How does glucose enter tissue fluid?

(Original post by coconut64)
the question is Describe and explain how substances that are dissolved in the blood plasma, such as oxygen or glucose, enter the tissue fluid from the capillaries.

The mark scheme mentions hydrostatic pressure, diffusion and fluid being forced out but I don't get how diffusion fits into the answer.

My answer is : at the arterial end, hydrostatic pressure is higher than plasma's solute potential, so solutes and plasma are forced out the capillary pores. This forms tissue fluid containing glucose.

I don't understand where the diffusion part comes from ... When they get forced out, do they diffuse out? is it what it is suggesting? But mark scheme says Emphasis here is on glucose/ oxygen being carried out as aresult of mass flow of fluid (not diffusion)


Thanks.

(Original post by Jpw1097)
In the arterial end of the capillary, there is a high blood pressure/hydrostatic pressure. The hydrostatic pressure exerted is greater than the osmotic pressure and so water in the blood plasma along with amino acids, glucose and other soluble molecules will be "squeezed" out of the capillary into the surrounding tissue fluid (or interstitial fluid) through gaps between endothelial cells. Large proteins such as albumin and large structures such as red blood cells are too large to be forced out of the capillary. Once these molecules have been forced out of the capillary, the blood remaining in the capillary now has a lower water potential than the surrounding tissue fluid and a much lower hydrostatic pressure (since a lot of water has been squeezed out and distance from heart increases). This means that the osmotic pressure is greater than the hydrostatic pressure and so majority of the water will be reabsorbed through gaps between endothelial cells in the venous side of the capillary. The remaining water and amino acids and other nutrients forms tissue fluid, which essentially bathes the tissues and provides them with nutrients while also removing waste products such as carbon dioxide. This tissue fluid is then emptied into lymphatic vessels where it is known as lymph and returns to the circulation via skeletal muscle contraction which squeezes these lymph vessels (which has valves) and returns the lymph to the subclavian vein where it is then emptied into circulation.

I understand what you're saying, it seems as though these substances are forced out (known as filtration) rather than diffusion. However, remember that water and other solutes will diffuse across the membrane, or alternatively move by osmosis, where there is a concentration of water potential gradient. Substances like oxygen, which is lipid soluble, will passively diffuse across the endothelium and into the surrounding tissues.

(Original post by coconut64)
but if they diffuse out why does it say they don't in the mark scheme. The last part emphasises specifically that they are forced out because of the pressure not diffusion. Thank you.

(Original post by Jpw1097)
Ah right, I thought you said that the mark scheme said that it was diffusion but you didn't understand why. That makes a lot more sense. Glucose, amino acids, oxygen and other materials do not diffuse across the endothelium in the capillary, they are forced out of the capillary through small gaps between endothelial cells due to the hydrostatic pressure in the arterial side of the capillary. Yes, the glucose which is in the tissue fluid will then enter the cells via facilitated diffusion and active transport and the oxygen in the tissue fluid will diffuse across the plasma membrane into the cell, as it is lipid soluble. However, this is not how these substances exit the capillary and enter the interstitial space to form tissue fluid - it is filtration instead.

(Original post by coconut64)
Okay, so solute and plasma are forced out. then oxygen diffuses into the cell down the concentration gradient? Thanks.