Clase 44 Fisiología Gastrointestinal - Flujo Sanguíneo (IG:@doctor.paiva)
Introduction to Gastrointestinal Physiology
Overview of the Class
- The 44th class on gastrointestinal physiology is introduced by Eduardo Paiva, focusing on blood flow in the gastrointestinal system.
- Topics include generalities, anatomy of gastrointestinal irrigation, variations in blood flow, and neural control of this flow.
Blood Circulation in the Digestive System
- The digestive system's blood vessels are part of systemic circulation; they transport blood from the digestive tract and pancreas to the liver via the portal vein.
- Arterial blood reaches the liver through celiac trunk and mesenteric arteries before returning to the inferior vena cava via hepatic veins.
Gastrointestinal Blood Flow Dynamics
Unique Features of Gastrointestinal Irrigation
- The gastrointestinal irrigation system is uniquely structured with an arterial system that opposes mesenteric insertion.
Variations in Blood Flow
- Blood flow varies with local activity; for instance, during absorption post-meal, intestinal blood flow can increase up to eight times normal levels.
- After eating, motor activities and secretion increase significantly but return to resting levels within two to four hours.
Factors Influencing Increased Blood Flow
Vasodilators Released During Digestion
- The digestive tract releases vasodilators like cholecystokinin and secretin during digestion which enhance blood flow.
Oxygen Concentration Effects
- A decrease in oxygen concentration in intestinal walls can lead to a 50% to 100% increase in intestinal blood flow due to vasodilatory mechanisms.
Mechanism of Countercurrent Blood Flow
Structure of Intestinal Wall Vessels
- Examination of intestinal wall cross-section reveals structures called villi composed of arteries and veins that connect without reaching their ends.
Oxygen Utilization
- Approximately 80% of oxygen from arterial blood bypasses metabolic functions in villi under normal conditions; however, reduced intestinal blood flow can lead to necrosis.
Neural Control Over Gastrointestinal Blood Flow
Role of Autonomic Nervous System
- Both sympathetic and parasympathetic nervous systems regulate gastrointestinal functions; parasympathetic stimulation increases glandular activity through acetylcholine release leading to vasodilation.
Sympathetic Influence on Blood Flow
Vasoconstriction and Blood Flow Regulation
Mechanisms of Blood Flow Restoration
- After several minutes of vasoconstriction, blood flow returns to normal due to autoregulatory escape mechanisms. This indicates that local metabolic vasodilator mechanisms generated by ischemia are stronger than sympathetic vasoconstriction.
- In extreme cases of low blood volume, such as hemorrhagic shock, there is a significant sympathetic discharge leading to arterial vasoconstriction. This response is critical for preserving vital organ function.
- The resulting vasoconstriction in intestinal and mesenteric vessels redirects approximately 200 to 400 ml of blood to maintain overall circulation, aiming to prevent cell death in essential organs like the brain and heart.
Reference Material