digestive system主要由胚胎時期之endoderm內(nèi)胚層分化而成
four basic digestive processes: digestion, absorption, secretion, and motility
Gastrointestinal (GI) tract：Mouth → pharynx → esophagus → stomach → small intestine
→ colon → rectum → anus
Wall of GI tract (內(nèi)層→外層): 有四層
? Mucosa黏膜層：Lining of the lumen管腔的襯裡
? Submucosa 黏膜下層：Connective tissue
? Muscularis externa 外肌層：Smooth muscle
? Serosa 漿膜層：Connective tissue
The mucosa is composed of three layers:
Mucous membrane：an innermost layer of epithelial cells
- Separates GI lumen from internal environment分隔消化道管腔與內(nèi)在環(huán)境
- Enterocytes (腸上皮細(xì)胞) are classified as absorptive cells (吸收細(xì)胞), exocrine cells (外分泌細(xì)胞), endocrine cells (內(nèi)分泌細(xì)胞)
- Goblet cells (杯狀細(xì)胞) (exocrine cells) secrete mucus (分泌黏液) throughout the length of the GI tract
Lamina propria (固有層)：a middle layer of connective tissue
- Contained within this layer are small blood vessels (小血管), nerves (神經(jīng)), and lymphoid tissue (淋巴組織)
- Lymph nodules (淋巴結(jié)) and Peyer’s patches (培氏斑) are important in defending the body against bacteria (保護(hù)人體免受細(xì)菌感染)
Muscularis mucosae (黏膜肌層)：an outer layer of smooth muscle
- Severs primarily to contract the mucosa into folds, which stirs the lumenal contents and promotes contact with the mucosal surface (使黏膜收縮產(chǎn)生皺摺，攪動管腔內(nèi)容物，增加與粘膜表面的接觸)
Submucosa黏膜下層：a thick layer of connective tissue that provides the GI tract with much of its distensibility (可膨脹性或可擴(kuò)展性) and elasticity
At its outer border (外圍) is a network of nerve cells (神經(jīng)細(xì)胞網(wǎng)路) known as the submucosal plexus (黏膜下神經(jīng)叢) (Meissner’s plexus 麥斯納神經(jīng)叢), which communicates with another nerve cells network in the muscularis externa (外肌層) called the myenteric plexus (腸肌神經(jīng)叢) (Auerbach’s plexus 歐爾巴克神經(jīng)叢)
Muscularis externa is largely responsible for the motility of the GI tract(使胃腸道具有運(yùn)動性) and contains two separate layers of smooth muscle (含兩層獨(dú)立的平滑肌)
Inner layer (內(nèi)層) of circular muscle (環(huán)狀肌) consists of single-unit smooth muscle (單一單位平滑肌) capable of generating spontaneous depolarizations (能夠產(chǎn)生自發(fā)性去極化), called slow-wave potentials(慢波電位)：(changes diameter 改變直徑)
Outer layer 外層 of longitudinal muscle (縱狀肌) consists of multi-unit smooth muscle (多單位平滑肌) that depends on neural input for contraction (依賴神經(jīng)訊息來收縮)：(changes length 改變長度)
multi-unit smooth muscle:
- Located in large airways (呼吸道) and arteries (動脈)
- Few if any gap junctions
- Each fiber acts individually receives own innervation (每根纖維單獨(dú)作用，由自己神經(jīng)支配)
single-unit smooth muscle：
- Location: gastrointestinal tract (消化道); uterus (子宮)
- Muscle fibers activated synchronously (協(xié)同性) fibers connected by gap junctions contract together as a single unit
Serosa is the outermost layer (最外層) of the GI tract
- Inner layer of fibrous connective tissues (內(nèi)層: 纖維結(jié)締組織) provides structural support (提供結(jié)構(gòu)性支撐)
