Lesson Explainer: Kidney Failure Biology

In this explainer, we will learn how to describe and evaluate the treatments for kidney failure.

Our kidneys are fist-sized, bean-shaped organs found near the bottom of the rib cage on either side of the spine. Properly functioning kidneys constantly maintain the perfect balance of ions and water in the bloodstream. They also remove soluble waste products, like urea. The kidneys carry out this function by constantly filtering our blood. Our kidneys are so important that they are among the few internal organs that come in a pair! You can see a pair of kidneys and their approximate position in the human body in Figure 1. This is fortunate for us since a healthy human can survive with just one properly functioning kidney.

Key Term: Kidney

The kidney is a bean-shaped, fist-sized organ located near the spine. The kidneys reabsorb water and salt and filter waste from the blood, which is excreted as urine.

Many factors can interfere with the healthy functioning of our kidneys. Chronic metabolic diseases like diabetes and high blood pressure are some of the most common causes of kidney failure. The kidneys can also be impacted by infection. Damage done to the kidneys over time causes kidney disease, which can eventually cause kidney failure.

Kidney (or renal) failure occurs when the kidneys stop functioning properly and the blood does not get filtered like it should. When the kidneys have dropped to 15% or less of their typical function, a patient is said to be in ESRD, end stage renal disease, also called kidney failure.

Key Term: Kidney Failure (ESRD)

Kidney failure occurs when damage or disease has caused the kidneys to drop below 15% of their normal function. Kidney failure is also called end stage renal disease.

Kidney failure causes a disruption of homeostasis. Homeostasis is the maintenance of a constant, normal internal environment within the body. Kidney failure will cause water and ions to become imbalanced and wastes like urea to build up in the blood.

The symptoms of kidney failure can include nausea, abdominal pain, an abnormally large or small amount of urine, back pain, joint swelling, muscle cramps, and shortness of breath.

Example 1: Identifying a Consequence of Kidney Failure

If the kidneys do not function properly, some substances will not be removed from the body. Which waste substance would you expect to build up in the blood of someone with kidney failure?

  1. Urea
  2. Carbon dioxide
  3. Glucose
  4. Dead red blood cells
  5. Bile

Answer

The kidneys are organs responsible for the production of urine. They make urine by filtering the blood and removing wastes like excess water and ions and soluble waste products like urea. When the kidneys fail, they no longer filter the blood efficiently. This leads to a buildup of certain waste products and a disruption of homeostasis. The kidneys are the primary organ responsible for the removal of urea from the bloodstream.

So, we can conclude that urea would build up in the blood of someone with kidney failure.

Our kidneys are incredibly hardworking organs. They carry out the excretory function of producing urine. Our kidneys produce urine by filtering our blood to remove excess salts, water, and urea that is produced in the liver. Urine production is a complex process that occurs in the kidneys within microscopic structures called nephrons.

Key Term: Nephron

A nephron is a microscopic structure within the kidney made of a twisted tubule and many associated capillaries. The nephron is the functional unit and the site of urine production in the kidney.

The nephrons are complicated structures composed of tiny blood vessels and tubules. In the nephron, almost all the small molecules are filtered out of the bloodstream. This includes water, ions, glucose, and soluble wastes but generally not the cells or large proteins. Then, the necessary or useful materials are reabsorbed into the bloodstream. This includes almost all the glucose, just the right amount of ions, and almost all the water. The wastes, excess ions, and water that are left behind are excreted in the form of urine. The basic steps of urine production can be seen in Figure 2.

Our kidneys are amazingly efficient at their jobs. Each kidney has about a million tiny nephrons! Together, they filter between 160–200 L of blood every day and produce about 1.5 L of urine. At any given moment, our kidneys contain about a fifth of our total blood volume.

Our kidneys are able to automatically adjust to the constantly changing conditions in our bodies to maintain the ideal levels of water and ions at all times. When an individual’s kidneys fail, their function is extremely hard to replace.

There are two treatments for kidney failure.

One is a kidney transplant. This is when the kidney of a recently deceased person, or a living donor, is removed and implanted into a patient with kidney failure. The other treatment option is called kidney dialysis. In kidney dialysis, also called renal dialysis, hemodialysis, or just dialysis, an external mechanism is used to filter the blood in place of the kidneys.

Let’s take a closer look at each of these treatments, their benefits, and drawbacks.

