Kidney Dialysis

Function of the Normal Kidney

• Humans have 2 kidneys which are bean-shaped organs that are 12 cm long and have a mass of 150 g.
• The kidneys perform two major functions:
  • excrete end products of metabolism
  • control the concentration of the constituents of the bodily fluids
• Each kidney contains approximately a million nephrons.
• A nephron is composed of 2 parts (as shown in Figure 1):
  • glomerulus - filters the circulating plasma from the capillaries within Bowman's capsule. This filtration is a passive event because the blood pressure in the capillary loops is 60 mmHg in addition to 25% of the cardiac output is sent to the kidneys.
  • tubule - is the area where important nutrients are extracted from the filtrate and are reabsorbed into the blood depending upon bodily need.

Figure 1: The diagram of the nephron which is composed of the glomerulus and the tubule (Brown Fig22.12).

• The output filtrate is about 1 mL/s/kidney (about 180 L/day).
• The total plasma volume is about 3 L and therefore the plasma is filtered through the kidneys about 60 times/day.
• The total length of the tubules in each kidney is approximately 50 km.
• The total urine output is about 1-2 L/day due to the active process of maintaining the correct fluid and electrolyte balance in the body by the filtration, reabsorption, and secretion of electrolytes, glucose, and water.
• The health of an individual is dependent upon the electrolyte levels within the body. For instance:
  • If sodium levels are elevated in the blood serum, the blood pressure will increase.
  • If there is an increase in the serum potassium levels above 6 mmol/L, the individual will have cardiac arrest without warning because potassium has a large effect on the excitability of cells.
  • If there is an increase in the calcium level, internal bleeding will result from peptic ulcers. Increases in calcium levels cause an increase in the amount of acid released into the stomach.
  • If there is a decrease in calcium levels, this can lead to bone diseases.
  • Regulation of bicarbonate concentration is very important because it directly effects many metabolic processes which are sensitive to changes in blood pH levels.
• Diffusion of substances across the dialysis membrane decreases with increasing molecular weight.
  • Electrolytes have low molecular weights such as sodium is 23, and potassium is 40.
  • Organic chemicals in the blood have higher molecular weights such as urea is 60 and bilirubin is 600.
  • Proteins have very high molecular weights such as albumin 60,000 and fibrinogen 400,000.
• Uraemia is the failure of the kidneys which results in retention of urea in the blood.
• Kidney dialysis performs three important functions:
  1. Maintains the correct electrolyte balance within the body.
  2. Maintains the blood pH levels.
  3. Maintains fluid balance.

History of Dialysis

• 90 years ago the first haemodialysis was performed in an animal, however it was not until the creation of cellophane (cellulose acetate) membranes in the 1920-1930's and the production of heparin (an anticoagulant) in the mid 1930's was haemodialysis safe enough to perform on humans. The first human haemodialysis was performed by Kolff in 1943.
• The artificial kidney has been used for 50 years to treat patients with severe renal failure.
• In haemodialysis,
  1. Heparin is administered to the individual and blood is removed from the individual's body. The blood does not clot in the extracorporeal circuit due to the presence of heparin.
  2. Blood then passes through the artificial kidney which separates the dialysis fluid from the blood by a semi-permeable membrane.
  3. Electrolytes and small molecules pass freely through the pores in the membrane while large molecules and blood cells are unable to pass through the pores.
• There are two types of dialysers are used in routine clinical settings:
  • Parallel plate dialyser in which blood flows between parallel sheets of membrane
  • Hollow fibre dialyser in which blood flows through hollow fibres which are bathed in dialysate.

Diffusion

• Artificial kidneys have semi-permeable membranes which are typically about 10-20 micrometers thick, have a surface area of about 1 m², and have pores which are about 500 nm in diameter.
• Molecules with a molecular weight less than 5,000 pass through the the membrane fairly easily, molecules of intermediate weights (5,000 to 40,000) will pass slowly.
• Dialysis can only remove a small quantity of amino acids and drugs. Even though amino acids have molecular weights of 75-204, both amino acids and drugs bind to plasma protein and as a result they do not pass through the membrane.

Dialysis machines

The block diagram of a single-pass dialysis system is shown in Figure 2. In the single-pass system, the dialysis fluid is used used at a rate of 500 ml/min.

Figure 2: The block diagram of a kidney dialysis machine (Brown 22.15).

• In this system a large amount of fluid is needed and therefore the dialysis fluid is continuously created from a concentrate in a mixing tank with a proportionating pump.
• The concentration of the fluid is checked by measuring the conductivity of the fluid in addition to the temperature and pressure.
• This fluid then passes across one side of the dialyser membrane towards the blood leak detector which checks to make sure that the patient is not losing blood through the membrane.
• The dialysis fluid is then removed as waste.
• The blood which is removed from the patient is pumped across the opposite side of the membrane in countercurrent flow. This blood contains heparin to prevent clotting.
• Since the same vein is used to remove and return the blood, there is no pressure difference to drive the blood through the extracorporeal circuit and a roller pump must be used.
• The blood passes through a bubble trap. This removes air trapped within the blood in addition to measuring the venous pressure.
• Administration of the anticoagulant heparin must be stopped 30 minutes prior to the end of the dialysis period.

Summary

The kidneys serve two important functions: excrete wastes and regulate body fluid levels. Glomeruli are semipermeable areas in the capillary tubes in which water, nutrients, salts, and wastes are removed from the blood. The capillaries in the glomeruli have a high permeability to small molecules due to the high blood pressures experienced in them. Glomerular capillaries receive blood from the aorta and renal arteries. Important nutrients are extracted from the tubules and reabsorbed into the blood depending upon bodily needs. Reabsorption of glucose, amino acids, and sodium require active transport to pump these against their concentration gradients. The pumping of sodium is followed by the passive movement of chlorine, bicarbonate, and the osmosis of water. Wastes pass from the glomeruli to the tubules and are either reabsorbed poorly or not at all. In the absence of high pressures to cause filtration, tubular secretion takes place in the distal tubules. Secretion is used to adjust the levels of potassium ion, ammonia, and hydrogen ion in the blood.

Material is from Brown, B H et al. Medical Physics and Biomedical Engineering. Institute of Physics Publishing, Philadelphia 1999, pp703-709.

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