Convection, the movement of water from the blood to the dialysate through the membrane, is accompanied by solvent drag, the movement of
molecules. The proportion of a solute that is transported by convection depends on the size of the molecule as well as the size and quantity of
membrane pores. Large molecules move more slowly while small molecules move freely and swiftly. The amount of a specific solute that flows
through the membrane from the blood into the dialysate is referred to as the ""sieving coefficient of a membrane."" A sieving value of 1.0 means
that 100% of a given solute passed through the membrane by a convective mechanism, whereas a sieving coefficient of 0.4 means that only 40%
of a given solute did so.
Water moves from one body fluid compartment to another according to osmotic forces. During hemodialysis, water is pushed through the
membrane by ultrafiltration to reduce the blood's solute concentration. Because the concentration of cellular osmotically active solutes is now
higher than that in the blood, water is then returned to the cells and tissues from the blood via osmotic pressures. Hypotension and a drop in blood
volume could arise from this. It is possible to add sodium to the dialysate, which will raise the blood's osmolality and cause the cells and tissues to
lose water. Later on in the dialysis procedure, the salt content of the dialysate is reduced to enable the removal of more water from the blood. Later
on in the dialysis procedure, the salt content of the dialysate is reduced to enable the removal of more water from the blood. The dialyzer
experiences ultrafiltration, whereas the bodily compartments do not. Ion exchange across cellular membranes may be driven by active transport,
although water flow is passive and controlled by osmotic pressures.
The blood passes through thousands of fiber tubes inside a hollow fiber dialyzer. Dialysate is around them, and the membrane is what separates
them. Through a phenomenon known as countercurrent, which involves the blood and dialysate flowing in the opposing directions, molecular
exchange is facilitated. This is due to the minimal change in concentration gradients from one end of the fiber to the other. As a result of the
hollow fibers' extreme fragility and rigidity, the membrane's compliance (deformability or volume change) is minimal. Because ultrafiltration rates
are predictable, exact fluid removal may be carried out. There is not much volume variation between low and high pressure since the hollow filters
have low blood flow resistance.
Each dialyzer membrane has a molecular weight cutoff (in daltons) that determines the size of the molecules that can pass through it. These may
range from 3000 to 15,000 daltons. Small molecules (e.g., sodium, potassium, phosphate, urea, water) pass through the filter easily while large
molecules, such as proteins (e.g., albumin with a molecular weight of 15,000 daltons), such as urea, are difficult to pass
The membrane's relationship to the components of the blood is referred to as biocompatibility. Blood touching a membrane may cause particular
cellular or protein components to become active, resulting in immunologic reactions including allergic reactions or anaphylaxis.
The ultrafiltration rate of the fluid transfer can be controlled by the dialysis machine by adjusting the hydraulic pressures in the blood and dialysate
compartments. The manufacturer assigns an ultrafiltration coefficient (Kuf) to each dialyzer. This is the amount of fluid (measured in milliliters) that
can move through the membrane in one hour at a specific pressure difference. As a result, 250 mL (5 x 50) of fluid can be transferred in 1 hour of
dialysis using a dialyzer with a Kuf of 5 and a transmembrane pressure of 50 mm Hg. Dialysis removal is an effective method of managing fluid
volume and weight because renal failure patients are frequently edematous and have too much water in the interstitial compartment. Diuretic
medications are used because the patient's kidneys are not working. Diuretic medications are not very helpful because the patient's kidneys are not
functioning.
Clearance (K) is the amount of blood that can be cleared of a specific solute in a given amount of time. Based on blood and dialysis flow rates,
membrane properties, the solute's molecular weight, size, and charge, and other factors, the manufacturer determines the clearance for a given
solute. Diffusion from the high-concentration to the low-concentration side of the membrane removes the majority of low-molecular weight solutes
during dialysis, with the rate also being temperature-dependent. Large molecules typically traverse the membrane via solvent drag (convection). The
amount of solute that passes through the membrane while the remainder is rejected or adsorbed is indicated by the sieving coefficient (SC). A SC
of 0.4. As a result, a SC of 0.4 indicates that 40% of a specific solute will flow through the membrane. Although cellulose membranes have a
tendency to absorb more than synthetic hydrophobic ones, the majority of tiny proteins are eliminated from the circulation via adsorption to the
membrane. Adsorbed membranes are more biocompatible, however they may reduce convection and diffusion.
