How does a glucose molecule enter the cell from blood vessel?
I think I understand your question, Natasha. In short, your own answer #2 is correct.
There are 3 spaces, and 2 pathways for glucose to pass from one to the next:

intracapillary plasma
extracellular fluid
the cytosol.

Ways glucose gets into the cell:

(2-&gt;3) To get from the ECF to the cytosol , glucose always needs a transport protein. These are the GLUTs. In two cases, the small intestine and kidney, these are part of a secondary active transport system based on the Na/K-ATPase. In the pancreas, it's GLUT2.

(1-&gt;2) To get from the capillary plasma to the ECF requires filtration, the process of applying hydrostatic pressure to the plasma and literally squeezing it like a sponge. The boundary of the &quot;blood sponge&quot; is the basement membrane. The membrane holds in the proteins, and lets anything dissolved in the watery serum (like glucose) through.


The Filtration Constant Kf is proportional to the percentage of the BM that is exposed in a given capillary, which varies by the type and other factors like histamine release.

Type 1 (continuous) has the lowest exposure of the BM (only the intercellular clefts). Type 2 (fenestrated)has the clefts and the fenestrations to expose the BM. Type 3 (sinusoids) have huge gaps between the cells, and importantly, an incompetent BM that allows proteins and cells through along with the watery serum.
Dynamic factors that change filtration rates:

Histamine causes the post-capillary venule's endothelial cells to contract, and exposes more of the BM allowing more serum filtration, but it also allows neutrophil extravasation, during which the neutrophils punch holes in the BM through which white cells can squeeze, which is why you get a proteinaceous exudate in Type I hypersensitivity.

Increase in blood flow Like a water balloon increasing its surface area with more blood flow, skeletal muscle capillary beds (Type I) increase their volume and surface area when engorged with blood during exercise. This doesn't change the Kf (the only exit for serum is still the intercellular clefts, but they're bigger now).


P.S. A 1985 morphometric study of pancreatic capillaries shows that the idea that endocrine glands need a boatload of bloodflow (high serum filtration) is valid. Even within a single capillary that feeds both an exocrine acinus and and endocrine islet, the side facing the endocrine islet had more fenestrae.