Transcript 食物的吸收

食物的吸收
Absorption
夏 强，PhD
浙江大学医学院生理学系
医学院科研楼C座518室
电话：88208252
Email：[email protected]
学习目标
• 了解人体消化系统与吸收有关的结构
• 能比较消化道对各类营养物质的吸收形式
和机制
• 能应用消化道吸收知识明确促进与抑制一
种营养物质吸收的作用靶点
消化道四大过程:
digestion, secretion, absorption, and motility.
消化道的液体平衡
Digestive secretions
are mostly water,
with the average
amounts indicated
here. Note that only
100 ml are excreted
in feces, so the
mechanisms for water
absorption are efficient
(recall the kidneys’
primary role in water and
osmotic homeostasis).
消化道与吸收有关的结构
The gut wall has a layered organization, with the absorptive cells lining the lumen
and neural and muscular components below. Blood and lymph vasculature is
abundant to transport absorbed nutrients.
nutrients
与吸收有关的结构
By projecting
into the lumen,
the villi increases
the surface area
for absorption of
nutrients.
Microvilli [aka brush
border] fringe the
villi to further increase
surface area.
消化与吸收的一般机制 General mechanisms of digestion and absorption
营养物质的吸收部位 Sites of nutrient absorption
Major gastrointestinal diseases and
nutritional deficiencies
Disease
Organ Site of
Defects in Nutrient
Predominant Disease Digestion/Absorption
Celiac sprue（脂肪痢）
Duodenum and jejunum
Fat absorption, lactose
hydrolysis
Chronic pancreatitis（慢性
Exocrine pancreas
胰腺炎）
Fat digestion
Surgical resection of ileum
（回肠手术切除）
Ileum
Crohn disease of ileum
（Crohn氏病）
Cobalamin and bile acid
absorption
Primary lactase deficiency
Small intestine
（原发性乳糖酶缺乏）
Lactose hydrolysis
糖的吸收
Carbohydrates
The three monosaccharide
products of carbohydrate
digestion— glucose, galactose,
and fructose—are absorbed by
the small intestine in a two-step
process involving their uptake
across the apical membrane into
the epithelial cell and their
coordinated exit across the
basolateral membrane.
The Na/glucose transporter 1
(SGLT1) is the membrane protein
responsible for glucose and
galactose uptake at the apical
membrane. The exit of all three
monosaccharides across the
basolateral membrane uses a
facilitated sugar transporter
(GLUT2).
蛋白质的吸收
Proteins
Action of luminal, brush border, and
cytosolic peptidases. Pepsin from the
stomach and the five pancreatic
proteases hydrolyze proteins—both
dietary and endogenous—to single
amino acids, AA, or to oligopeptides,
(AA)n . These reactions occur in the
lumen of the stomach or small intestine.
Various peptidases at the brush borders
of enterocytes then progressively
hydrolyze oligopeptides to amino acids.
The amino acids are directly taken up by
any of several transporters. The
enterocyte directly absorbs some of the
small oligopeptides through the action
of the H /oligopeptide cotransporter
(PepT1). These small peptides are
digested to amino acids by peptidases in
the cytoplasm of the enterocyte. Several
Na -independent amino acid
transporters move amino acids out of
the cell across the basolateral
membrane
Ann N Y Acad Sci. 2000;915:171-83.
Role of M cells in intestinal barrier function.
Kucharzik T1, Lügering N, Rautenberg K,
Lügering A, Schmidt MA, Stoll R, Domschke W.
完整蛋白质的吸收 Absorption of whole proteins. Both enterocytes and specialized M cells can take up intact
proteins. The more abundant enterocytes can endocytose far more total protein than can the M cells. However,
the lysosomal proteases in the enterocytes degrade ∼90% of this endocytosed protein. The less abundant M
cells take up relatively little intact protein, but approximately half of this emerges intact at the basolateral
membrane. There, immunocompetent cells process the target antigens and then transfer them to lymphocytes,
thus initiating an immune response
脂肪的吸收
Lipids
乳化脂滴分解成混合微粒 The breakdown of emulsion
droplets to mixed micelles
微粒转运脂肪分解产物到肠细胞表面 Micellar transport of lipid breakdown products to the surface of the
enterocyte. Mixed micelles carry lipids through the acidic unstirred layer to the surface of the enterocyte. 2-MAG,
fatty acids, lysophospholipids, and cholesterol leave the mixed micelle and enter an acidic microenvironment
created by an apical Na-H exchanger. The acidity favors the protonation of the fatty acids. The lipids enter the
enterocyte by (1) nonionic diffusion, (2) incorporation into the enterocyte membrane (collision), or (3) carriermediated transport.