- Outer layer of epithelial tissues called the mesothelium (間皮) secretes a watery lubricating fluid (分泌水性的潤滑液) that makes it easier for organs to slide past one another (使器官間更容易相互滑過)
- The mesothelium continuous with mesenteries (腸繫膜), a system of clear, thin membranes that interconnects most of the abdominal organs and houses nerves and blood vessels running to them 腸繫膜把腹腔大多數(shù)的器官及其神經(jīng)和血管相互結(jié)合起來
mastication 咀嚼
lubricates the food 潤滑食物
upper esophageal sphincter (上食道括約肌), a ring of skeletal muscle surrounding the esophagus at its upper end. 上食道括約肌是骨骼肌
Fundus 胃底—the wall is thin and easily expands (壁薄且容易膨脹) to accommodate increases following a meal (容納飯後的容量增加)
Body 胃體—the gastric mucosa (胃黏膜) is thrown into longitudinal folds(縱狀皺褶) called rugae (皺褶), which flatten as the stomach expends 胃粘膜被扔進(jìn)稱為皺褶的縱狀皺褶，隨著胃的擴(kuò)張而變平
Antrum 胃竇 —has the thickest muscle layer (厚的肌肉層) and produces strong contractions (產(chǎn)生很強(qiáng)的收縮) responsible for gastric mixing and emptying (負(fù)責(zé)混合及胃排空)
Contraction of the antrum propel chyme from the stomach into the small intestine, a process called gastric emptying (胃排空) is regulated by pyloric sphincter (幽門括約肌), a surrounding ring of smooth muscle
胃竇的收縮推進(jìn)食糜從胃進(jìn)入小腸，稱為胃排空，此過程由幽門括約肌 (平滑肌環(huán)) 調(diào)節(jié)
Both the fundus (胃底) and body contain gastric pits (胃小凹), which contain cells that secrete the products found in gastric juice 胃底和胃體都有胃小凹，胃小凹中有細(xì)胞可分泌胃液中的成分
Chief cells (主細(xì)胞), which secreted pepsinogen (胃蛋白酶原), the precursor for a proteolytic enzyme called pepsin (胃蛋白酶)
Hydrogen ions secreted by parietal cells (壁細(xì)胞)
Intrinsic factor (內(nèi)生性因子) secreted by parietal cells necessary for absorption of vitamin B12
Gastrin (胃泌素) (endocrine) secreted from G cells
Mucus (黏液) secreted from neck cells (頸細(xì)胞)
The small intestine is divided into three major regions: an initial portion called the duodenum (十二指腸), a middle portion called the jejunum (空腸), and a terminal portion called ileum (迴腸)
bile膽汁
Absorption is typically completed within the first 20% of the small intestine’s length (在小腸的前20%就幾乎完成所有營養(yǎng)物的吸收)
Epithelial cells in crypts of Lieberkuhn (李培昆氏凹窩；小腸隱窩) secrete bicarbonate-rich fluid succus entericus 腸液
Together, the cecum 盲腸, colon 結(jié)腸, and rectum 直腸 constitute what is often referred to as the large intestine 大腸
Relaxation of both sphincters, which are normally closed, allows fecal material to be eliminated from the body, a process called defecation
肛門內(nèi)、外括約肌都舒張時，使糞便從體內(nèi)排出，這一過程稱為排便
Bicarbonate (碳酸氫鹽) neutralized acid (中和酸)
Pancreatic amylase (胰澱粉酶) breaks down starch and glycogen
Pancreatic lipases (胰脂肪酶) breaks down fats
Proteases (蛋白水解酶) breaks down proteins
Nucleases (核酸水解酶) breaks down nucleic acids
- 正常狀況下，肝臟釋放血管收縮素原(Angiotensinogen)，經(jīng)腎臟分泌的腎素(Renin)作用變成血管收縮素I (Angiotensin I);
