The lymphatic nodules are arranged as sleeves around the blood vessels, bringing blood into the spleen. Within the white pulp are splenic nodules called Malpighian corpuscles, which are rich in B-cells, so this portion of lymphoid tissue is quick to respond to foreign antigenic stimulation by producing antibodies. The walls of the meshwork of sinuses in the red pulp also contain phagocytes that engulf foreign particles and cell debris, effectively filtering and removing them from circulation.
The haemoglobin is then split apart into haem and globin. The globin is broken down into its constituent amino acids, which can be utilised in the synthesis of a new protein. Haem consists of an iron atom surrounded four non-iron pyrrole rings. The iron is removed and transported to be stored as ferritin, then reused to make new haemoglobin in the red bone marrow; macrophages convert the pyrrole rings into the green pigment biliverdin and then into the yellow pigment bilirubin.
Both are transported to the liver bound to plasma albumin. Bilirubin, the more toxic pigment, is conjugated in the liver to form a less toxic compound, which is excreted in bile. In some animals — particularly athletic mammals such as horses, greyhounds and foxes — the spleen is also an important reservoir of blood, which is released into circulation during times of stress to improve aerobic performance.
In humans, however, the spleen contributes only a small percentage of blood cells into active circulation under physiological stress; the total stored blood volume is believed to be only ml Bakovic et al, The capsule of the spleen may contract following haemorrhage, releasing this reserve into circulation in the body.
As the spleen is the largest collection of lymphoid tissue in the body, infections that cause white blood cell proliferation and antigenic stimulation may cause germinal centres in the organ to expand, resulting in its enlargement splenomegaly. This happens in many diseases — for example, malaria, cirrhosis and leukaemia. The spleen is not usually palpable, but an enlarged spleen is palpable during deep inspiration.
Enlargement may also be caused by any obstruction in blood flow, for example in the hepatic portal vein. The anatomical position of the spleen coincides with the left tenth rib. Given its proximity to the abdominal wall, it is one of the most commonly injured organs in blunt abdominal trauma. The spleen is a fragile organ and, due to its highly vascularised nature, any injury causing rupture will rapidly lead to severe intraperitoneal haemorrhage; death may result due to massive blood loss and shock.
A moderate splenic injury may be managed conservatively, but an extensively burst or ruptured spleen may be treated by complete and prompt removal splenectomy. However, current data supports successful non-operative management of many traumatic splenic injuries, with the intention of reducing the need for complete removal Armstrong et al, Patients being treated for certain malignant diseases may also require a partial or total splenectomy and, although other structures such as the bone marrow and liver can take over some of the functions that are usually carried out by the spleen, such patients may be at increased risk of infection.
With an overwhelming post-splenectomy infection, there is also an increased risk of sepsis, which is associated with significant morbidity and mortality. Infection is usually with encapsulated pathogens, including Streptococcus pneumoniae, Haemophilus influenzae and Neisseria meningitidis.
Clinical guidelines to help reduce the risk of infection advocate education about infection prevention, vaccination and antibiotic prophylaxis Arnott et al, Lymph nodes vary in size and shape, but are typically bean-shaped structures found clustered at specific locations throughout the body. Although their size varies, each node has a characteristic internal structure Fig 4. The central portions of the lymph node are essential to its function; here, there are large numbers of fixed macrophages, which phagocytose foreign material such as bacteria on contact, and populations of B- and T-cells.
Lymph nodes are crucial to most antibody-mediated immune responses: when the phagocytic macrophages trap pathogenic material, that material is presented to the lymphocytes so antibodies can be generated. Each lymph node is supplied by one or more afferent lymphatic vessels, which deliver crude, unmodified lymph directly from neighbouring tissues.
A healthy, fully functioning node removes the majority of pathogens from the lymph before the fluid leaves via one or more efferent lymphatic vessels. In addition to its lymphatic supply, each lymph node is supplied with blood via a small artery; the artery delivers a variety of leucocytes, which populate the inner regions of the node.
When infection is present, the lymph nodes become increasingly metabolically active and their oxygen requirements increase. A small vein carries deoxygenated blood away from each node and returns it to the major veins. In times of infection, this venous blood may carry a variety of chemical messengers cytokines that are produced by the resident leucocytes in the nodes.
