4-Digestion and Absorption of Fluids & Electrolytes

Primary function of secretion in the intestines

Adding substances to the lumen.

Primary function of absorption in the intestines

Removing substances from the lumen.

Part of the intestines that absorbs all nutrients in adult mammals

The small intestine.

Structures in the small intestine responsible for absorption and secretion

Villi (absorptive cells) and crypts (secretory cells).

Structures in the large intestine responsible for absorption and secretion

Surface epithelium (absorptive cells) and colonic crypts (gland cells).

Location and function of progenitor cells

They are found at the base of crypts in both the small and large intestine and contribute to cell turnover.

Effect of starvation on intestinal cell turnover

It decreases turnover.

Total daily fluid movement in the intestines

Approximately 8-9 L/day.

Fluid received and absorbed by the small intestine daily

It receives 7.5 L/day and absorbs about 6.5 L/day.

Fluid received and absorbed by the large intestine daily

It receives 2.0-2.5 L/day and absorbs 1.9 L/day.

Absorptive capacity of the large intestine

Up to 5 L/day.

Net ion movement

The difference between ion movement into and out of the lumen.

Three types of heterogeneity affecting ion transport in the intestines

Segmental heterogeneity, surface heterogeneity, and cellular heterogeneity.

Ions absorbed and secreted in the small intestine

Absorbed: Na+, Cl-, K+; Secreted: HCO3-.

Ions absorbed and secreted in the large intestine

Absorbed: Na+, Cl-; Secreted: K+, HCO3-.

Difference between transcellular and paracellular movement

Transcellular: solute moves through cells, crossing two membranes; Paracellular: solute moves passively between epithelial cells through tight junctions.

Determination of mucosal resistance in the intestines

Paracellular resistance, which depends on tight junction permeability.

Secretagogues and their function

They induce secretion by increasing second messengers.

Examples of secretagogues

Bacterial toxins, hormones, neurotransmitters, immune regulatory products, and laxatives.

Absorptagogues and their function

They induce absorption through neural, endocrine, or paracrine factors.

Osmotic diarrhea and its cause

It occurs when dietary components are not absorbed, such as lactose intolerance.

Secretory diarrhea and its cause

It is caused by the secretion of fluids and electrolytes from the intestine, often induced by enterotoxins.

How oral rehydration solution (ORS) helps in secretory diarrhea

ORS contains Na+ and glucose, which are absorbed together, counteracting fluid loss.

Primary function of the Na-K pump in sodium absorption

It moves Na+ across the basolateral membrane, maintaining a gradient for Na+ entry.

Four types of sodium absorption

Nutrient-coupled Na+ transport, Na-H exchanger, electroneutral NaCl absorption, and electrogenic Na+ absorption.

Type of sodium absorption not inhibited by cAMP

Nutrient-coupled Na+ transport.

Bacterial infection affecting NaCl absorption

Enterotoxigenic E. coli (ETEC), which increases cAMP and inhibits NaCl absorption.

Amiloride

It inhibits Na-H exchangers and electrogenic Na+ absorption.

Chloride Absorption Types

Voltage-dependent Cl- absorption, Cl-HCO3 exchanger, and electroneutral NaCl absorption.

Cl-HCO3 Exchanger

Absent in congenital chloride diarrhea.

Chloride Concentration in Stool

Cl- is in high concentration.

Cystic Fibrosis Transmembrane Regulator (CFTR)

It serves as a Cl- channel in the apical membrane.

Passive Potassium Absorption Location

In the small intestine via solvent drag.

Active K+ Absorption Mechanism

The H-K pump.

Factors Regulating K+ Secretion

Aldosterone and cAMP.

Aldosterone Effect on K+ Secretion

It activates the Na-K pump and increases apical K+ channels.

Pump-Leak Model of K+ Secretion

It describes K+ uptake via the Na/K/Cl cotransporter and its secretion through apical K+ channels.

Active Calcium Absorption Location

In the duodenum.

Calcium Absorption Regulator

Vitamin D.

Calbindin Function

It binds intracellular calcium and facilitates its transport.

Calcium Extrusion Transporters

The Na-Ca exchanger and Ca2+ ATPase pump.

Forms of Dietary Iron

Heme iron and nonheme iron.

Ferric Reductase Dcytb Function

Converts ferric iron (Fe3+) to ferrous iron (Fe2+) for absorption.

Nonheme Iron Absorption Transporter

The divalent metal transporter (DMT1).

Hepcidin Function

Regulates systemic iron levels by controlling ferroportin.

Anemia vs. Hemochromatosis

Anemia is caused by iron deficiency, whereas hemochromatosis results from excessive iron absorption.

Na+/K+ Pump Location

Basolateral membrane.

Na+/H+ Exchanger (NHE3) Location

Apical membrane (small intestine and proximal colon).

Na+-glucose and Na+-amino Acid Cotransporter (SGLT1) Location

Apical membrane (small intestine only).

Epithelial Na+ Channel (ENaC) Location

Apical membrane (distal colon).

Na+/K+/2Cl- Cotransporter (NKCC1) Location

Basolateral membrane (crypt cells, secretory function).

Cl-/HCO3- Exchanger Location

Apical membrane (ileum and colon for Cl- absorption).

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Location

Apical membrane (crypt cells, Cl- secretion).

Passive K+ Channel for Secretion Location

Apical membrane (distal colon).

K+/Cl- Cotransporter (KCC1) Location

Basolateral membrane (absorptive function in colon).

H+/K+ ATPase Exchanger Location

Apical membrane (distal colon, for K+ absorption).

TRPV6 Calcium Channel Location

Apical membrane (duodenum, active absorption).

Ca2+ ATPase (PMCA1) Location

Basolateral membrane (duodenum, transports Ca2+ into blood).

Na+/Ca2+ Exchanger (NCX1) Location

Basolateral membrane (duodenum, secondary active transport of Ca2+).

Divalent Metal Transporter 1 (DMT1) Location

Apical membrane (duodenal enterocytes, non-heme iron uptake).

Ferroportin Location

Basolateral membrane (duodenal enterocytes, iron export into circulation).

Hephaestin Location

Basolateral membrane (duodenal enterocytes, works with ferroportin to oxidize Fe2+ to Fe3+).