- 再經(jīng)過肺臟製造的血管收縮素轉(zhuǎn)換酵素(Angiotensin Converting Enzyme, ACE)作用後，變成血管收縮素II (Angiotensin II)，接著會：使血管收縮→血壓上升刺激腎上腺皮質(zhì)分泌醛固酮(Aldosterone) →造成Na+與水分滯留→血壓上升接著會產(chǎn)生負(fù)回饋，以抑制Renin的釋放，導(dǎo)致Angiotensin II 的釋出減少，所以血壓不會一直上升
Common bile duct 總膽管 transports bile (運(yùn)送膽汁) from liver and/or gallbladder to duodenum
從肝臟或膽囊運(yùn)送到十二指腸
Common bile duct joins with pancreatic duct to form Ampulla of Vater
總膽管與胰管匯集形成壺腹
Sphincter of Oddi 歐迪氏括約肌,which is normally closed (平常關(guān)閉) but opens when food is present (有食物時打開) regulates flow from pancreas and gallbladder to duodenum (調(diào)控胰液及膽汁至十二指腸的流速)
The liver contains numerous blood-filled cavities 充血的空腔 called sinusoids 竇狀隙, which take the place of true capillaries 微血管
the cells take from the blood the raw materials for making bile and then secrete the bile into the
bile canaliculi (膽小管) bile ducts (膽管) common hepatic duct (總肝管)
The functional unit of the liver is the liver lobule 肝小葉, consisting of liver cells called hepatocytes 肝細(xì)胞
colon結(jié)腸
maltose 麥芽糖
Dextrinase 糊精酶 – limit dextrins to glucose
Glucoamylase 葡萄糖澱粉酶 – straight-chain glucose polymer to glucose
Sucrase 蔗糖酶 – sucrose to fructose(果糖) + glucose
Lactase 乳糖酶 – lactose to galactose(半乳糖) + glucose
Maltase 麥芽糖酶 – maltose to 2 個 glucose
Fructose 果糖： facilitated diffusion
Glucose & galactose： sodium-linked secondary active transport
Secondary Active Transport次級主動運(yùn)輸
- Energy stored in Na+ (or H+) concentration gradient established by Primary Active Transport
- “Transporter” membrane protein binds to Na+ and another substance at same time so shape change moves both ions or molecules
Two types of transporters
- Symporters: move two substances in same direction
- Antiporters: move substances in opposite directions
次級主動運(yùn)輸 (協(xié)同運(yùn)輸)時， Na+移動之動力是來自Na+/K+ ATPase pump主動運(yùn)輸形成之Na+濃度梯度，此情況下， Na+擴(kuò)散進(jìn)入細(xì)胞內(nèi)，往往可驅(qū)使另一分子或離子與Na+同時移動，以便進(jìn)入或離開細(xì)胞。這些分子或離子若與Na+之運(yùn)輸方向相同，則此種協(xié)同運(yùn)輸稱為同向運(yùn)送(Symport or cotransport) ；若與Na+之運(yùn)輸方向相反則稱為逆向運(yùn)送 (antiport or counter-transport)
Endopeptidases 內(nèi)勝肽酶—split polypeptides at interior peptide bonds small peptides 將多勝肽從內(nèi)切開形成小的勝肽片段
Exopeptidases 外勝肽酶—cleave off amino acids from one end of the polypeptides amino acids 將多勝肽從末端把胺基酸一個一個切開
To protect the epithelial cells from the proteolytic action of these enzyme, the enzymes are stored in epithelial cells in an inactive form, called zymogens 酶原 secreted by exocytosis
為了保護(hù)上皮細(xì)胞不受到酵素的蛋白水解作用，這些酵素都以去活化的酶原型態(tài)儲存在上皮細(xì)胞的小泡內(nèi)，再以胞吐的方式分泌出去
Chief cells 主細(xì)胞 secrete pepsinogen胃蛋白酶原, which is partially activated 部分活化by hydrochloric acid (HCl) secreted by parietal cells 壁細(xì)胞.
Pepsin 胃蛋白酶 can then start digestion of protein 開始消化蛋白質(zhì) in the lumen of the stomach 胃, but it cannot complete digestion 不能完全消化為單個氨基酸 to individual amino acids because its actions are limited to splitting bonds between certain amino acids only受限於胃蛋白酶只可切除某些胺基酸間的鍵結(jié).