Definition: Kidney Transplant

A kidney transplant is when one or two kidneys are removed from a living or deceased donor and implanted into a host with kidney failure.

Definition: Kidney Dialysis (Renal Dialysis/Hemodialysis)

Kidney dialysis is the use of a dialyzer to filter the blood outside the body, replacing the function of a healthy kidney.

The first step in successful kidney transplantation is the location of a compatible donor kidney. The new kidney must be compatible with the patient receiving the transplant, also called the host. The donor and the patient will probably need to have the same blood group and tissue type, and for this reason, close relatives are often considered good matches for kidney donations. A kidney that is not a close match will be rejected and attacked by the host’s immune system, which will damage the new kidney.

In order to avoid transplant rejection, kidney donors who are close family members are best. Finding an unrelated match is much more difficult.

In order to transplant a kidney, it is removed from the donor, then implanted into the abdomen of the host. The artery and the vein of the transplanted kidney are attached to a vein and artery lower in the body than the existing kidneys, and the ureter of the transplanted kidney is attached to the bladder. You can see an abdomen with a transplanted kidney in Figure 3. The existing kidneys are usually not removed, so the recipient of a kidney transplant may have three kidneys in their body!

A transplanted kidney will likely function properly for around 10 to 15 years. During this time, the recipient will lead a healthy, normal life with few dietary restrictions and no symptoms of kidney failure. Kidney transplantation is also considered to be generally more cost effective than long term kidney dialysis. For these reasons, kidney transplant is the preferred, life-prolonging treatment for kidney failure.

One of the drawbacks of kidney transplant is that the transplanted kidney can be rejected by the patient’s body. For this reason, the recipients of kidney transplants must take immunosuppressant drugs for the rest of their lives. Immunosuppressants suppress, or decrease, the overall activity of the immune system. These drugs may protect the new kidney, but they also leave the patient vulnerable to infections.

The other drawback to the transplant solution is that there simply are not enough spare kidneys to go around. There are many more patients needing kidney transplants than there are available donor kidneys. Without a willing, healthy, related donor, patients with kidney failure may spend their entire life waiting for a kidney to become available.

Example 2: Recalling One of the Long-Term Solutions for Kidney Failure

Kidney failure can be treated with a long-term solution involving a medical procedure. What medical procedure can be used to treat a small amount of kidney failure patients?

  1. Limb amputations
  2. Stents
  3. Kidney transplant
  4. Hormone replacement surgery
  5. Pancreas transplant

Answer

There are two treatments for kidney failure. One is dialysis and the other is kidney transplant. Kidney dialysis is when a patient is hooked up to a machine that filters the blood in the place of the failing kidneys. Dialysis is not limited in the number of patients who can receive it because it is performed by an apparatus that is easily accessible and in wide supply.

A kidney transplant is when a healthy kidney of a donor is removed and implanted in the body of a kidney failure patient, or host. The new, donor kidney in the host takes over the function of the failing, diseased kidneys. Kidney transplant is severely limited by the number of donors available. There are significantly more patients in need of kidney transplants than there are available donors.

This means that the medical procedure that can only be used to treat a small amount of kidney failure patients is kidney transplant.

Example 3: Identifying Trends in Kidney Transplant Data from a Graph

The graph provided shows the number of people requiring a kidney transplant compared to the number of transplants carried out.

Which of the following statements about the data correctly describes the trend?

  1. The number of transplants carried out is significantly higher than the demand for kidneys transplant.
  2. The demand for kidney transplants is consistently higher than the number of transplants carried out.
  3. The number of kidney transplants carried out was higher in 1990 than in 2010.
  4. The demand for kidney transplants has slowly decreased over time.

Answer

This graph presents two sets of data using the same scales and axes. Before trying to interpret the data in a graph, it is helpful to look at the key and the axes. The key says that the blue bars represent the number of transplants carried out. The red line marked with x’s represents the number of patients awaiting kidney transplants. The 𝑥-axis shows us the years, in 5-year increments, from 1990 to 2015. The 𝑦-axis shows us the number of patients. Now that we have figured out how the data is being presented, we can look at the trends. If we focus on the blue bars, we can see that the number of transplants carried out has not changed much over time. There were around 2‎ ‎000 kidney transplants performed in 1990, 1995, 2000, and 2005. By 2010 and 2015, that number has only increased slightly and is still less than 2‎ ‎500. The number of kidney transplants performed is limited by the number of donor kidneys available. Patients who need a transplant must be matched with a healthy kidney from an available donor in order to receive the treatment they need.

