Spleen: red pulp circulation

The path by which blood enters splenic sinuses varies according to species.  Blood may flow directly into the sinuses from the capillaries (closed circulation) or the capillaries may open into the red pulp where the blood would percolate through the pulp before entering the sinuses (open circulation).  Open circulation is thought to predominate in the human.

The path by which blood enters splenic sinuses varies according to species.  Blood may flow directly into the sinuses from the capillaries (closed circulation) or the capillaries may open into the red pulp where the blood would percolate through the pulp before entering the sinuses (open circulation).  Open circulation is thought to predominate in the human.

The path by which blood enters splenic sinuses varies according to species.  Blood may flow directly into the sinuses from the capillaries (closed circulation) or the capillaries may open into the red pulp where the blood would percolate through the pulp before entering the sinuses (open circulation).  Open circulation is thought to predominate in the human.

The path by which blood enters splenic sinuses varies according to species.  Blood may flow directly into the sinuses from the capillaries (closed circulation) or the capillaries may open into the red pulp where the blood would percolate through the pulp before entering the sinuses (open circulation).  Open circulation is thought to predominate in the human.

The path by which blood enters splenic sinuses varies according to species.  Blood may flow directly into the sinuses from the capillaries (closed circulation) or the capillaries may open into the red pulp where the blood would percolate through the pulp before entering the sinuses (open circulation).  Open circulation is thought to predominate in the human.

The path by which blood enters splenic sinuses varies according to species.  Blood may flow directly into the sinuses from the capillaries (closed circulation) or the capillaries may open into the red pulp where the blood would percolate through the pulp before entering the sinuses (open circulation).  Open circulation is thought to predominate in the human.

The path by which blood enters splenic sinuses varies according to species.  Blood may flow directly into the sinuses from the capillaries (closed circulation) or the capillaries may open into the red pulp where the blood would percolate through the pulp before entering the sinuses (open circulation).  Open circulation is thought to predominate in the human.

Two types of capillaries supply the red pulp, sheathed and unsheathed; both are branches of penicillar arterioles.  In the closed pathway, unsheathed capillaries make direct connection with the splenic sinuses and, thus, blood is confined in a vessel throughout its path.  Macrophages migrate in and out of the sinus to provide immune surveillance and phagocytosis of red blood cells.

In the open pathway, sheathed capillaries are surrounded by a sleeve of macrophages (not diagrammed here).  Sheathed capillaries open directly into the reticular connective tissue of the splenic cord, allowing blood to be exposed to cells of the splenic cord. The open pathway is thought to predominate in humans.  Blood eventually percolates back into the splenic sinuses.