The pancreas 胰臟 secretes several zymogens into the duodenum 十二指腸, including trypsinogen 胰蛋白酶原, chymotrypsinogen 胰凝乳蛋白酶原, and procaroboxypeptidase 羧基端勝肽酶原
Activation of proteases in the small intestine. Trypsinogen 胰蛋白酶原is converted to trypsin 胰蛋白酶 by the brush border enzyme enterokinase 腸激酶.
Trypsin then converted other zymogens to their active forms in the lumen of the small intestine and also converts other trypsinogen to trypsin
Absorption of amino acids
- Cross apical membrane by sodium-linked secondary active transport
- Cross basolateral membrane by facilitated diffusion
Absorption of di- and tri-peptides
- Cross apical membrane by active transport
- Broken down inside cell to amino acids
- Amino acids cross basolateral membrane by facilitated diffusion
Because lipids are hydrophobic 疏水性, but lipases are water-soluble 水溶性
lipids do not mix readily with chyme脂質(zhì)不容易與食糜混合→ form fat globules 脂肪球
Efficient digestion 有效率的消化 of lipids is made possible by the action of bile 膽汁 it simple facilitates the action of lipases by breaking fat globules down into smaller droplets 小顆粒, a process called emulsification 乳化作用
膽酸 (bile acids)是膽汁中的主要成分，傳統(tǒng)上認(rèn)定的功用是消化脂肪所必需的乳化劑，以及膽固醇的代謝廢棄物。在人類的膽汁中，結(jié)合型膽酸是主要的存在形式，在膽汁的pH環(huán)境下，幾乎均以鈉鹽或鉀鹽的形式存在，稱為膽鹽（bile salts）。
肝細(xì)胞將膽固醇代謝成膽酸，隨著膽汁儲存在膽囊，進(jìn)食時隨著膽汁排入小腸，乳化飲食中的油脂以利吸收；同時被小腸細(xì)胞吸收的膽酸會進(jìn)入肝門靜脈，流回肝臟再供利用，這是膽酸的腸肝循環(huán)，只有5%膽酸會從糞便排出體外。每天大約有200-600 mg的膽固醇會用來合成膽酸。
In the ileum 迴腸, the bulk of the bile salts that were secreted into the duodenum 十二指腸 (about 95%) are absorbed into the circulation
these bile salts are eventually recycled by the liver and secreted again in the bilevia a pathway referred to as the enterohepatic circulation 腸肝循環(huán)
Fatty acids 脂肪酸 and monoglycerides 單酸甘油酯 absorbed into enterocytes 腸細(xì)胞 are
synthesized into triglycerides, which are then packaged into chylomicrons 乳縻微粒.
After exocytosis 胞吐 into interstitial fluid, chylomicrons enter the lymphatic system 淋巴系統(tǒng) via lacterals 乳縻管and eventually reach the general circulation 全身血液循環(huán)
Pernicious anemia 惡性貧血 which can result from a lack of vitamin B12 in the diet or a lack of intrinsic factor
Sodium (Na+)
- Solvent drag 拖 with water reabsorption in duodenum, jejunum
- Actively absorbed in jejunum, ileum, and colon
Chloride (Cl-)
- Passively follows sodium absorption 被動性的跟隨鈉離子而吸收(並非以被動擴(kuò)散的方式吸收) electro-neutrality 電中性
Potassium (K+)
- Passively absorbed depending on electrochemical gradients
- Secreted when lumenal concentrations very low (<25 mM)
Bicarbonate (HCO3-)
- Passively absorbed in jejunum
- Secreted exchange for chloride ions in ileum, and colon
The acid secreted by the stomach is generated inside parietal cells by the reaction of carbon dioxide (CO2) with water (H2O), which is catalyzed by the enzyme carbonic anhydrase 碳酸酐酶 (CA) and generates hydrogen ions (H+) and bicarbonate (HCO3-)
胃分泌胃酸是透過二氧化碳與水在壁細(xì)胞內(nèi)部反應(yīng)產(chǎn)生的，該反應(yīng)由碳酸酐酶催化，並產(chǎn)生氫離子和碳酸氫根