The red line shows us values much higher than the blue bars. We see a steady increase in this line, which represents patients on the kidney recipient waiting list, from 1990 to 2015. The number of patients on the waiting list climbs over this period from 6‎ ‎000 to around 8‎ ‎000 people. This means that the number of patients who need kidney transplants steadily increases, while the number of transplants carried out each year remains almost unchanged.

Using this information, we can conclude that the demand for kidney transplants is consistently higher than the number of transplants carried out.

While patients wait for a kidney donor to become available, the function of the kidney can be replaced by dialysis.

In dialysis, a patient is hooked up to a dialyzer, also sometimes called an “artificial kidney,” a basic outline of which you can see in Figure 4. A dialyzer is controlled by a computer called a “dialysis machine.” During dialysis, the patient is attached to the dialyzer through two needles. Blood flows out of an artery of their body through one needle and into the dialysis machine. Within the dialyzer, the blood passes through a network of tubing that filters waste from the blood and restores water and ion balance. The cleansed blood is then pumped back into the patient via a vein through the second needle.

Key Term: Dialyzer (Artificial Kidney)

A dialyzer is an apparatus that filters the patient’s blood outside the body, performing the function of a healthy kidney.

Within the dialyzer, the patient’s blood flows through semipermeable tubing, or "membrane," as shown in Figure 5. Because the dialysis tubing is semipermeable, small molecules can pass through freely. Outside of the tubing, there is a specially designed fluid called dialysis fluid. Dialysis fluid is also called dialysate, dialysis solution, or bath.

You might remember that because of the principle of diffusion, molecules naturally move from areas of high concentration to areas of low concentration.

The dialysis fluid is designed to create concentration gradients that filter wastes from the patient’s blood and restore water and mineral balance. A concentration gradient is a difference in concentration of a particular substance between one place and another. Substances will automatically diffuse down their concentration gradient. Diffusion will stop when the concentrations in all areas are equal; this is shown in the diagrams in Figure 6.

The dialysis fluid has most of the things found in healthy blood plasma. This includes the right balance of sterilized water and ions and no metabolic wastes such as urea or organic acids. Glucose is present in dialysis fluid, and this prevents the patient from losing too much glucose from their blood through diffusion during dialysis.

Key Term: Dialysis Fluid (Dialysate)

Dialysis fluid is a balanced fluid used in kidney dialysis. It is made of water, ions, and oftentimes glucose as well.

When the blood comes into contact with the dialysis fluid through the semipermeable tubing, waste products (like urea) diffuse out because their concentration is high in the blood and they should not be present at all in the dialysis fluid. If the concentration of ions is higher in the blood, they will diffuse into the dialysis fluid. If the concentration in the blood is lower, they will diffuse in. The same is true of the water.

As the blood passes through the dialysis tubing, the wastes are removed and the ion and water concentrations are balanced, much like they would be in a functioning kidney. The cleansed blood is then pumped back into the patient.

In order to maintain necessary concentration gradients to filter the blood efficiently, a constant, fresh supply of dialysis fluid must be pumped through the dialyzer. The dialysis fluid flows in the opposite direction to that the blood is moving in, which is called “countercurrent.”

This countercurrent setup maintains a concentration gradient along the entire length of the dialysis tubing, as demonstrated in Figure 7. If the blood and dialysate flowed in the same direction, the concentrations of waste would approach equilibrium very quickly. Therefore, movement of waste from the blood and into the dialysate would only occur in a small portion of the dialyzer.

Dialysate must also be continually removed from the dialysis machine, as indicated in Figure 7, to prevent waste products from building up in the dialysate and moving back into the blood.

Example 4: Analyzing the Function of a Dialyzer

The diagram provided shows a simplified section of a dialyzer.

  1. Complete the statement with “high” and “low” to correctly describe the difference in urea concentrations: The concentration of urea is in the blood and in the dialysis fluid.
  2. Which of the following best explains why the blood flows in the opposite direction to the dialysis fluid?
    1. The opposite direction of flow maintains a steep concentration gradient so useful products continually diffuse out of the blood.
    2. The opposite direction of flow maintains a steep concentration gradient so waste products continually diffuse into the blood.
    3. The opposite direction of flow maintains a steep concentration gradient so waste products continually diffuse out of the blood.
    4. The opposite direction of flow has no benefit and is just a coincidence.

