Lymphatic system
Updates to Article Attributes
The lymphatic system (also known as the lymphoid system, lymph vascular system or
A portion of the blood entering any capillary bed does not return via the veins, but instead forms interstitial fluid in the extracellular space, which is returned to the circulation via the lymphatic vessels (a.k.a. lymphatics). When the fluid is in the lymphatic system it is known as lymph. It is known as chyle when the lymph draining the gut also contains chylomicrons from a meal.
Terminology
Occasionally the lymphatic system is considered with the reticuloendothelial system, with the combined whole termed the lymphoreticular system.
Lymphatic vessels
A portion of the blood entering any capillary bed does not return via the veins, but instead forms interstitial fluid in the extracellular space, which is returned to the circulation via the lymphatic vessels. When the fluid is in the lymphatic system it is known as lymph. It is known as chyle when the lymph draining the gut also contains chylomicrons from a meal.
The lymphatic vessels (a.k.a. lymphatics) form a lymph vascular system to return the interstitial fluid, as lymph, to the systemic circulation
The lymphatic capillaries (initial lymphatics) are the smallest vessels in the system and arise in the tissue capillary beds as blind-ending single layer endothelial tubes. Compared to capillaries, the lymphatic capillaries are relatively dilated with greater variability in calibre. Dynamic collapse of the capillaries is prevented by anchoring filaments binding their walls to the surrounding structural tissues 5.Ultrastructurally, the lymphatic endothelium of the capillaries lacks fenestrations with a rather sparse/absent basal lamina. Tight junctions are absent between individual cells of the endothelium 3,4. These cytological features allow larger macromolecules to pass with ease from the extracellular space to the lymphatics, which includes proteins, cellular debris and microorganisms.
As the lymphatics enlarge, their walls become more like veins, although unlike the venous wall, the various tunica layers are harder to distinguish. Like veins, these collecting lymphatics also have valves, but in a much larger number. Smooth muscle in their walls creates peristaltic waves ensuring unidirectional flow of the lymph 3. This flow is also supported by skeletal muscle action, inspiratory decrease in intrathoracic pressure and a pressure gradient towards the veins into which they drain 3.
Finally, the lymph returns to the main systemic circulation via the thoracic duct on the left, and on the other side of the body, the right lymphatic duct.
For the most part, subcutaneous lymphatic vessels accompany veins, whilst deeper lymphatics accompany arteries 2-4. The initial lymphatics in the villi of the gut wall are known as lacteals.
Lymphatic vessels are present throughout the body with the following exceptions:
Until recently this list would also have included the CNS, but recent work shows that both the brain and meninges have their own lymphatics.
Overall lymph flow is estimated to be 2-4 litres per day 3.
Functions
The primary function of the lymphatic system is to return exuded capillary fluid and protein to the systemic circulation. Approximately 25-50% of the plasma proteins re-enter the central veins every day from the lymph 3. In the GI tract, the draining lymphatics have a key role in transporting cholesterol and long chain fatty acids (as chylomicrons) to the liver.
Radiographic features
In a non-disease state, only the larger lymphatic vessels are visible on cross-sectional imaging, generally, the thoracic duct, cisterna chyli and right lymphatic duct.