- Parietal cells produce acid by active transport of H+ into lumen of stomach & HCO3- transported into interstitial fluid in exchange for Cl-
- Cl- diffuses into lumen through channels
- H+ and Cl- secreted into lumen & HCO3- enters blood
壁細(xì)胞透過將氫離子主動運(yùn)輸?shù)轿盖粊K將碳酸氫根轉(zhuǎn)運(yùn)到組織液中以交換氯離子而產(chǎn)生酸
Calcium (Ca2+) is actively absorbed in duodenum and jejunum →
binds to brush border protein (calcium-binding protein 鈣結(jié)合蛋白)→ taken into epithelial cell by an unknown mechanism →transported out of cell across basolateral membrane by Ca2+ pump
透過鈣幫浦通過基底外側(cè)膜轉(zhuǎn)運(yùn)出細(xì)胞
The absorption of iron (Fe2+), like that of calcium and vitamin B12, requires the aid of proteins 需要蛋白的幫助
transferrin 運(yùn)鐵蛋白 secreted by enterocytes into lumen of small intestine
transferrin binds iron transferrin-Iron complex binds to a receptor→ taken into cell by receptor-mediated endocytosis
腸細(xì)胞分泌的運(yùn)鐵蛋白進(jìn)入小腸腔→運(yùn)鐵蛋白結(jié)合鐵→運(yùn)鐵蛋白-鐵複合物與受體結(jié)合→透過受體介導(dǎo)的內(nèi)吞作用進(jìn)入細(xì)胞
The iron is either stored in enterocyte as ferritin 鐵蛋白 or transported across the basolateral membrane into the blood transported in blood is carried by another form of transferrin
鐵要麼以鐵蛋白的形式儲存在腸細(xì)胞中，要麼就跨過基底外側(cè)膜運(yùn)輸?shù)窖褐?/font>
Transferrin 是一種由肝臟製造的 β 球蛋白，每一個Transferrin 分子能和二個鐵原子結(jié)合
GI function not regulated based on concept of homeostasis 恆定
regulate GI function to maximize absorption, regardless of whether nutrients needed
胃腸道功能的調(diào)控並不是基於維持身體內(nèi)在環(huán)境恆定的原則，而是不管是否有營養(yǎng)的需求，都要竭盡所能的達(dá)到最大的吸收
- Mechanoreceptors 機(jī)械性接受器, which detect the degree of distension of the wall 偵測管壁膨脹的程度
- Chemoreceptors 化學(xué)接受器, which monitor the concentrations 濃度 in the lumen of specific substances such as hydrogen ions or fats
- Osmoreceptos 滲透性接受器, which monitor the osmolarity of the luminal contents
Gastrin胃泌素 stimulate gastric secretion and motility.
Cholecystokinin(CCK)膽囊收縮素, Secretin腸促胰泌素, and Glucose-dependent insulinotropic peptide(gastric inhibitory peptide,GIP) inhibit gastric secretion and motibility.
Cephalic Phase 頭期—stimuli originate in head 頭
- thoughts, taste, smell of food… requires input from CNS (long reflexes)
Gastric Phase 胃期—stimuli originate in stomach 胃
- via long and short reflex and GI hormones
Intestinal Phase 腸期—stimuli originate in small intestine 小腸
- via long and short reflex and GI hormones
Satiety factors 飽足因子 (such as insulin 胰島素, CCK 膽囊收縮素, neural input from mechanoreceptors 機(jī)械性接受器 and chemoreceptors 化學(xué)接受器) acts on the hypothalamus 下視丘to reduce the sensation of hunger
Leptin 瘦素 is a hormone released from adipose cells 脂肪細(xì)胞 when calories exceeds demands
Leptin acts on appetite-control centers in the hypothalamus to reduce the sensation of hunger
Leptin also promotes a general increases in the body’s metabolic rate , which reduces fat storage
α-MSH（α黑色素細(xì)胞刺激素）和CART（可卡因和苯丙胺相關(guān)轉(zhuǎn)錄本）到達(dá)外側(cè)視丘區(qū)以引起飽足感，並到達(dá)室旁核以刺激CRH和TRH的釋放
Parasympathetic activity stimulates copious 大量的 secretion of a watery 水性的 saliva.
