S2: Consequences of Fluid Loss from GI Tract Flashcards
Sites of water loss
- Skin
- Respiratory Passageways
- GIT
- Urinary Tract
List ways we lose water and gain water
Water loss:
- Defacation; Diarrhoea
- Vomiting
- Urination
- Ventilation
- Sweating
- Menstruation
Water gain:
- Drinking
- Metabolic
What is diarrhoea and what causes it?
This is characterised by both an increase in frequency and weight of the stool.
This occurs due to:
1. Lack of absorption in villus cells - enterocytes lose ability to reabsorb e.g. due to toxins
- Increased secretion (HCO3-, enzymes, Na+) due to crypt cell hyperplasia
Describe how decreased reabsorption causes osmotic diarrhoea
This is where an individual consumes an osmotically rich solution (e.g. mannitol, sugary or salty drinks) , which increases the osmolality of the tubular fluid. This means there is reduced reabsorption, as water is drawn into the fluid.
- Defect in absorption process also means that water potential is due to reduced absorption. e.g. Dissacharidase deficiency which is found in lactose intolerance (lack of brush border enzyme lactase)
Describe how deranged motility causes decreased reabsorption
Where the motility of the small intestine increases, meaning there is reduced time for reabsorption. This means that the volume of the faecal content will increase.
Describe how increased secretion causes secretory diarrhoea
This is where there is increased secretion of ions from the crypt cells.
For example, substances such as; Acetylcholine and Substance P and Cholera act to increase the rate of secretion (via increasing the intracellular levels of Cl-).
What disease causes secretory diarrhoea?
Secretory Diarrhoea is a very common result in cholera. The cholera Toxins result in increased release of Cl- from the crypt cells and hence, uncontrolled water secretion.
What are the two parasites that cause diarrhoea?
Entamoeba histolytica Giardia Lamblia
Describe diarrhoea that is caused by Entamoeba histolytica
- Asymptomatic or amoebic dystentery
- Gradual onset with systemic symptoms (anorexia, headache)
- Treatment is with metronizadole
Describe diarrhoea that is caused by Giardia Lamblia
- Steatorrhoea and abdominal pain
- Maldigestion and malabsorption of lipids, CHOs vitamin A, B12, folic acid
- Treatment is with Metronidazole or tinidazole
What is dysentery?
Infection of intestines that causes painful, blood, low volume in diarrhoea
Causes of bloody diarrhoea
- Chronic disease
- Ulcerative colotis
- Neoplasm
Major consequence for severe diarrhoea
- Hypovolaemia
- Hyponatremia
- Metabolic acidosis
What is vomiting?
Vomiting is retrograde giant contraction with oral expulsion of gastric contents and bile.
It allows the removal of ingested toxic substances.
Symptoms of vomiting
Symptoms include increase in salivation, HR and sweating accompanied with pallor and nausea.
What is the vomiting centre (VC)?
Brainstem medulla oblongata
What inputs to vomiting centre?
- Neural inputs (NTS and chemoreceptor trigger zone)
Role of the vomiting centre
Stimulates the vomiting reflex
Examples of inputs that can initiate vomiting
- Distension of stomach or small intestine
- Action of some substances on chemoreceptors in brain or intestine
- Increase pressure in skull
- Rotating movements of head (motion sickness)
- Intense pain
- Tactile stimuli to the back of the throat (gag reflex)
- Sight, smell, emotional circumstances
Describe mechanism of vomiting
- Nausea (sweating, pallor, hypersalivation)
- Deep inspiration cause closure of epiglottis
- Increase in abdominal pressure causes retrograde giant contractions
- Breath held, chest fixes
- Decrease in oesophageal pressure causes relaxation of oesophageal sphincters
- More increase in abdominal pressure causes forceful expulsion and retching. Bile may be expelled from contractions of upper portion of intestine.
How do 5-HT blocking drugs e.g. Ondansetron work?
Serotonin is released in response to damage to cells in the intestine (e.g. radiotherapy, chemotherapy). 5-HT3 receptor agonsits block these receptors so serotonin cannot bind.
