Tubes

I feel like my ANHB2212 posts are dragging out too long, and I need to be more concise. Sorry about that! Will try to be more concise here...

Describe the pattern of layers in bodily tubes

Hmm, I know how I can get through this concisely! Here's a list of the layers, from outside in.

• Adventitia/serosa- Adventitias are anchoring layers that help anchor the tube down to something else. Serosas are serous membranes, as mentioned in my post about the coelomic cavity.
• Muscularis externa- This is the muscle layer, which is usually two muscle layers: an outer longitudinal layer and an inner circular layer.
• Submucosa- Loose connective tissue with blood vessels and nerves.
• Mucosa- Consists of the muscularis mucosa (a thin layer of muscle tissue), the lamina propria (which supports the epithelium) and the epithelium itself (derived from endoderm if it's the gut, or from mesoderm if it's a blood vessel).
• Lumen- The cavity in the middle of the blood vessel.

Some tubes have glands- these can be found in the mucosa, submucosa or external layers.

Understand how this pattern is adapted to suit different functions in the gut

I'm going to whip out a table here. For conciseness. Or something. I'll only write stuff in the cells if there's something interesting or unique to write about. (I omitted "adventitia/serosa" because I didn't have anything special to talk about for either of them.)

Section	Main function(s)	Muscularis externa	Submucosa	Mucosa
Oesophagus	Transfer food to stomach- peristalsis		Mucous glands	Mucous glands, stratified squamous epithelium to protect from abrasion
Stomach	Some storage of food, control of food progression	Sphincters- pyloric and cardiac sphincters. Additional oblique muscle layer to mix food.		Thick and glandular, protects stomach. Glands secrete acids that kill pathogens and alkaline mucus that protects from acid. Columnar epithelium for absorption.
Small intestine	Absorption of nutrients, chemical digestion	Duodenal glands secrete alkaline mucus via intestinal pits	Spiral "plicae circulares" to extend contact time, villi, microvilli. Simple columnar epithelium, highly vascularised. Ileum has lymphoid tissue ("Peyer's Patches" to protect against colon bacteria.
Large intestine	Reabsorption of water, propulsion to rectum	3 "taeniae coli" that bunch intestine into sacculations/haustra		Increasing number of mucous cells. Smooth (no villi). Columnar epithelium.

Understand how this pattern is adapted to suit other bodily tubes

Blood Vessels

Blood vessels have three main layers:

• Tunica adventitia- similar to adventitia. Made of loose connective tissue that anchors the blood vessel in place. Sometimes it contains vasa vasorum, or "blood vessels of the blood vessels," which are especially important in ensuring that large, thick vessels are supplied with blood.
• Tunica media- similar to mucsularis mucosa. This is the main structural layer, made up of smooth muscle, elastin and/or connective tissue.
• Tunica intima- similar to mucosa. Made of simple squamous endothelium and supporting connective tissue. Nourished by diffusion from the blood in the lumen.

Now let's look at arteries in particular:

• Elastic arteries (e.g. carotid artery)- have a thick internal elastic lamina to keep the blood pressure constant. They have a vasa vasorum but very little muscle. They may also have baroreceptors and/or chemoreceptors.
• Muscular arteries (e.g. splenic artery)- have a thick wall of muscle (75% of wall thickness is muscle). This allows them to control blood flow to organs.
• Arterioles- there's lots of them, so their collective large cross-sectional area reduces the blood pressure before the blood gets to the capillaries (more on this when I write up on physiology).

And veins, which tend to have larger lumens than arteries:

• Large veins (e.g. IVC): no valves, but some longitudinal smooth muscle
• Medium veins (e.g. femoral vein)- have valves, which are just flaps of connective tissue lined with endothelium
• Venules- the veins' answer to arterioles
• Sinuses (e.g. coronary sinus)- super dilated veins

And then there's the capillaries between them. They are made of a single layer of endothelial cells, which makes it easy for stuff to diffuse through. They also have pericytes that serve as little muscular sphincters for the capillaries. Capillaries differ in the size of the pores between cells. Fenestrated capillaries allow large molecules like hormones to pass through, whereas sinusoids have massive gaps that allow entire cells to pass through.

Respiratory Tubes

Trachea, bronchi and bronchioles all have adventitia to anchor them. The trachea and bronchi also have cartilage "horseshoes" and respiratory epithelium consisting of mucous glands and cilia. The bronchioles do not have cartilage, but they are elastic and have smooth muscle that can adjust airway size. Bronchioles do not have glands in their mucosa.

Urinary Tubes

Urinary tubes (ureters and urethra) are retroperitoneal and have an adventitia. The urethra has an additional inner longitudinal muscular layer as well as a skeletal muscle sphincter. Ureters and urethra all have mucous glands to protect against corrosive urine, as well as transitional epithelium that becomes stratified squamous near the external opening of the urethra.

Reproductive Tubes

The ductus deferens/ vas deferens has an adventitia and 3 muscular layers. Its mucosa is elastic and has stereocilia, which are cilia that don't "waft" things but instead absorb things.

The uterine/fallopian tubes have a serosa rather than an adventitia, but they also have 3 muscular layers. Their mucosa has cilia that help waft the egg along, as well as glands that secrete nourishing fluid.

...Okay, that post was still quite long. Damn.