Table of Contents
- Circulatory System
- Lymphatic System
- What are Lymphatic Capillaries?
- Where Do All Lymphatic Vessels End?
- Lymphatic Capillaries Function
- How are Lymphatic Capillaries Different from Blood Capillaries?
- Lesson Summary
The circulatory system is the collection of organs and tissues that transport blood around the body. The main organs in the circulatory system are the heart and the blood vessels. The circulatory system is important because blood helps transport oxygen and nutrients to cells and remove metabolic waste like carbon dioxide. The circulatory system is named such because it is a closed loop. Blood travels in a closed loop from the the heart to the lungs, back to the heart, out to the body, and then back to the heart again.
Blood flow starts from the right side of the heart, where it then goes to the lungs to become oxygenated. Blood returns back to the left side of the heart and is pumped through the arteries out to the body. The arteries become more narrow as they turn into arterioles and eventually capillaries. Capillaries are the smallest blood vessels of the circulatory system and are responsible for gas and nutrient exchange. The walls of capillaries are only a single cell layer thick and thus, allow for this exchange. Capillaries condense into larger vessels called veins, which bring blood back to the heart to start the process again.
The circulatory system works with the lymphatic system to regulate blood pressure and tissue swelling and to screen for pathogens. The lymphatic system includes the lymph fluid, lymph vessels, and lymph nodes. During circulation, the pressure exerted from the heart causes blood plasma to leak from the capillaries and enter the tissues. This fluid is called interstitial fluid. Interstitial fluid must be collected and returned to the blood in order to prevent swelling in the tissues, called edema. This is the job of the lymphatic system. The lymphatic system also sends this collected interstitial fluid, called lymph fluid, through lymph nodes, which contain high levels of white blood cells that screen the fluid for pathogens.
The lymphatic capillaries definition is the tiniest vessels of the lymphatic system that initially collect interstitial fluid. The lymphatic capillaries are similar to the blood capillaries; however, there are important differences. The lymphatic capillaries have a closed end which faces the capillary beds of the circulatory system. Lymphatic capillaries are also made of a single layer of endothelial cells connected with a basement membrane. At junctions between the endothelial cells, there are one-way flaps that act as mini-valves, allowing interstitial fluid from the tissues to diffuse into the lymphatic capillaries. Elastin fibers connect the outer portion of the mini-valves to fibroblasts in the connective tissue. This helps to anchor the lymphatic capillaries in the capillary bed.
Lymphatic capillaries bring interstitial fluid in the tissues from the capillary beds back to the heart. Since the valves in the capillary beds only allow for one-way transfer of fluid, fluid continues to enter the lymphatic capillaries until the pressure is greater inside the vessel than in the tissue. When the pressure is greater inside the lymphatic capillaries, the endothelial cells adhere more closely together and prevent back-flow of the fluid. As the pressure rises in the capillaries, this drives the interstitial fluid further up into the capillaries, which eventually merges into lymphatic vessels. Once the interstitial fluid enters the lymphatic vessels, it is called lymph.
Lymphatic vessels are larger than the capillaries and have valves to prevent back-flow and layers of smooth muscle to help move the fluid through the vessel. Movement of the skeletal muscles and compression of the chest from breathing also helps keep lymphatic fluid moving through the lymphatic vessels. Eventually, the lymphatic vessels return the lymph fluid to the heart where it can be recycled into the blood through two main vessels, the thoracic duct and the the right lymphatic duct.
As lymph flows through the lymphatic vessels back to the heart, it also travels through lymph nodes. Lymph nodes are small, bean-shaped organs that exist throughout the lymphatic system. They have a high number of immune cells and filter the lymph, looking for pathogens, such as bacteria, or other harmful agents, like cancer cells.
Lymphatic capillaries are found in every capillary bed in the body. All areas of the circulatory system experience plasma leak and thus, require lymphatic capillaries to prevent edema and return blood to the heart.
The main function of the lymphatic capillaries is to collect lost interstitial fluid and return it to the rest of the lymphatic system and ultimately to the heart. There are several results of this function. First, the lymphatic capillaries help protect the body against edema. Edema is a swelling that can occur in tissues when interstitial fluid leaks into the tissue faster than it can be removed by the lymphatic capillaries. Edema typically is a symptom of a larger, different medical issue. However, untreated edema itself can lead to stiffness, decreased mobility, problems with circulation, and more.
The lymphatic capillaries also help regulate interstitial pressure in the tissues. They do this by removing excess fluid that would otherwise increase interstitial pressure, which can damage other tissues and organs.
Lymphatic capillaries are different from blood capillaries in several ways. First of all, lymphatic capillaries have a dead end and are not connected to the circulatory system. Lymphatic capillaries send lymph fluid one way, from the tissues to the heart. The blood capillaries connect arteries and veins and thus, are part of a complete, closed circulatory system. Blood capillaries carry blood, complete with red blood cells. Lymphatic capillaries carry lymph fluid, which does not include red blood cells and does not transport nutrients or oxygen to cells. Blood capillaries are permeable both ways and allow for materials to diffuse from the blood into the cells and from the cells into the blood. However, lymphatic capillaries only allow for diffusion of interstitial fluid from the tissue into the lymphatic capillary. The differences between lymphatic capillaries and blood capillaries are summarized in the table below:
|Characteristic||Lymphatic Capillaries||Blood Capillaries|
|Composed of||Endothelial cells||Endothelial cells|
|Function||Transport interstitial fluid back to the heart||Transport oxygen, nutrients, and metabolic waste around the body|
|Directionality||One way, from the tissues back to the heart||Transport blood between the arteries and veins in a closed loop|
|Diffusion||One way, from the tissues into the lymphatic capillaries||Both ways, from the capillaries to the cells and from the cells to the capillaries|
The circulatory system is the collection of tissues and organs that transport blood around the body. The circulatory system consists of the heart, the arteries, veins, and capillaries, which are the smallest blood vessels and the site of gas exchange between the blood and cells. The circulatory system works closely with the lymphatic system, which consists of the lymph nodes, lymph, and lymphatic vessels, including the lymphatic capillaries. Lymphatic capillaries are the dead-end vessels that absorb interstitial fluid that has leaked from the blood capillaries during circulation. Their role is to regulate interstitial pressure and prevent swelling, called edema.
Lymphatic capillaries transport interstitial fluid to the larger lymphatic vessels, where it becomes lymph fluid. The lymph fluid flows through lymph nodes, which scan the lymph for pathogens, such as bacteria, and cancer cells or other harmful agents prior to returning it to the heart. Lymph is moved through the lymphatic vessels by movement of the skeletal muscles and the chest cavity during breathing.
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The difference between lymphatic capillaries and blood capillaries is that lymphatic capillaries are designed to absorb interstitial fluid, and blood capillaries are designed for the exchange of gases, nutrients, and metabolic waste. Lymphatic capillaries have a closed end and thus allow for fluid movement in one direction. Blood capillaries are open at both ends and connect arteries to veins.
Lymphatic capillaries are tiny vessels that are needed to take up excess interstitial fluid. They pool into larger lymphatic vessels that deliver the fluid back to the heart.
The function of lymphatic capillaries in capillary beds is to collect fluid that has leaked from blood vessels and return it to the heart. This fluid is called interstitial fluid.
Lymphatic capillaries absorb interstitial fluid. Interstitial fluid is plasma that has leaked from blood vessels due to pressure from the heart.
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