Answer

Part 1

A dialyzer operates using the principle of diffusion. Diffusion occurs because molecules have a natural tendency to move from areas of high concentration to areas of low concentration. The dialyzer is used to filter, cleanse, and balance the blood of patients with kidney failure. Kidney failure causes the concentration of certain waste materials that build up in the blood. In order to remove those materials, the patient’s blood is pumped through a dialyzer.

Within the dialyzer, it passes through a tube made of a semipermeable membrane. Outside this membrane is a specially designed fluid called dialysis fluid. The concentration of materials in the dialysis fluid is set up to create a concentration gradient, or a difference in concentration, between the blood and the dialysis fluid.

Specifically, the concentration of waste like urea is high in the blood of a kidney failure patient. There is no urea present in fresh dialysis fluid. So, a concentration gradient exists where the concentration of urea in the blood is much higher than the concentration of blood in the dialysis fluid. Since diffusion will cause molecules to move from areas of high to areas of low concentration, down their concentration, urea will diffuse out of the blood, through the semipermeable membrane, and into the dialysis fluid in the dialyzer.

Therefore, our statement should read: “The concentration of urea is high in the blood and low in the dialysis fluid.”

Part 2

As the blood passes through the dialyzer, the concentration of urea in the blood must steadily decrease in order to remove the waste from the blood as efficiently as possible. When the difference in concentration for a substance is large between one area and another, we say that substance has a steep concentration gradient. Molecules diffuse more quickly down a steeper concentration gradient than a less steep one.

Therefore, in order to keep the urea diffusing out of the blood, it is important to maintain a concentration gradient along the entire path of the dialyzer. If the dialysis fluid flowed in the same direction as the blood, the concentration gradient for urea at the beginning of the dialyzer would be extremely steep. The blood coming from the patient would have a lot of urea, and the fresh dialysis fluid would have none. So, lots of urea would diffuse out in the beginning.

However, as the fluids move through the dialyzer, the concentration of urea in the dialysis fluid would increase, while the concentration of urea in the blood would decrease. This would bring the two concentrations closer to each other, decreasing the concentration gradient and the amount of urea that continues to diffuse out of the bloodstream. To avoid this, the dialysis fluid flows in the opposite direction of the blood, which maintains a steep concentration gradient throughout the dialyzer, not just in one place.

Using this information, we can conclude that the opposite direction of flow maintains a steep concentration gradient so waste products continually diffuse out of the blood.

Dialysis is a much more common treatment for kidney failure than transplant. There are a limited number of donor kidneys available, but we can produce as many dialysis machines as needed.

The life expectancy of a patient on dialysis is anywhere from 5 to 30 years depending on the overall health of the patient and the compliance with treatment guidelines.

Patients on dialysis must follow strict dietary restrictions to prevent the buildup of wastes and ions in the blood in between dialysis treatments. This involves avoiding foods high in salt or potassium. They also must restrict fluid intake since their kidneys are not efficiently removing excess water from the bloodstream.

Dialysis is also an uncomfortable and time-consuming process. Dialysis patients are typically treated 3 to 4 times a week for 3 to 6 hours at a time. The time commitment of dialysis can interfere with a patient’s daily life. A scene showing patients in a typical dialysis center is shown in the photograph below.

Patient having dialysis treatment at Dialysis Centre

Figure8

Dialysis is an imperfect replacement for a functioning kidney. A kidney can maintain homeostasis constantly, but dialysis is only working when the patient is attached to the dialyzer.

Some of the most common causes of kidney failure are preventable. The best way to avoid kidney failure is to maintain a healthy diet, drink lots of water, look after your cardiovascular health, and avoid destructive practices like smoking, drinking alcohol, or using drugs. If you keep your kidneys healthy, not only will they serve you for your entire lifetime, but they could save someone else’s life in the future.

Let’s review what we have learned about kidney failure in this explainer.

Key Points

  • Kidney failure occurs when the kidneys drop below 15% of their typical function.
  • The two treatments for kidney failure are kidney transplant and dialysis.
  • A kidney transplant is when a donor kidney is implanted into the body of a patient with kidney failure.
  • Dialysis is when an artificial kidney, or dialyzer, is used to filter the blood outside the body.

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