Related pathology
See also
-<![endif]--><!--StartFragment-->The <strong>lymphatic system</strong> (also known as the <strong>lymphoid system</strong>, <strong>lymph vascular system</strong> or <strong>systema lymphoideum</strong> in <a title="TA (anatomy standard)" href="/articles/terminologia-anatomica-1">TA</a>) is the collective term given to the lymphatic vessels and <a title="Lymphoid organ" href="/articles/lymphoid-organ-2">lymphoid tissues</a> in the body <sup>1,4</sup>.</p><p>A portion of the blood entering any capillary bed does not return via the veins, but instead forms interstitial fluid in the extracellular space, which is returned to the circulation via the lymphatic vessels (a.k.a. lymphatics). When the fluid is in the lymphatic system it is known as <a title="Lymph fluid" href="/articles/lymph-fluid">lymph</a>. It is known as <a title="Chyle" href="/articles/chyle">chyle</a> when the lymph draining the gut also contains chylomicrons from a meal.</p><h4>Terminology</h4><p>Occasionally the lymphatic system is considered with the reticuloendothelial system, with the combined whole termed the <strong>lymphoreticular system</strong>.</p><h4>Lymphatic vessels</h4><p>The <strong>lymphatic capillaries</strong> (<strong>initial lymphatics</strong>) are the smallest vessels in the system and arise in the tissue capillary beds as blind-ending single layer endothelial tubes. Compared to capillaries, the lymphatic capillaries are relatively dilated with greater variability in calibre. Dynamic collapse of the capillaries is prevented by anchoring filaments binding their walls to the surrounding structural tissues <sup>5</sup>.<br><br>Ultrastructurally, the lymphatic endothelium of the capillaries lacks fenestrations with a rather sparse/absent basal lamina. Tight junctions are absent between individual cells of the endothelium <sup>3,4</sup>. These cytological features allow larger macromolecules to pass with ease from the extracellular space to the lymphatics, which includes proteins, cellular debris and microorganisms.</p><p>As the lymphatics enlarge, their walls become more like veins, although unlike the venous wall, the various tunica layers are harder to distinguish. Like veins, these <strong>collecting lymphatics</strong> also have valves, but in a much larger number. Smooth muscle in their walls creates peristaltic waves ensuring unidirectional flow of the lymph <sup>3</sup>. This flow is also supported by skeletal muscle action, inspiratory decrease in intrathoracic pressure and a pressure gradient towards the veins into which they drain <sup>3</sup>.</p><p>Finally, the lymph returns to the main systemic circulation via the thoracic duct on the left, and on the other side of the body, the right lymphatic duct.</p><p>For the most part, subcutaneous lymphatic vessels accompany veins, whilst deeper lymphatics accompany arteries <sup>2-4</sup>. The initial lymphatics in the villi of the gut wall are known as <strong>lacteals</strong>.</p><p>Lymphatic vessels are present throughout the body with the following exceptions:</p><ul>- +<![endif]--><!--StartFragment-->The <strong>lymphatic system</strong> (also known as the <strong>lymphoid system</strong> or <strong>systema lymphoideum</strong> in <a href="/articles/terminologia-anatomica-1">TA</a>) is the collective term given to the lymphatic vessels and <a href="/articles/lymphoid-organ-2">lymphoid tissues</a> in the body <sup>1,4</sup>.</p><h4>Terminology</h4><p>Occasionally the lymphatic system is considered with the reticuloendothelial system, with the combined whole termed the <strong>lymphoreticular system</strong>.</p><h4>Lymphatic vessels</h4><p>A portion of the blood entering any capillary bed does not return via the veins, but instead forms interstitial fluid in the extracellular space, which is returned to the circulation via the lymphatic vessels. When the fluid is in the lymphatic system it is known as <a href="/articles/lymph-fluid">lymph</a>. It is known as <a href="/articles/chyle">chyle</a> when the lymph draining the gut also contains chylomicrons from a meal.</p><p>The lymphatic vessels (a.k.a. lymphatics) form a <strong>lymph vascular system </strong>to return the interstitial fluid, as lymph, to the systemic circulation</p><p>The <strong>lymphatic capillaries</strong> (<strong>initial lymphatics</strong>) are the smallest vessels in the system and arise in the tissue capillary beds as blind-ending single layer endothelial tubes. Compared to capillaries, the lymphatic capillaries are relatively dilated with greater variability in calibre. Dynamic collapse of the capillaries is prevented by anchoring filaments binding their walls to the surrounding structural tissues <sup>5</sup>.