Functions and Structure of the Digestive System

Functions of the Small Intestine

• The small intestine is a critical component of the gastrointestinal (GI) tract.

  • Almost all digestion and absorption occur here.

  • It is the longest region of the GI tract, measuring between 5 to 7 meters in length.

Structure of the Small Intestine

• The small intestine is divided into three main regions:

  1. Duodenum:

  • Shortest region of the small intestine.

  • The term "duodenum" translates to "12 finger breadths", indicating its length.

  • This is the section where chyme (partially digested food) enters and where secretions from the pancreas and bile from the gallbladder are introduced.

  1. Jejunum:

  • Named from the Latin word meaning "empty" as it is often found empty during autopsies.

  • The primary site for most digestion and some absorption occurs here.

  1. Ileum:

  • The longest part of the small intestine.

  • The term "ileum" means "twisted".

  • Involved mostly in absorption, with some digestion occurring here.

Surface Area Importance

• The small intestine has a significantly large surface area needed for effective absorption:

  • Total surface area is approximately 200 square meters, comparable to the floor space of a typical Southern California home.

  • Imagery: Imagine spreading a meal over that floor area to conceptualize the extent of surface required for absorption.

Structural Features Contributing to Surface Area

• Plicae Circulares:

  • Circular folds in the intestinal lining that increase surface area.

• Villi:

  • Finger-like projections that protrude from the mucosal lining of the intestine, enhancing absorption.

  • Each villus has a rich supply of blood vessels, enhancing nutrient uptake.

• Microvilli:

  • Tiny hair-like structures on the surface of the epithelial cells that further increase surface area, termed the brush border.

Mucosal Structure

• The mucosal epithelium is composed of:

  • Simple columnar epithelium with goblet cells that produce mucus for protection.

  • Intestinal Crypts:

  • In the mucosa that house:

    • Cells producing digestive enzymes and hormones.

    • Rapidly dividing stem cells necessary for quick cell replacement (lifespan of 12-24 hours).

Vascular Structures in the Villi

• Each villus contains:

  • A large lymphatic capillary referred to as a lacteal for absorbing fats.

  • Fat is absorbed and sent into these lacteals to prevent clogging of standard capillaries.

Peristalsis and Movement

• The muscularis mucosae consists of:

  • An inner circular layer and an outer longitudinal layer which facilitate peristalsis (the movement of contents).

Transition to the Large Intestine

Function of the Large Intestine

• The large intestine stores intestinal bacteria that are essential for health.

  • These bacteria produce important vitamins (e.g., Vitamin B12 and K).

  • They also break down materials not digested in the small intestine, which can include fibers.

• Implications of bacterial action:

  • Example of lactose intolerance: those who cannot digest lactose may experience gas production from bacteria digesting it.

  • Foods high in undigestible sugars (like beans) similarly feed intestinal bacteria.

Structure of the Large Intestine

• Composed of layers including:

  • Mucosa with prominent goblet cells for mucus secretion.

  • Muscularis mucosa with submucosa featuring many lymph nodes to contain bacteria.

• The outer longitudinal layer of muscle is only present in strips (teniae) rather than throughout, since strong peristalsis is not required.

Impact of Diet on the Large Intestine

• Dietary Fiber:

  • Essential for bulk in the diet, allowing material to be pushed through the intestines.

• Consequences of Low Fiber:

  • Lack of fiber can lead to constipation and potential intestinal blockages.

• Diarrhea Mechanism:

  • Insufficient water absorption leads to fluids in feces, often in response to infection; it can result in dehydration and potentially be fatal within 24 hours if severe.

Conditions Related to the Large Intestine

Appendicitis

• Treatment involves either removing the appendix or addressing complications if it has ruptured.

Diverticulitis

• Painful condition involving diverticula in the large intestine, often requires surgical intervention.

Sigmoid Colon and Defecation

• Sigmoid Colon: Functions to retain feces temporarily while allowing gas to escape.

• Levator Ani and Anal Sphincter: Muscles involved in controlling defecation.

Colon Cancer and Prevention

• Polyps, non-cancerous growths that may become cancerous, can be detected through colonoscopies.

  • The presence of abnormal DNA may indicate cancer.

• Statistics on colon cancer: Rising cases in younger populations (20s-30s), traditionally seen in older individuals.

Liver Anatomy and Function

• Liver: The largest internal organ, weighing about 3 pounds; located mostly in the upper right abdomen.

Functions of the Liver

• Over 500 functions involving:

  1. Production of Blood Proteins:

  • All blood proteins, excluding antibodies, are produced here.

  1. Cholesterol Production:

  • Major source for body cholesterol; dietary intake is minor.

  1. Bile Production:

  • Bile as an emulsifier aiding in fat digestion.

  • It breaks fats into smaller globules for easier digestion.

  1. Processing Nutrients:

  • All nutrient-rich blood from the GI tract passes through the liver, allowing for nutrient regulation.

  1. Detoxification:

  • The liver takes toxins out of blood, including alcohol, but can also be damaged in the process.

Liver Structure

• Consists of two lobes (right and left) with lobules serving as functional units.

• Each lobule contains hepatocytes, the primary liver cells, and is surrounded by sinusoids (capillaries). Kupffer cells (macrophages) are present to break down old red blood cells.

Jaundice

• A sign of liver disease indicated by yellowing of the skin, resulting from bile accumulation due to liver dysfunction (e.g., hepatitis).

• Important symptom to recognize, particularly in newborns.