Venous bed and two types of microcirculation
One of the most important morphological features of the spleen is the structure of its venous bed. Blood outflow begins from venous sinuses, which transition directly into trabecular veins (venae trabeculares). The histological specificity of these veins lies in the complete absence of a muscular layer. At the same time, the vascular endothelium is tightly fused with the rigid connective tissue stroma of the trabeculae themselves. Due to this, the vascular lumen does not collapse, which ensures an unhindered and constant outflow of blood from the organ. Subsequently, the trabecular veins merge to form the large splenic vein (v. lienalis), which leaves the spleen via its hilum.
In addition to the specific venous outflow, the uniqueness of the organ's blood circulation lies in the parallel functioning of two microcirculation variants:
- Closed type: Blood from terminal capillaries flows directly into the lumen of the venous sinuses, forming a classic continuous vascular bed.
- Open type: Terminal capillaries open not into vessels, but directly into the reticular tissue of the red pulp. Only after passing through this stroma does the blood collect into the venous sinuses.
The physiological role of open circulation is extremely important: the release of blood into the reticular tissue leads to a sharp deceleration of blood flow velocity. This creates the necessary conditions for close contact between erythrocytes and macrophages, which recognize and phagocytose old or damaged blood cells.
Lymphatic system and neural apparatus
The structure of the splenic lymphatic bed differs in that the organ completely lacks afferent lymphatic vessels. The system is represented exclusively by efferent pathways (vasa lymphatica efferentia). These vessels begin to form in the white pulp zones, then travel along the connective tissue trabeculae and exit through the hilum, heading toward regional lymph nodes.
The neural apparatus of the organ is represented by the autonomic nervous system, including sympathetic and parasympathetic fibers. Nerve trunks penetrate the spleen along with the splenic artery, after which they form dense perivascular plexuses. They provide innervation to both blood vessels and cellular elements of the white and red pulp.
Dynamics of age-related involution
Throughout a person's life, the ratio of splenic tissues undergoes significant changes, known as age-related involution:
- Newborns: The proportion of lymphoid tissue (white pulp) accounts for 5 to 20% of the total organ mass.
- 10–20 years: This is the period of maximum development and the largest volume of white pulp.
- 20–30 years and older: The process of gradual age-related involution begins. The volume of lymphoid tissue steadily decreases, while the proportion of red pulp and the connective tissue framework (trabeculae) increases to compensate.
- Elderly and senile age: Involution reaches its peak. Pronounced atrophy of lymphoid tissue occurs, accompanied by its replacement with connective tissue (sclerosis) and a general decrease in organ mass.
Regenerative potential and splenectomy
The spleen possesses varying capacities for recovery depending on the nature of the damage:
- Physiological regeneration proceeds at a stably high level due to the constant natural renewal of the pulp's cell composition.
- Reparative regeneration is significantly limited. In extensive trauma or resection of part of the organ, its initial anatomical shape is not restored. However, the remaining tissue is capable of undergoing compensatory hypertrophy to replenish the lost function.
In the event of a splenectomy (complete surgical removal of the organ), systemic adaptation mechanisms are triggered. The loss of the spleen leads to vicarious (compensatory) hypertrophy of other organs of the immune system and hematopoiesis. The main compensatory load is assumed by the lymph nodes and red bone marrow.
To remember the types of circulation: 'Closed type — like a metro (blood flows strictly along the vascular tunnel), Open type — like a square (release into the red pulp toward macrophages).'
Class and exam questions
Why don't the trabecular veins of the spleen collapse?
Their wall completely lacks a muscular layer, and the endothelium is tightly fused with the rigid connective tissue stroma of the trabeculae, which keeps the lumen always open.
What is the physiological significance of the open type of circulation?
With the open type, blood is released into the reticular tissue of the red pulp. This sharply slows down blood flow, giving macrophages time to recognize and phagocytose old erythrocytes.
Are there afferent lymphatic vessels in the spleen?
No, the lymphatic system of the spleen is represented only by efferent vessels, which form in the white pulp.
Which organs assume the function of the spleen after its complete removal?
Following splenectomy, vicarious (compensatory) hypertrophy of the lymph nodes and red bone marrow occurs.
What else to study on this topic
- Cellular composition and structure of the red pulp reticular tissue
- Histological structure and zones of the white pulp
- Mechanisms of damaged erythrocyte recognition by macrophages
- Detailed mechanisms of red bone marrow vicarious hypertrophy
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