Absorption of water and ions Flashcards
How are ions absorbed?
• Example ion=Na+
1. Na+/K+ ATPase (pump) will actively transport Na+ into the blood and K+ into the cell to create an Na+ concentration/electrochemical gradient
2. Na+ and another molecule (glucose/ animo acid) enter the cell by facilitated diffusion using a co-transporter
3. Na+ can also enter cell alone by facilitated diffusion
4. Na+ will diffuse down a concertation gradient into the blood by a transport protein
How is water absorbed?
• Establishing the Na+ concentration/electrochemical gradient is the most important thing for absorbing water
• The movement of Na+ out of the lumen will cause the water potential gradient in the blood to decrease below (establishes an osmotic gradient) that of the lumen so water will enter the blood as it will move by osmosis down a water potential/osmotic gradient
○ The movement of water out of the lumen will cause the concertation of K+, Cl-, amino acids and glucose in the lumen
○ This means that glucose can diffuse into cell by facilitated diffusion until there is no net movement of glucose (then Na+ glucose co-transporter will be needed)
○ K+ and Cl- will diffuse down their electrochemical gradients via a paracellular route
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○ When large molecules are broken down into smaller molecules (osmolytes) the concentration inside the lumen increases/ water potential decreases. This causes water to be drawn out from the blood and into the lumen. Crypt cell can also pump Na+ into lumen to decrease water potential
○ This water will make it easier for the movement down the small intestine
○ When the osmolytes (e.g. ions, glucose, animo acids etc….) are absorbed the water potential of the lumen increases above the blood so water will move by osmosis down a water potential gradient into the blood. This is the re-absorption of the secreted water
How is diarrhoea made?
• Lots of water is lost though faces can cause dehydration (major cause of death especially in children)
○ >500ml per day
• Could be due to
○ Inhibition of Na+ transport - no Na+ concertation gradient so not Na+ moves into blood. Water will not follow by osmosis as a water potential gradient hasn’t been set up
§ This could be caused by- inflammation of the bowel/ irritation (e.g. by bile if not absorbed)
○ Massive secretion of fluid
§ Could be due to bacteria-vibrio cholerae, E.coli
□ Cholera lives in salty water- will be more floods
□ Cholera causes crypt cells to secret more Na+ so more water in gut. Colon cant keep up and process large amounts of water
○ increased motility-
§ Too much peristalsis pushes food too quickly into large bowel faster than water can be reabsorbed
○ ‘osmotic’ diarrhoea
§ Increased concentration of slowly absorbed substances e.g. Mg^2+, SO4^2-, PO4^2-
§ This draws water out of the blood and into the lumen
○ Dumping syndrome
§ Gastric emptying very quickly e.g. due to gastric by surgery-
§ e.g. No stomach to break down, store and gradual release of food
§ Food is broken down in small intestines but not gradual release of food into small intestine so lots of osmolytes.
§ so Lots of water in small intestine. Food moves though very quickly
§ Lots of water being removed from blood stream- pressure in blood will drop so heart will beat faster to try increase blood pressure (multiple symptoms within 10-30 minutes, common in 20-50% of patients)
§ Advice patients to eat less and eat smaller portions
What is oral rehydration therapy?
• Death from infection related to diarrhoea due to dehydration
• Hydration with salt (Na+) and sugar (glucose) can increase the water absorbed in the small intestine therefore reducing diarrhoea and preventing dehydration
• Na+Cl-/glucose causes water to enter blood by osmosis
• Cheap, safe and effective
How is calcium absorbed?
• Ca2+ is essential for bones and teeth
○ Needed especially for growing children and lactating women
• Regulated according to the need and is related to the blood levels of Ca2+
• Regulated by vitamin D and PTH (parathyroid hormone)
• Absorbed along the whole length of the small intestine
1. Ca2+ is removed by active transport from the cell and put into the blood to create a Ca+ concentration gradient
2. Ca2+ will move by facilitated diffusion into the cell down a concentration gradient
3. Ca2+ binds to calbindin to stop Ca2 to stop precipitating to allow safe transport though cytoplasm
4. Vitamins D allows all proteins in diagram to be made