在肠细胞内的再酯化及乳糜微粒的形成与分泌Re-esterification of digested lipids by the enterocyte and the
formation and secretion of chylomicrons. The enterocyte takes up short- and medium-chain fatty acids and glycerol
and passes them unchanged into the blood capillaries. The enterocyte also takes up long-chain fatty acids and 2MAG and resynthesizes them into TAG in the SER. The enterocyte also processes cholesterol into cholesteryl esters
and lysolecithin into lecithin. The fate of these substances, and the formation of chylomicrons, is illustrated by steps
1 to 8.
钙的吸收
Calcium
Calcium
钙在十二指肠的主动吸收 Active Ca uptake in the duodenum. The small intestine absorbs Ca by two mechanisms.
The passive, paracellular absorption of Ca occurs throughout the small intestine. This pathway predominates, but it is
not under the control of vitamin D. The second mechanism—the active, transcellular absorption of Ca —occurs only in
the duodenum. Ca enters the cell across the apical membrane through a channel. Inside the cell, the Ca is buffered by
binding proteins, such as calbindin, and is also taken up into intracellular organelles, such as the endoplasmic
reticulum
铁的吸收
Iron
Iron
非血红素铁与血红素铁在十二指肠的吸收 Absorption of nonheme and heme iron in the duodenum. The
absorption of nonheme iron occurs almost exclusively as Fe2+, which crosses the duodenal apical membrane
through DMT1, driven by a H gradient, which is maintained by Na-H exchange. Heme enters the enterocyte by an
unknown mechanism. Inside the cell, heme oxygenase releases Fe3+, which is then reduced to Fe2+. Cytoplasmic
Fe2+ then binds to mobilferrin for transit across the cell to the basolateral membrane. Fe2+ probably exits the
enterocyte through basolateral ferroportin. The ferroxidase activity of hephaestin converts Fe to Fe for carriage
in the blood plasma bound to transferrin.
钠的吸收
Sodium
钠主动吸收模式 Modes of active Na absorption
by the intestine. A, Nutrient-coupled Na
absorption occurs in the villous cells of the
jejunum and ileum and is the primary mechanism
for postprandial Na absorption. The thickness of
the arrows in the inset indicates the relative
magnitude of the Na absorptive flux through this
pathway. B, Electroneutral Na-H exchange at the
apical membrane, in the absence of Cl-HCO
exchange, is stimulated by the high pH of the
HCO -rich luminal contents. C, Na-H and Cl-HCO
exchange is coupled by a change in intracellular
pH that results in electroneutral NaCl absorption,
which is the primary mechanism for
interdigestive Na absorption. D, In electrogenic
Na absorption, the apical step of Na movement
occurs through the ENaC. CA, carbonic anhydrase.
氯的吸收
Chloride
Modes of Cl absorption by the intestine. A, In voltage-dependent
Cl absorption, Cl may passively diffuse from lumen to blood
across the tight junctions, driven by the lumen-negative
transepithelial voltage (paracellular route). Alternatively, Cl may
diffuse through apical and basolateral Cl channels. The thickness
of the arrows in the inset indicates the relative magnitude of the
Cl absorptive flux through this pathway. B, In the absence of a
parallel Na-H exchanger, electroneutral Cl-HCO exchange at the
apical membrane results in Cl absorption and HCO secretion. C,
Electroneutral NaCl absorption can mediate Cl absorption in the
interdigestive period. pH couples the two exchangers. CA,
carbonic anhydrase.
腺细胞生电性氯分泌的机制 Cellular mechanism of electrogenic Cl secretion by crypt cells. The basolateral
Na/K/Cl cotransporter brings Cl into the crypt cell; the Cl exits across the apical Cl channel. Secretagogues may
open preexisting Cl channels or may cause subapical vesicles to fuse with the apical membrane, thus delivering
new Cl channels. The paracellular pathway allows Na movement from blood to lumen, driven by the lumennegative transepithelial voltage. The thickness of the arrows in the inset indicates that the magnitude of the Cl
secretory flux through this pathway is the same throughout the intestine.
钾的吸收与分泌
Potassium
钾分泌与吸收的细胞机制 Cellular mechanisms of K secretion and
absorption. A, This mechanism pertains only to the small intestine, which is
a net absorber of K through solvent drag across tight junctions. The
thickness of the arrows in the inset indicates the relative magnitude of the K
flux through this pathway. B, The colon is a net secretor of K . The primary
mechanism is passive K secretion through tight junctions, which occurs
throughout the colon. The driving force is a lumen-negative transepithelial
voltage. C, Another mechanism of K secretion throughout the colon is a
transcellular process that involves the basolateral uptake of K through the
Na-K pump and the Na/K/Cl cotransporter, followed by the efflux of K
through apical K channels. D, Confined to the distal colon is a transcellular
mechanism of K absorption that is mediated by an apical H-K pump.
谢谢专注！