Anatomy Relevant to the Hematological System

by Adam Lawson BA, MSc and Terra Doucette Hiller BA, BSN, RN

Structures Related to Hematopoiesis

Hematopoiesis is the production and maturation of all cellular blood components.

Red Bone Marrow - actively generates erythrocytes (red blood cells), platelets, and most leukocytes (white blood cells).
Red marrow is clearly visible in the cross-sections. It appears as red spongy material within the center of bones. The red color is from the immature erythrocytes it contains.
- Large flat bones contain red marrow: the pelvis, sternum, skull, ribs, vertebrae, scapula, and the proximal ends of the femur and humerus.
- All marrow is red at birth; between the age of 8 and 18 years, the marrow of the limbs becomes yellow.
Use the slider bar in the dissection window to slide to different bones and identify the red marrow.
Yellow Bone Marrow - inactive fat cells which can indirectly facilitate leukopoiesis.
- It can quickly revert to red bone marrow in emergent situations such as rapid blood loss or anemia. The process can take place in only a few hours.
- The shafts of long bones contain yellow bone marrow: the humerus, femur, and tibia.
Multipotent stem cells, known as "mesenchymal stem cells" or "marrow stem cells," are found in bone marrow. They have been shown to give rise to a number of cells (e.g., fat cells, osteoblasts, chondrocytes, fibroblasts, and muscle cells) which, when extracted, can be used to regenerate tissue. Many studies have suggested these mesenchymal stem cells have as much potential as embryonic stem cells.
Kidneys - release erythropoietin, a hormone which acts as a precursor in bone marrow to initiate erythropoiesis.
Failed kidneys will not produce erythropoietin; bone marrow will cease its production of RBCs, leading to anemia in patients with chronic kidney failure. Erythropoietin can be injected into patients with renal failure to stimulate red blood cell production. Many athletes inject erythropoietin ("blood doping") to increase red blood cell production and thereby improve aerobic capacity and endurance.
Liver - produces and breaks down a large variety of blood components.
- Component produced include: albumin, cholesterol, glucose, and several essential coagulation factors.
- It will break down old erythrocytes with phagocytes in the liver.
After about 120 days, an aged cell will undergo changes in its plasma membrane which mark it for breakdown by phagocytes in the spleen, liver, and bone marrow.

Broken down erythrocytes release heme molecules which are further broken down into iron and biliverdin.
- Biliverdin is transported to the liver by albumin and converted to bilirubin before it is finally excreted in bile.
- Iron is transported by the transferrin protein for re-use in new erythrocytes.
- During the fetal stage of development, the liver is the main site of erythropoiesis. Bone marrow gradually begins to the perform the task as it develops and, by 32 weeks of gestation, takes over the task entirely.
Spleen - serves several hematological functions; it produces lymphocytes, stores blood platelets, and filters and breaks down erythrocytes.

Structures Related to Immunological Function and the Lymphatic System

Several organs produce, conduct, or store leukocytes throughout the body to protect against antigens and the development of tumors.

Spleen - it also serves an immunological function; it filters and breaks down many blood-borne antigens. It is the largest lymphoid organ in the body.
The spleen is highly vascularlized and susceptible to injury during trauma. If a splenic hemorrhage is life-threatening, the organ will often be surgically removed. Macrophages of the bone marrow and liver will take over splenic function, but the immune system will be slightly compromised.
Thymus - where immature lymphocytes develop into T lymphocytes.
- It lies posterior to the sternum over the aortic arch.
- It releases thymosin and thymopoietin, which cause T lymphocytes to gain immunocomptence.
- It is active during childhood, but begins to atrophy gradually in late adolescence. Functional tissue is replaced by fatty fibrous tissue, and the production of immunocompetent cells subsequently declines.
- It is the only lymphoid organ that does not directly fight antigens. A "blood-thymus barrier" lines the inside of thymic capillaries and protects the thymus from blood-borne antigens.
Tonsils - four groups of lymphoid tissue which line the pharyngeal cavity and protect it from antigens acquired through inspired air and swallowed food.
1. Palatine tonsils - lie directly posterior to the mouth and palate on the lateral sides of the pharyngeal wall. These tonsils are the most likely to be removed due to recurrent or chronic infection.
2. Lingual tonsils - lie on the posterior surface of the tongue.
3. Pharyngeal tonsils (a.k.a., adenoids) - lie on the pharyngeal roof.
4. Tubal tonsils - lie just posterior to the pharyngotympanic tubes as they enter the pharynx.
Appendix - samples many different antigens from within the digestive system and generates a wide variety of memory lymphocytes to protect the body. Nearly half of its wall is covered with dense lymphatic tissue.
Lymphatic System - a series of nodes and vessels which conduct lymph through the body and, ultimately, back into the bloodstream.

Lymph is a combination of elements from blood and interstitial fluid. It usually resembles blood plasma with a higher concentration of white blood cells (especially in lymph nodes).

Lymph which contains fatty acids and emulsified fats absorbed from the small intestine is called "chyle," and has a whiter appearance than lymph.
- Lumbar trunks - lie along the sides of the aorta in the inferior abdomen and receive lymph from the lower limbs, pelvic organs, and from the anterior abdominal wall.
- Intestinal trunk - receives lymph from the stomach and intestines.
- Bronchomediastinal - run alongside the trachea and receive lymph from the thoracic viscera, lungs and thoracic wall.
- Subclavian trunks - located at the base of the neck, they receive lymph from the upper limbs, inferior neck and superior thoracic wall.
- Jugular trunk - located at the base of each internal jugular vein, they drain lymph from the head and neck.
- Cisterna Chyli - the most inferior part of the thoracic duct lying around the area of L1 and L2. It is the area of the union of the lumbar and intestinal trunks of the lymphatic trunks.
- Thoracic duct - returns excess tissue, fat and protein to the blood stream. Lymph ascends upward through it, along the ventral surface of the vertebrae, and empties into the left subclavian vein near its junction with the jugular vein.

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