Sympathetic activity stimulates minute 少量的 secretion of proteinrich,viscous 黏性的 saliva.
Acid secretion is controlled by cephalic-phase 頭期, gastric-phase 胃期, and intestinal-phase 腸期 stimuli and is generally stimulated when food is present in the stomach and suppressed when the food leaves.
Because the stimuli that affect acid secretion also tend to affect the secretion of pepsinogen in the same manner, the secretion of pepsinogen generally rises and falls in parallel with changes in acid secretion.因?yàn)橛绊懰岱置诘拇碳の镆矁A向於以同樣的方式影響胃蛋白酶原的分泌，所以酸分泌增加時， 胃蛋白酶原也跟著分泌增加
Secretin acts on the liver to stimulate bile secretion
secretin 刺激肝臟分泌膽汁
CCK promote gallbladder contraction and relaxation of the sphincter of Oddi
CCK促進(jìn)膽囊收縮及歐迪士括約肌鬆弛
In any given region of the GI tract, slow waves occur at regular intervals at a fairly constant frequency, a pattern that is referred to as the basic electrical rhythm(BER)基礎(chǔ)電性節(jié)律 and is affected by neural activity and hormones:
parasympathetic→ excitation
在胃腸道的任何給定區(qū)域中，慢波均以固定的頻率在固定間隔發(fā)生，這種模式被稱為基礎(chǔ)電性節(jié)律BER，並受副交感神經(jīng)和激素的影響
Myenteric plexus腸肌神經(jīng)叢 (Auerbach’s plexus歐爾巴克神經(jīng)叢)：Second part of enteric nervous system
Neural activity and hormonal signals generally affect the height of slow waves rather than their
frequency神經(jīng)活動和激素訊號通常會影響慢波的高度，而不是其頻率
In the stomach, smooth muscle generates force in a grade fashion漸進(jìn)的形式 that varies according to the degree of depolarization 去極化的程度
The basic electrical rhythm (BER) set up a wave of contraction 收縮波 in the muscularis externa 外肌層 that travels longitudinally down the intestinal tract→these waves called peristalsis 蠕動
Peristalsis propels the contents forward推動內(nèi)容物往前 at a rate dependent on the BER
推動的速率依BER而定
In the proximal 近端 GI segment
circular muscle contracts; longitudinal muscle relaxes→diameter decreases
In the distal 遠(yuǎn)端 GI segment
circular muscle relaxes ; longitudinal muscle contracts→ diameter increases
prepares it to receive the contents from the proximal segment準(zhǔn)備接收從前端送來的內(nèi)容物
We can control the rate and force of chewing motions voluntarily & unconscious chewing is orchestrated協(xié)調(diào) by a chewing reflex 咀嚼反射
normally, jaw muscles 顎肌 active → hold mouth closed
food enters mouth → stimulate pressure receptor 壓力接受器→ inhibits jaw muscles →relax
Chewed food + saliva = bolus 食團(tuán) → tongue 舌 moves bolus to pharynx 咽 → initiates swallowing reflex 開始吞嚥反射, a series of muscle contractions coordinated by the swallowing center in the medulla 延腦
Bolus descends in pharynx, pressing on epiglottis 會厭 → epiglottis covers glottis 聲門 so food does not enter trachea → inspiration inhibited 吸氣受到抑制
Upper esophageal sphincter括約肌 relaxes → bolus enters esophagus → sphincter closes behind bolus → bolus stretches esophagus, triggering peristalsis
Peristalsis propels bolus to stomach (~ 9 seconds) →bolus arrives at stomach→triggers relaxation of lower esophageal sphincter→ bolus enters stomach
Substances (such as copper sulfate) stimulate vomiting are known as emetics. 刺激嘔吐的物質(zhì)被稱為催吐劑
