How do the molecules get into blood vessels?

There are several ways in which molecules can enter blood vessels:

1. Simple diffusion: This is the most common way for small, lipid-soluble molecules to enter blood vessels. Lipid-soluble molecules can easily pass through the phospholipid bilayer of cell membranes, including the membranes of blood vessel endothelial cells. Simple diffusion occurs along a concentration gradient, from an area of higher concentration to an area of lower concentration.

2. Facilitated diffusion: This process involves the use of membrane transport proteins to move molecules across cell membranes. Facilitated diffusion is also concentration-dependent, but it is more efficient than simple diffusion and allows for the transport of larger, hydrophilic molecules that cannot easily diffuse through the lipid bilayer.

3. Active transport: This process requires energy in the form of ATP to move molecules across cell membranes against a concentration gradient. Active transport is used to move molecules from an area of lower concentration to an area of higher concentration.

4. Endocytosis: This process involves the engulfment of extracellular material by a cell membrane, forming a vesicle that is then transported into the cell. Endocytosis can be divided into three main types: phagocytosis (ingestion of solid particles), pinocytosis (ingestion of fluid), and receptor-mediated endocytosis (ingestion of specific molecules that bind to receptors on the cell surface).

5. Exocytosis: This process is the reverse of endocytosis and involves the release of material from a cell by the fusion of a vesicle with the cell membrane. Exocytosis is used to secrete hormones, enzymes, and other molecules from cells.

The type of process by which a molecule enters a blood vessel depends on the size, polarity, and charge of the molecule, as well as the concentration gradient and the presence of specific transport proteins or receptors on the cell surface.