<br><br>Ultrastructurally, the lymphatic endothelium of the capillaries lacks fenestrations with a rather sparse/absent basal lamina. Tight junctions are absent between individual cells of the endothelium <sup>3,4</sup>. These cytological features allow larger macromolecules to pass with ease from the extracellular space to the lymphatics, which includes proteins, cellular debris and microorganisms.</p><p>As the lymphatics enlarge, their walls become more like veins, although unlike the venous wall, the various tunica layers are harder to distinguish. Like veins, these <strong>collecting lymphatics</strong> also have valves, but in a much larger number. Smooth muscle in their walls creates peristaltic waves ensuring unidirectional flow of the lymph <sup>3</sup>. This flow is also supported by skeletal muscle action, inspiratory decrease in intrathoracic pressure and a pressure gradient towards the veins into which they drain <sup>3</sup>.</p><p>Finally, the lymph returns to the main systemic circulation via the thoracic duct on the left, and on the other side of the body, the right lymphatic duct.</p><p>For the most part, subcutaneous lymphatic vessels accompany veins, whilst deeper lymphatics accompany arteries <sup>2-4</sup>. The initial lymphatics in the villi of the gut wall are known as <strong>lacteals</strong>.</p><p>Lymphatic vessels are present throughout the body with the following exceptions:</p><ul>
-<li><a title="Placenta" href="/articles/placenta">placenta</a></li>-<li><a title="Cornea" href="/articles/cornea">cornea</a></li>-<li><a title="Teeth" href="/articles/teeth">teeth</a></li>-</ul><p>Until recently this list would also have included the CNS, but recent work shows that both the brain and meninges have their own lymphatics.</p><p>Overall lymph flow is estimated to be 2-4 litres per day <sup>3</sup>.</p><h4>Functions</h4><p>The primary function of the lymphatic system is to return exuded capillary fluid and protein to the systemic circulation. Approximately 25-50% of the plasma proteins re-enter the central veins every day from the lymph <sup>3</sup>. In the GI tract, the draining lymphatics have a key role in transporting cholesterol and long chain fatty acids (as chylomicrons) to the liver.</p><h4>Radiographic features</h4><p>In a non-disease state, only the larger lymphatic vessels are visible on cross-sectional imaging, generally, the thoracic duct, cisterna chyli and right lymphatic duct.</p><h4>Related pathology</h4><ul><li><a title="Lymphoedema" href="/articles/lymphoedema-1">lymphoedema</a></li></ul><p><!--EndFragment--></p>- +<li><a href="/articles/placenta">placenta</a></li>
- +<li><a href="/articles/cornea">cornea</a></li>
- +<li><a href="/articles/teeth">teeth</a></li>
- +</ul><p>Until recently this list would also have included the CNS, but recent work shows that both the brain and meninges have their own lymphatics.</p><p>Overall lymph flow is estimated to be 2-4 litres per day <sup>3</sup>.</p><h4>Functions</h4><p>The primary function of the lymphatic system is to return exuded capillary fluid and protein to the systemic circulation. Approximately 25-50% of the plasma proteins re-enter the central veins every day from the lymph <sup>3</sup>. In the GI tract, the draining lymphatics have a key role in transporting cholesterol and long chain fatty acids (as chylomicrons) to the liver.</p><h4>Radiographic features</h4><p>In a non-disease state, only the larger lymphatic vessels are visible on cross-sectional imaging, generally, the thoracic duct, cisterna chyli and right lymphatic duct.</p><h4>Related pathology</h4><ul><li><a href="/articles/lymphoedema-1">lymphoedema</a></li></ul><h4>See also</h4><ul>
- +<li><a href="/articles/glymphatic-pathway">glymphatic system</a></li>
- +<li><a href="/articles/lymph-nodes-general">lymph nodes</a></li>
- +</ul><p> </p><p><!--EndFragment--></p>
References changed:
- 1. William Alexander Newman Dorland. Dorland's Illustrated Medical Dictionary. (2007) ISBN: 9781416023647 - <a href="http://books.google.com/books?vid=ISBN9781416023647">Google Books</a>
- 2. Chummy S. Sinnatamby. Last's Anatomy. (2011) ISBN: 9780702033940 - <a href="http://books.google.com/books?vid=ISBN9780702033940">Google Books</a>
- 3. Kim E. Barrett, Susan M. Barman, Scott Boitano et al. Ganong's Review of Medical Physiology 25th Edition. (2015) ISBN: 9780071848978 - <a href="http://books.google.com/books?vid=ISBN9780071848978">Google Books</a>
- 4. John W. Heath (Ph. D.). Wheater's Functional Histology. (1993) <span class="ref_v4"></span>
- 5. Susan Standring. Gray's Anatomy E-Book. (2015) ISBN: 9780702068515 - <a href="http://books.google.com/books?vid=ISBN9780702068515">Google Books</a>
Sections changed:
- Anatomy
Systems changed:
- Haematology
- Vascular
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