These cytokines act as general warning signals, alerting the body to the potential threat and activating a variety of specific immune reactions. The structure of a lymph node is not unlike that of the spleen. Each lymph node is divided into several regions:. During infection, antibody-producing B-cells begin to proliferate in the germinal centres, causing the affected lymph nodes to enlarge and become palpable and tender.
Some of the cytokines released are pyrogenic meaning they cause fever and act directly on the thermoregulatory centre in the hypothalamus to increase body temperature.
Swollen lymph nodes and a fever are both sure signs that the body is mounting an effective immune response against the offending pathogen; this will be discussed in more detail in part 3 of this series. Other types of lymphatic tissue also exist. Mucosa-associated lymphoid tissue MALT is positioned to protect the respiratory and gastrointestinal tracts from invasion by microbes.
The following are made up of MALT:. The tonsils are aggregates of lymphatic tissue strategically located to prevent foreign material and pathogens from entering the body. The palatine tonsils are in the pharynx, the lingual tonsils in the oral cavity and the pharyngeal tonsils adenoids are at the back of the nasal cavity; as a result of this, the tonsils themselves are at high risk of infection and inflammation tonsillitis.
This will also be discussed further in part 3. Sign in or Register a new account to join the discussion. You are here: Immunology. The lymphatic system 2: structure and function of the lymphoid organs.
Abstract This article is the second in a six-part series about the lymphatic system. This article has been double-blind peer reviewed Scroll down to read the article or download a print-friendly PDF here if the PDF fails to fully download please try again using a different browser Click here to see other articles in this series Assess your knowledge and gain CPD evidence by taking the Nursing Times Self-assessment test.
Key points The lymphoid organs include the red bone marrow, thymus, spleen and clusters of lymph nodes Blood and immune cells are produced inside the red bone marrow, during a process called haematopoiesis The thymus secretes hormones that are essential for normal immune function and develops T-lymphocytes The spleen mounts the immune response and removes micro-organisms and damaged red blood cells from circulation Lymph nodes are clustered throughout the body and filter pathogens from lymph, swelling when mounting an immune response.
Also in this series The lymphatic system 1: structure, function and oedema The lymphatic system 3: its role in the immune system The lymphatic system 4: allergies, anaphylaxis and anaphylactic shock The lymphatic system 5: vaccinations and immunological memory The lymphatic system 6: the history and function of immunotherapies.
References Armstrong RA et al Successful non-operative management of haemodynamically unstable traumatic splenic injuries: 4-year case series in a UK major trauma centre.
European Journal of Trauma and Emergency Surgery ; 5, Clinical Infectious Diseases ; 4, Clinical and Experimental Pharmacology and Physiology ; 11, Garnitschnig L et al Postprandial dynamics of splenic volume in healthy volunteers.
Physiological Reports ; 8: 2, e Gujar S et al A cadaveric study of human spleen and its clinical significance. National Journal of Clinical Anatomy ; 6: 1, Polish Journal of Radiology ; 4, Pivkin IV et al Biomechanics of red blood cells in human spleen and consequences for physiology and disease.
Severa M et al Thymosins in multiple sclerosis and its experimental models: moving from basic to clinical application. Multiple Sclerosis and Related Disorders ; The posterior lymph sacs join the cisterna chyli and lose their connections with adjacent veins.
With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and are converted into groups of lymph nodes. The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach. The thymus arises as an outgrowth of the third pharyngeal pouch.
Lymph node structure : A lymph node showing afferent and efferent lymphatic vessels, capsule, sinus, efferent lymphatic vessel, backflow valve, nodule, cortex, and hilum.
Privacy Policy. Skip to main content. Search for:. Development of Lymphatic Tissues. Lymphatic Tissue Development Lymphatic tissue development begins by the end of the fifth week of embryonic development. Learning Objectives Describe lymphatic tissue development. Key Takeaways Key Points The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and converted into groups of lymph nodes.
Key Terms mesoderm : One of the three tissue layers in the embryo of a metazoan animal. Through embryonic development, it produces many internal organs of the adult, e.
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