11.3.6 Kidney Failure & Urinalysis

• Kidney failure can occur in one or both kidneys for a variety of reasons, such as
  • Physical damage from an injury
  • High blood pressure
  • Diabetes
  • Overuse of certain drugs (e.g. aspirin)
  • Infection
• Kidney failure is dangerous and can be fatal within a relatively short time period
  • Humans can survive with one functioning kidney
• If the kidneys fail urea is not excreted; this leads to a build-up of urea in the blood which can become toxic at high concentrations
• Kidney failure can also lead to disruption in the balance of water and solutes in the blood
  • This can lead to problems relating to osmosis and cell damage as well as more specific problems relating to excess quantities of certain mineral ions
• There are two forms of treatment for kidney failure
  • Dialysis
    • Toxins, metabolic waste products and excess substances are removed from the blood by diffusion through a dialysis membrane
  • Kidney transplant
    • The non-functioning kidneys are replaced with a functioning kidney from a donor
    • Note that the non-functioning kidneys are usually left in place while the new kidney is attached to the blood supply elsewhere in the abdomen

Haemodialysis

• Dialysis is a process used to separate small and large molecules with a partially permeable membrane
• Haemodialysis, also spelled hemodialysis, is a form of dialysis treatment that needs to be carried out several times a week and that requires a dialysis machine
  • Another form of dialysis is known as peritoneal dialysis and involves use of the patient’s own internal membranes rather than a machine
• Blood flows via a tube from the patient to the dialysis machine
• Inside the dialysis machine partially permeable dialysis membranes separate the patient's blood from dialysis fluid
• Small molecules such as urea and salts can fit through pores in the dialysis membrane so exchange of substances can take place
  • The dialysis fluid contains no urea, so there is always a urea diffusion gradient causing urea to diffuse out of the blood and into the fluid
  • The dialysis fluid contains a salt concentration similar to the ideal blood concentration, so diffusion of salts across the membrane only occurs when there is an imbalance
    • If the blood is too high in salts they will diffuse out of the blood and if the blood is too low in salts they will diffuse in
  • The fluid contains a glucose concentration equal to normal blood sugar levels, preventing the outward diffusion of glucose across the membrane when blood glucose levels are normal
    • If blood sugar levels are elevated the glucose will diffuse out of the blood into the fluid
• The blood and fluid flow in opposite directions to ensure a concentration gradient along the whole length of the membrane
• The fluid in the machine is also continually refreshed so that concentration gradients are maintained between the dialysis fluids and the blood
  • This means that each time blood circulates through the machine some more of the urea it contains passes into the dialysis fluid, until almost all of it is removed
  • Each haemodialysis session takes 3-4 hours to complete
• Patients are given a drug that prevents the formation of blood clots during dialysis
  • Such drugs are known as anticoagulants

Kidney transplant

• An alternative to potentially restricting dialysis treatments is to have a kidney transplant
• This involves taking a single, healthy kidney from a donor and transplanting it into a patient with kidney failure
• Kidney transplants are considered to be a better long term solution to kidney failure than dialysis
  • The patient has more freedom as they no longer need to have dialysis several times a week
  • Patients often feel ill after dialysis and again as toxins start to accumulate a few days later; a transplant enables a patient to be healthy for an extended period
  • Diet can be much less restricted than it needs to be when a patient is on dialysis
• There are still some risks associated with kidney transplants
  • Donors won’t have the same antigens on their cell surface membranes as the patient so there will be some immune response to the new kidney
  • Immunosuppressant drugs need to be taken for the rest of a patient’s life to reduce the risk of organ rejection; these can leave the patient vulnerable to infections
  • A kidney will often be rejected over time, so a new kidney transplant is often needed after several years
  • There are not enough donors to cope with the demand, and waiting lists are long

• Analysing the composition of urine by carrying out urinalysis can tell us a great deal about the health of an individual; urine contains
  • The products of metabolism
  • Molecules that are present in the blood in high concentrations and that fit through the membranes in the glomerulus and Bowman's capsule
  • Drugs that may have been taken into the body
• Urinalysis shows up any deviations from normal urine composition, aiding with medical diagnoses or detecting drug use
• Tests that can be carried out during urinalysis include
  • pH testing
    • The pH of urine may influence the development of kidney stones
    • Test strips containing indicator chemicals may change colour when dipped in a urine sample
  • Testing for glucose concentration
    • High glucose levels in the urine can be a sign of diabetes
    • Test strips similar to those used in pH testing can change colour to indicate the glucose concentration of a urine sample
  • Testing for the presence of proteins
    • In normal circumstances proteins are too large to filter through from the glomerulus, so if they are present in the urine this can be a sign of high blood pressure, kidney damage, or diabetes
  • Drugs testing
    • Drugs can be tested for using monoclonal antibodies which bind to specific drugs due to their complementary structure, showing up as a line on a test strip
      • This kind of test works in a similar way to the lateral flow tests or rapid antigen tests that you may have come across when testing for COVID19
    • This is important when testing for e.g. doping in sport
  • Testing for the presence of white blood cells
    • This can be a sign of infection in the urinary tract
  • Pregnancy testing
    • The hormones that circulate in the blood during pregnancy can be detected in the urine using complementary monoclonal antibodies
• An example of a type of urinalysis that involves a test strip and the use of monoclonal antibodies is the pregnancy test
  • Pregnancy testing sticks contain antibodies that are specific to human chorionic gonadotropin (hCG), a hormone produced during pregnancy