As pointed out in mytutor.co.uk in the basic biology section
When we breathe we inhale oxygen from the air into our lungs. The alveoli are tiny air sacs in the lung where oxygen diffuses into the blood via small blood vessels, known as capillaries. The blood in these capillaries has a low concentration of oxygen which allows oxygen in the alveoli to diffuse down the concentration gradient and into the blood. When we eat food, it is broken down by enzymes, such as amylase, in the digestive system. Glucose is formed by the breakdown of carbohydrates in the small intestine. Glucose is then absorbed into the blood from the small intestine via the villi by active transport. Glucose and oxygen travel in the bloodstream and are taken up into cells. Respiration takes place in the mitochondria, producing energy.
Glucose transporters are a wide group of membrane proteins that facilitate the transport of glucose across the plasma membrane, a process known as facilitated diffusion (Wikipedia).
Role of Diffusion: Glucose and other small molecules (but not macromolecules) diffuse in and out of capillaries through the liquid filled spaces between the cells, not by diffusing across the cell membrane. Most proteins are too big to enter or leave capillaries by diffusion (Colombia University).
The blood glucose levels are expressed as millimoles per litre (mmol/l) and stay stable amongst people without diabetes at around 4-8mmol/L.
When the body processes sugar, glucose in the bloodstream naturally attaches to haemoglobin, a protein within red blood cells that carries oxygen throughout your body.
The amount of glucose that combines with this protein is directly proportional to the total amount of sugar that is in your system at that time.
Because red blood cells in the human body survive for 8-12 weeks before renewal, measuring glycated haemoglobin (or HbA1c) can be used to reflect average blood glucose levels over that duration, providing a useful longer-term gauge of blood glucose control. (Diabetes UK)
Normal (non-diabetic) HbA1c levels will give a measurement of below 42 mmol/mol (below 6%).