13) The Intestines Flashcards
Describe the features of chyme when it reaches the intestines:
Isotonic
Neutral
Digestion nearly complete
What is the intestines role?
Absorb nutrients
Absorb water/electrolytes
How can these substances being absorbed move from gut lumen into blood (generally)?
Transcellular - apical and basolateral
Paracellular
How is the small intestine adapted for absorption?
Large surface area by villi and microvilli
Slow transit time by plicae circularis
Which cells make up the epithelium of the small intestine?
Enterocytes and goblet cells
How does the intestine ensure it’s function of absorption across the epithelium in this harsh environment?
Mucosa constantly shed (3-6 days) and replaced by stem cells at base of crypt
What cells are located in the crypt (intestinal gland)?
Stem cells
Enteroendocrine gland - hormones
Paneth cells - antibacterial
What is involved in the final breakdown of carbohydrates?
Brush border hydrolases
How are carbohydrates absorbed?
As monosaccharides
What does starch consist of?
Amylose (alpha 1-4 bonds)
Amylopectin (alpha 1-4 and 1-6 bonds)
What enzyme breaks alpha 1-4 bonds and what is the product?
Amylase forming glucose or maltose
What enzyme breaks alpha 1-6 bonds and what is the product?
Isomaltase forming glucose or alpha dextrins
What enzymes break down disaccharides?
Maltase, sucrase and lactase
How does glucose move into enterocytes?
Through SGLT-1, sodium symporter
What other sugar can move through SGLT-1?
Galactose
How does fructose enter the cell?
Through GLUT-5 by facilitated diffusion
How does monosaccharides move into blood?
Through GLUT-2 by facilitated diffusion
What does oral rehydration fluid contain and what is its goal?
Mixture of glucose and salt to stimulate maximum water uptake in diarrhoea
How are proteins absorbed?
As AAs, dipeptides and tripeptides
How does the stomach contribute to protein digestion?
Acid unravels proteins
Pepsin starts break down on proteins into smaller peptides
How is trypsinogen converted into its active form?
By enteropeptidase
What is the function of exopeptidases?
Give an example
Break bonds at end of polypeptide to produce dipeptides and AAs
Carboxypeptidase (A+B)
What is the function of endopeptidases?
Give an example
Break bonds within a polypeptide to produce shorter peptides
Trypsin, chymotrypsin, elastase
How are amino acids transported into the cell?
Co-transport with sodium
How are di/tripeptides transported into the cell?
H+ co-transporter (PepT1)
What happens to di/tripeptides inside enterocytes?
Cytostolic peptidases convert them to amino acids
Why is it more difficult for large intestine to absorb water?
Less water so smaller osmotic gradient
What hormone can help large intestine to absorb water?
Aldosterone - induces insertion of Na+ channels
How is calcium absorbed is intake is low?
Active transcellular absorption by facilitated diffusion (apical) and Ca2+ ATPase (basolateral)
What substances are required for active transcellular absorption of Ca2+?
Vit D, calbindin
Stimulated by PTH
How is calcium absorbed is intake is normal/high?
Passive paracellular absorption
How is iron transported across apical membrane?
Co-transported with H+
What happens to iron inside cells when levels are low?
Binds to transferrin and transported to stores in Hb, BM, liver and spleen
What happens to iron inside cells when levels are high?
Iron contained in ferritin complexes (trapped in cell) and lost when enterocyte is replaced
How are water soluble vitamins absorbed?
Na+ co-transport
How is Vit B12 absorbed?
In terminal ileum bound to intrinsic factor
What can cause Vit B12 deficiency and what can this lead to?
Gastritis or terminal ileal removal (Crohn’s)
Leading to pernicious anaemia (type of megaloblastic anaemia)
What is segmentation?
Mixing of contents in small intestine
How are contents moved along the small intestine?
Small intestine divided into sections, each with a pacemaker. Frequency of pacemaker higher proximally
What is haustra and why do they form?
Sacs of circular muscle which from as taenia coli (longitudinal muscle running length of LI) is shorter than intestine
What does haustral shuttling do?
Agitates and mixes food allowing remaining water to be absorbed
What is mass movement?
1-3 times a day, contents move rapidly from transverse colon to rectum
What triggers mass movement?
Often eating (gastro-colic reflex)
How much does rectum fill before we get the urge to defecate?
25%
How is faeces expelled? (think sphincters)
Internal sphincter relaxes (PSNS)
External sphincter relaxes (voluntary)
Intra-abdominal pressure rises
What is tenesmos?
Urge to defecate due to something stretching rectum
