DIGESTIVE SYSTEM LECTURE 2

Oesophagus

Muscular tube 25cm in length from the laryngopharynx to the stomach
Posterior to the trachea
Normally empty with lumen collapsed but expands for food & H₂O
Four layers usual except mucosa epithelium, muscularis externa, and serosa

Bile

Yellow, brownish, or olive-green liquid consisting mostly of H₂O, bile salts, cholesterol, lecithin, bile pigments (bilirubin), and several ions 
pH 7.6 - 8.6

Function of oesophagus

Transport (5s for food, 1s for fluid due to peristalsis)
Protection
No absorption, little secretion, no digestion 

Mucosa (Epithelium) of oesophagus

Thick stratified squamous epithelium w/ cells dividing in basal layers and pushing outwards towards lumen
Entire epithelium renewed every 7 days
Protection against abrasion from food

Muscularis externa of oesophagus

Skeletal muscle in upper third in addition to smooth muscle

Allows rapid contraction and voluntary swallowing

Serosa of oesophagus

Does not contact a body cavity so is referred to as adventitia

Fibrous and attaches to neighbouring organs such as trachea

Stomach

J-shaped bag on left side & enlargement of gut tube
Capacity 1.5L
Four regions; Cardia, fundus, body, pyloric part
Has pyloric sphincter and rugae when empty

Four layers are usual except mucosa epithelium and muscularis externa

Function of stomach

Primary storage (rate of ingestion > rate of digestion & absorption)
Secretion of acid, enzymes, mucous (2 - 3L per day, food + gastric juice = chyme)
Digestion of proteins by pepsin
Absorption of H₂O, ions, and some drugs (alcohol & aspirin)
Protection against own secretions and microbes 
Transport (mixing peristaltic waves every 20s)

Cardia

Surrounds opening of oesophagus into stomach
Mostly mucous glands

Fundus

Rounded portion superior to and to the left of cardia
Secretes acid, enzymes, and mucous

Body

Inferior to the fundus and large central portion of the stomach 
Secretes acid, enzymes, and mucous 

Pyloric part

Pyloric antrum connects to the body of stomach → pyloric canal → pylorus (mostly mucous glands) connects to duodenum

Pyloric sphincter

Well developed smooth muscle outlet connecting the pylorus to duodenum

Mucosa (Epithelium) of stomach

Epithelial cells extend down into lamina propria and muscularis mucosae to form many pits lined w/ mucus-secreting cells + gastric glands which open into pits

Superficial to deep: Surface mucous cells → Undifferentiated cells → Parietal cells → Mucous neck cells → Chief cells → Enteroendocrine cells

Surface mucous cells

Secrete insoluble alkaline mucous which protects the mucosa from acid and pepsin

Undifferentiated cells

Stem cells dividing to generate new epithelium

→ Surface mucous cells
→ Parietal cells
→ Mucous neck cells → Chief cells

Parietal cells

Secrete HCL to kill microbes and living cells + intrinsic factor which is needed to absorb Vitamin B₁₂

Mucous neck cells

Secrete soluble acid mucous while eating

Chief cells

Secrete pepsinogen and gastric lipase

Pepsinogen → Pepsin by acid → Catalyses protein splitting

Enteroendocrine cells

G cells located mainly in pyloric antrum secretes hormone gastrin into bloodstream

Gastrin → Stimulates secretion of acid and pepsinogen → Increases muscular contractions of stomach → Relaxes pyloric sphincter 

Muscularis externa of stomach

Three layers rather than two: outer longitudinal, middle circular, inner oblique

Propulsion

Peristaltic waves move gastric contents from the body of stomach down into antrum
Pyloric sphincter remains almost but not completely closed

Retropulsion

Food particles in the stomach are too large to fit through pyloric sphincter so are forced back into body of stomach 

Gastric emptying

Food particles in chyme that are small enough to pass through pyloric sphincter
Slow process - 3mL of chyme every propulsion

Liver

Enormous gland made of epithelial cells (Hepatocytes) derived from embryonic endoderm
All liver functions are carried out by hepatocytes

Function of the liver

Glycogen/glucose storage & release
Recycling RBC
Bile synthesis & secretion
Synthesis of plasma proteins and removal of toxins from blood

Hepatocytes requirements

Access to nutrient-laden blood drained from intestinal wall (Hepatic portal vein)
Access to oxygenated blood from systemic circuit (Hepatic artery)
Access to ducts which drain bile to gall bladder (Bile canaliculi)

Hepatic laminae

Plates of hepatocytes one cell thick bordered on either side by endothelial-lined vascular spaces (hepatic sinusoids) 
Highly branched and irregular structures

Bile canaliculi

Small ducts in grooves in cell membranes between hepatocytes that collect bile, flows opposite of blood

Bile canaliculi → Bile ductules → Bile ducts → Right/Left hepatic ducts → Common hepatic duct → Cystic duct → Common bile duct → Small intestine 

Hepatic sinusoids 

Highly permeable blood capillaries that receive oxygenated blood from hepatic artery and nutrient-rich deoxygenated blood from hepatic portal vein

Hepatic sinusoids → Central vein → Hepatic veins → IVC

Portal triad

Bile duct, branch of hepatic artery, and branch of hepatic portal vein

Liver lobules

Hepatic lamellae stacked together to form a hexagonal shape 2mm long by 1mm wide

Endothelial cell of hepatic sinusoids

Fenestrated to act as a filter to allow passage of lymph but not RBCs

Hepatic sinusoids → Endothelial cell → Lymph space of Disse → Microvilli → Hepatocyte → Tight junctions → Bile canaliculi