They block the transmission of vomit signals via nerves from the intestines to the brain. This stops stimulation of the vomiting centre so vomiting does not occurs.
5-HT blocking drugs are therefore anti-emetic
Consequence of excess vomiting
- Eosophagitis
- Hyponataemia
- Hypovoaemia
- Metabolic alkalosis (due to loss of acid in stomach)
What is lost in vomit?
- Food
- Mucus
- Ions - Na+, K+, Cl-. HCO3-
- Bile
- Blood
What are the overall consequences of fluid loss from the GI Tract?
- Hypovolemia - Hypovolaemia poses issues regarding the blood flow to organs, as the BP is reduced.
- Metabolic acidosis
- Hyponatemia
- Haemoconcentration (increased viscosity of the blood flow, meaning that the blood flow is reduced)
- Malnutrition
Hypercalcaemia.
Describe pathway by which sodium and water excretion are decreased in response to severe sweating
Severe sweating leads to loss of hypo-osmotic salt solution.
The same stimuli that results in the release of ADH, is the same stimuli that results in the feeling of thirst. This means that the hyper-osmolality and fluid volume contraction will result in the feeling of thirst (as well as the reduction of salivation in the mouth). Regarding thirst, normally, the main input is from the osmoreceptors.
Decrease in plasma volume stimulates decreased GFR, increased plasma aldosterone and vasopressin. Increase plasma osmolarity due to decrease H2O concentration also stimulates vasopressin. This decreases sodium excretion and H2O excretion.
Compare osmolality of sweat and plasma
The sweat is hypo-osmotic relative to the plasma osmolality. This means that there is hyper-osmotic contraction.
What inhibits thirst?
Metering of water intake by GI tract prevents overhydration
Factors stimulating thirst
- Drug mouth, throat
- Decrease in plasma volume (detected by baroreceptors)
- Angiotensin II (stimulated by baroreceptors)
- Increase in plasma osmolarity (detected by osmoreceptors)
Clinical consequences of dehydration
- Reduce skin turgor
- Confusion and lack of mental awareness due to brain cell shrinkage
- Dryness of lips
- Reflux
- Overloaded kidneys as toxins accumulate and cannot be flushed out
- Temparature - heat shock, fainting, headache
- Heart works harder due to higher blood viscosity, blood clotting
Explain how dehydration can cause reflux
Reflux occurs because the water secretions that come from the pancreas; to neutralized the duodenal chime, need large amounts of water. Since, there is reduced water, there is reduced neutralization. This means that the chime builds up, pressure builds up and it can then be refluxed out. The build up of acidic chime is present, because there are physiological mechanisms that actually prevent the movement of acidic chime, until it has been fully neutralized.
How does dehydration affect calcium levels?
Dehydration actually causes the levels of Ca2+ to increase but NaCl and K to decrease.
This increase in calcium levels can results in: kidney stones (calcified Bilirubin), kidney failure and arrhythmia).
Symptoms of hypercalcaemia
If mild= asymptomatic • Nausea • Vomiting • Loss of appetite • Constipation • Abdominal pain • Excessive thirst • Fatigue, lethargy, muscle weakness •Joint pain, confusion
What substance stimulates the thirst mechanism?
ADH
Describe how ADH regulates water loss
- ADH stimulates the thirst mechanism and is released when there is increased osmolarity of body fluids
- ADH inserts aquaporin-2 water channels in the collecting duct of the kidney which increases the permeability of the collecting duct to H2O and results in concentrated urine production
- When H2O is ingested, there is a decrease in osmolarity of blood and interstitial fluid which decreases ADH secretion and removal of water channels
Factors that regulate H2O release
- Large decrease in blood volume (detected by baroreceptors)
- Severe dehydration under this condition, GFR decreases causing less H2O in urine
- Intake of copious amounts of water results in high BP and increased GFR increasing H2O in urine
- Hyperventilation results in increased fluid loss (exhalation)
- Vomiting/diarrhoea causes increased fluid loss from GIT
- Fever, heavy sweating and burns cause large H2O loss
Where is the thirst centre?
In hypothalamus
