Absorption is the process whereby nutritive material, lymph, is transferred from tissues; serous cavities — pericardium, peritoneum, etc.; and mucous membranes into the blood. The lymph is absorbed from the mucous membrane of the alimentary canal, as it is the principal source of nutritive material used by the body for the maintenance of the quantity and quality of the blood; while the lymph absorbed from the serous cavities and tissues represents a reabsorption of the nutritive materials which have escaped through the capillary walls, and are returned to the veins through the lymphatic vessels. Were this lymph allowed to collect in the tissues, there would occur an excessive accumulation, and this condition would be readily accounted for in the swelling of the subcutaneous tissue and organs giving rise to a pathologic condition termed edema.
Under the chapter on digestion it was shown how the food we eat is reduced to a liquid condition by the action of the various gastric, pancreatic, and intestinal juices and their ferments. This nutritive material is taken up by the mucous membrane of the intestine and absorbed, then reaches the blood-current by way of the lymph channels — lacteals, and finally reaches the thoracic duct; or by way of the venous capillaries of the mesenteric veins, and is carried to the liver, and thence to the right side of the heart.
Before understanding the methods of food absorption, a description of the mucous membrane of the intestines is necessary.
THE FUNCTION OF THE VILLI 287 Structure of the Villi. — The mucous membrane of the small intestine is covered by tiny conical processes which extend from the end of the pylorus of the stomach to the end of the ileum. These, when examined under the microscope, show a conical process covered with columnar epithelial cells, each cell containing a nucleus, and here and there goblet cells which secrete mucus. The epithelial cells rest upon a basement membrane. In the body of the villus that portion within the mucous membrane contains a network of connective tissue supporting arterioles, capillaries, venules, and lymphatic vessels. In the centre of the villus is a lymph capillary, usually single, with epithelial cells in its wall.
The Function of the Villi. — The action of the cells of the villi during digestion is to absorb the nutritive products of digestion. These products are taken from the intestinal canal and transferred into the lymph spaces within the body of the villi, from which they are finally taken up by the blood capillaries and lymphatics. There are two routes by which the nutritive material passes into the general bloodstream. The capillaries which enter the villi and are in intimate relation with the lymph space give up their nutritive materials (blood plasma) by a transudation through their walls, which forms the lymph; meantime the nutritive material absorbed from the intestines by the cells of the villi undergoes metabolic changes and enters the arterial capillaries from the lymph spaces. This fenourished blood in the arterial capillaries passes into the venules and then into the larger veins of the intestines, to be conveyed to the liver, which uses it to maintain its functions. The lymph contained in the lymph spaces within the villi is called chyle (a milk-white fluid), which is absorbed by the lymphatic vessels called lacteals, and these empty into the large mesenteric lymphatics to drain into the thoracic duct, and the latter communicates with the left subclavian vein, and thus returns the excess of lymph back to the blood-stream.
The wonderful aspect of absorption is the apparently unsolvable problem in regard to how these numbers of cells in the villi of the intestinal mucous membrane can absorb the different constituents of the nutritive materials from the alimentary canal and transfer them into the lymph spaces to be absorbed by the blood capillaries and lymphatics. It is supposed to be due to a 'selective action' based on their organization and living condition, an action which is to a great extent conditioned and limited by the degree of diffusibility of the substances to be absorbed (Brubaker's Physiology, p. 225).
QUESTIONS 289
- Describe absorption.
- By what two sources does lymph reach the blood current?
- What is the function of the cells in the villi of the mucous membrane of the small intestines?
- Where are the lymph spaces found?
- What do you understand by the term lacteals?
- What is chyle?
- How does the nutritive material absorbed by the cells of the villi reach the arterial capillaries? Where is this material conveyed?
- Where is the lymph absorbed by the lacteals from the lymph spaces conveyed?
- Name the vessels found in a villi.
Key Takeaways
- Absorption is crucial for transferring nutrients into the bloodstream.
- Villi play a vital role in absorbing nutritive materials from the alimentary canal.
- Lymph and chyle are key components in this process.
Practical Tips
- Understand that proper digestion and absorption of food are critical for maintaining health and energy levels, even in survival situations.
- Recognize the importance of a healthy diet to support the body's ability to absorb nutrients effectively.
- Be aware of conditions like edema which can arise from poor lymphatic function.
Warnings & Risks
- Overeating or consuming foods that are difficult to digest can lead to bloating and discomfort, affecting absorption efficiency.
- Dehydration can impair the lymphatic system's ability to transport nutrients effectively.
- Inadequate nutrition can weaken the body’s immune response and overall health.
Modern Application
While the chapter provides a detailed understanding of how the human body absorbs nutrients from food, modern survival techniques have advanced in terms of nutritional science and emergency preparedness. Understanding these historical principles still holds value for ensuring optimal health during prolonged periods without access to conventional medical care or nutrition.
Frequently Asked Questions
Q: What is chyle and where does it come from?
Chyle is a milk-white fluid that contains absorbed nutritive materials. It comes from the lymph spaces within the villi of the small intestine, which are absorbed by the lacteals and then conveyed to the thoracic duct.
Q: How do cells in the villi absorb nutrients?
Cells in the villi absorb nutritive products directly from the intestinal canal. These products undergo metabolic changes within the lymph spaces before entering the arterial capillaries, which then carry them to the liver and right side of the heart.
Q: What are lacteals and what is their role?
Lacteals are lymphatic vessels that absorb chyle (milk-white fluid) from the lymph spaces within the villi. They then carry this fluid to the thoracic duct, which drains it into the left subclavian vein.
Q: What is edema and how does it relate to absorption?
Edema is a condition characterized by swelling of tissues due to excessive accumulation of lymph. Poor absorption can lead to this condition as lymph accumulates in the tissues instead of being properly absorbed into the bloodstream.
Q: How do the villi contribute to efficient nutrient absorption?
Villi increase the surface area for absorption by extending from the pylorus of the stomach to the end of the ileum. Their structure, including columnar epithelial cells and goblet cells that secrete mucus, facilitates the selective absorption of nutrients.