tight limb.
Digestion, Gas Exchange and Salt Balance FAQ
Why do herbivores lose weight when their gut microbes are killed?
Their own enzymes cannot break down plant cell walls, so they rely on symbiotic microbes housed in an enlarged gut chamber, typically the caecum or colon, where fermentation happens. Without those microbes, much of the fibre passes out undigested and far less energy is extracted from the same amount of food.
How does living at high altitude change the blood?
Lower air pressure reduces the partial pressure of inspired oxygen, so blood oxygen falls. The kidneys respond by releasing erythropoietin, which drives the red marrow to produce more erythrocytes. Over weeks, haemoglobin and oxygen-carrying capacity rise and exercise tolerance recovers.
Why does a marine fish use more oxygen when its gills become leaky to salt?
To keep blood osmolarity steady it must pump the extra salt back out, and the sodium-potassium ATPase that powers this uses ATP. Higher pump activity means more cellular respiration, so resting oxygen consumption climbs even though the blood looks normal.
Why does an osmoconformer swell when its pool is diluted?
An osmoconformer lets its internal osmolarity follow the water around it. When rain makes a rock pool more dilute than the animal's body fluids, water moves in by osmosis and its cells and tissues swell. An osmoregulator in the same pool would hold its internal osmolarity steadier, at an energy cost.
Exam move
Treat each system as a chain you can draw: chyme to secretin to bicarbonate, low oxygen to EPO to erythrocytes, pumped salt to medullary gradient to water recovery. Practise breaking one link at a time and writing the knock-on effects, then add the measurement that would confirm each step, because the week's practice answers reward tests as much as explanations.
Keep a short list of contrasts, open versus closed circulation and conformer versus regulator, with one example animal for each, and rehearse why extra ADH cannot fix a failed gradient.
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