Lecture 30 Flashcards
What is the process of absorption and where does it occur?
The process of absorption is the passage of substances from the GI lumen across the lining of the intestine into the interstitial fluid and then into the blood or lymph. It needs to be selective and only run one way. It occurs in the mouth, esophagus and stomach minimally fo lipid soluble substances and primarily (90% of water and sodium) in the small intestine (large surface area) for all nutrients and the large intestine slightly (9% of water and sodium).
What are the factors which affect absorption?
Factors which affect absorption are: motility (a correct rate of propulsion/peristalsis for digestion and absorption and also increasing exposure to absorptive surfaces), surface area (rate of absorption proportional to this, anatomical adaptations like villi and microvilli exist to increase this), the passage of the molecules across surface area (this is hard as intestinal epithelium is a barrier, two pathways past this is paracellular (between the cells) or cellular (across the cell membranes and through the cytoplasm)), removal of substances from the interstitial space (done via large blood flow, arrangement of the vessels within the villi and the position of the lacteal).
What are the differences between the paracellular and cellular pathways for absorption?
The paracellular pathway is when the solutes to not cross the cell membrane, the barrier is the tight junction (a relatively non selective barrier, small enough molecules can pass) and movement is passive via a concentration gradient.
The cellular pathway must cross two cell membranes (lipid bilayers), this means non lipid soluble molecules require transport proteins (which are selective and allow for active transport).
How do we maximise absorption across the available surface area?
We maximise absorption across the available surface are by ensuring the molecule is as small as possible (chemical digestion) and then using specific transport proteins for selective absorption
How much water does the small intestine recieve? How is it absorbed?
Roughly 9-10 L per day of water enters the small intestine (1.5 L from drinking water a day), it does this via osmosis, the passive movement of water from the lumen into the blood. The osmotic gradients are set up via the absorption of salts and nutrients.
What are the three sodium transport options? How do they work?
Na+ can be transported alone, coupled with glucose or coupled with amino acids. All three of these involve sodium/potassium pumps. The sodium potassium pump constantly pumps out sodium and brings in potassium, this keeps sodium levels low in the cell relative to the high levels in the lumen, this generates a sodium gradient which can be used to bring in monosaccharides or amino acids via cotransport. The monosaccharide or amino acid can then passively flow out of the cell via channels like GLUT2 for glucose due to the gradient.
(Some amino acids and monosaccharides can travel through paracellulary)
How does fat absorption occur?
Products of fat digestion are lipid soluble and as such can passively diffuse across the cell membrane into the cell and out without a transporter, however, they must be delivered to the brush border via micelles (only the fatty acids and monoglycerides diffuse into the cell). In the cell they are synthesised into triglycerides, packaged into chylomicrons and then exit the cell via exocytosis into lacteals.
How is absorption of vitamins and bile salt done?
most bile salts are reabsorbed in the distal portions of the small intestine to ensure they have been used, a transporter brings them back in the ileum and there is also passive transport in the jejunum.
Absorption of vitamins is done for vitamins A, D, E and K with the fats (fat soluble), B12 is absorbed by intrinsic factor which binds to B12, the receptors in the ileum bind to the inrinsic factor/B12 complex and the B12 is then actively absorbed and binded to another transport protein to transport it through
How does elimination occur?
The elimination job is largely done in the large intestine, faeces are formed here, they are transferred to the rectum via peristaltic contractions and eliminated from the body via the defecation reflex.
