Large intestine - rectum - macroscopic structure Flashcards
(28 cards)
What is the general function of the large intestine?
The large intestine’s primary function is to absorb water and electrolytes from indigestible food matter and form faeces for elimination.
What are the main parts of the large intestine?
The large intestine includes the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.
Where does the rectum begin and end?
The rectum begins at the end of the sigmoid colon (around S3 vertebra) and ends at the anal canal.
What is the shape and size of the rectum?
The rectum is about 12 cm long and appears straight, but it has three lateral curves (folds) inside that help support faeces.
What features distinguish the large intestine from the small intestine?
The large intestine has no villi (unlike the small intestine).
It has haustrations (pouches) and teniae coli (longitudinal muscle bands) that help move and segment the contents.
What is the function of haustra in the large intestine?
Haustra are pouches formed by the contraction of teniae coli, which help segment and mix the contents for more efficient water absorption.
What is the role of teniae coli in the large intestine?
The teniae coli are three longitudinal muscle bands that run along the length of the large intestine. They help create haustral contractions, which move and segment the contents.
What is the role of the rectum in the digestive system?
The rectum serves as a temporary storage site for faeces before defecation. It helps retain faeces until the body is ready to eliminate it.
What are the lateral curves in the rectum, and what is their function?
The three lateral curves in the rectum help to support faeces and assist in preventing leakage by providing a reservoir for the contents.
What is the difference between the rectum and the anal canal in terms of structure?
The rectum is a straight structure, with a smooth inner lining.
The anal canal has a striated muscle layer (external anal sphincter) for voluntary control over defecation.
How does the rectum transition from the sigmoid colon?
The rectum starts where the sigmoid colon ends, at the level of S3. It is marked by the absence of teniae coli and haustra, which are present in the rest of the colon.
What is the structure of the anal canal?
The anal canal is a short, cylindrical passage that connects the rectum to the outside of the body. It is about 2-4 cm long and contains two important sphincters:
Internal anal sphincter (involuntary)
External anal sphincter (voluntary)
What is the role of the internal anal sphincter?
The internal anal sphincter is made of smooth muscle and is responsible for keeping the anus closed at rest. It relaxes automatically during defecation to allow the passage of stool.
What is the role of the external anal sphincter?
The external anal sphincter is made of skeletal muscle and gives voluntary control over defecation, allowing the body to control when to eliminate faeces.
What is the significance of the pectinate line in the anal canal?
The pectinate line marks the boundary between the endodermal (upper) and ectodermal (lower) origins of the anal canal. It is clinically important because it separates areas with different nerve supplies, blood supply, and lymphatic drainage.
What structural feature distinguishes the rectum from the rest of the large intestine?
The rectum lacks teniae coli and haustra, which are present in the rest of the large intestine. Instead, the rectum has a smooth wall for continuity in pushing faeces toward the anal canal.
How does the rectum’s structure differ from the large intestine in terms of muscle arrangement?
The rectum has continuous smooth muscle in the muscular layer, unlike the large intestine, which has teniae coli (three longitudinal muscle bands). This continuous muscle in the rectum helps maintain its function for storing faeces and defecation.
How does the peritoneal covering of the rectum vary along its length?
The upper third of the rectum is covered by peritoneum on the front and sides.
The middle third is covered by peritoneum only on the front.
The lower third is extraperitoneal, with no peritoneal covering, which gives it a more fixed position in the pelvic cavity.
What are haustra, and where are they found in the large intestine?
Haustra are pouches or sacs in the wall of the large intestine formed by teniae coli. They are not present in the rectum, which has a continuous smooth wall instead.
What is the role of the rectal ampulla?
The rectal ampulla is the expanded portion of the rectum just before the anal canal. It stores faeces until defecation is triggered.
What is the role of teniae coli in the large intestine?
Teniae coli are three longitudinal muscle bands that run the length of the large intestine. They help create haustral contractions that move and segment the contents of the large intestine.
What is the transition between the rectum and anal canal marked by?
The transition is marked by the pectinate line, where the epithelial lining changes from simple columnar epithelium in the rectum to stratified squamous epithelium in the anal canal.
What role does the rectum’s smooth muscle layer play?
The smooth muscle layer of the rectum helps propel faeces toward the anal canal and plays a key role in maintaining continence by contracting to hold faeces until defecation is initiated.
How is the structure of the anal canal different from the rectum?
The rectum has smooth muscle for continuous peristalsis.
The anal canal has a striated muscle layer (external anal sphincter) that provides voluntary control over defecation.
