Human Anatomy and Physiology - N. I. Fedyukovich 2003

Cardiovascular system
Hematopoietic organs

Bone Marrow is the primary organ of Blood Cell production. Within it, stem Cells form and proliferate, giving rise to all types of blood and immune system cells. Consequently, bone marrow is also referred to as an immune organ. Stem cells have a high capacity for multiple divisions, forming a self-sustaining system.

As a result of numerous complex transformations and differentiation along three pathways (erythropoiesis, granulopoiesis, and thrombocytopoiesis), stem cells become formed elements. Stem cells also give rise to immune system cells—lymphocytes—which further differentiate into plasma cells (plasmacytes).

A distinction is made between Cytology/practical/86.html">Red bone marrow, located in the spongy substance of flat and short bones, and yellow bone marrow, which fills the cavities of the diaphyses of long tubular bones.

The total mass of bone marrow in an adult is about 2,5—3,0 kg, or 4,5—4,7 % of body weight.

Red bone marrow consists of myeloid tissue, which also includes reticular and hematopoietic tissue, whereas yellow bone marrow consists of adipose tissue that has replaced the reticular tissue. In the event of significant blood loss, yellow bone marrow is replaced by red bone marrow once again.

The Spleen (lien, splen) Functions as a peripheral organ of The Immune System. It is located in the Abdominal cavity, in the left hypochondrium, at the level of the IX to XI Ribs. The mass of the spleen is about 150—195 g, with a length of 10—14 cm, width of 6—10 cm, and thickness of 3—4 cm. The spleen is covered on all sides by the Peritoneum, which is tightly fused with the fibrous capsule and fixed by the gastrosplenic and phrenicosplenic ligaments. It has a reddish-brown color and a soft consistency. Connective Tissue septa, or trabeculae, extend from the fibrous capsule into the organ, with the parenchyma situated between them. The parenchyma is composed of white and red pulp. The white pulp consists of splenic lymphoid nodules and lymphoid tissue surrounding intraorgan Arteries. The red pulp is formed by loops of reticular tissue filled with erythrocytes, lymphocytes, macrophages, and other cellular elements, as well as venous sinuses.

The splenic hilum, housing Vessels and nerves, is located on its concave surface.

The destruction of erythrocytes and the differentiation of T- and B-lymphocytes take place in the spleen.

The Thymus (thymus), or Thymus gland, is a central organ of lymphocytopoiesis and immunogenesis. In the thymus, stem cells arriving from the bone marrow undergo a series of transformations to become T-lymphocytes, which are responsible for cell-mediated immune responses. These T-lymphocytes then enter the bloodstream and Lymph, leave the thymus, and migrate to the thymus-dependent Zones of the peripheral Organs of Immunogenesis. Within the thymus, epithelial stromal cells produce thymosin (a hematopoietic factor), which stimulates the proliferation of lymphoblasts. In addition, other BIOLOGICALLY ACTIVE SUBSTANCES are produced in the thymus (including factors with properties of Insulin and Calcitonin, as well as growth factors).

The thymus is an unpaired organ consisting of left and right lobes connected by loose connective tissue. The gland narrows superiorly and widens inferiorly. In many cases, the left lobe may be longer than the right.

The thymus is located in the anterior part of the superior Mediastinum, anterior to the upper part of the Pericardium, the arch of the aorta, and the left brachiocephalic and SUPERIOR VENA CAVA. Laterally, it is flanked by the right and left mediastinal pleurae. The anterior surface of the thymus connects to the Sternum. The organ is covered by a thin connective tissue capsule, from which septa extend inward, dividing the glandular tissue into small lobules. The parenchyma of the organ consists of a peripheral cortex and a central medulla. The thymic stroma is represented by reticular tissue. Thymic lymphocytes (thymocytes) and dendritic epithelial cells (epithelioreticulocytes) are situated between the fibers and Cells of the reticular tissue. In addition to its immunological and hematopoietic functions, the thymus also possesses endocrine activity.