Overview of Hemodialysis

By Sherri Bresn, BS, BSN, RN, CNN
Hemodialysis (HD) is a treatment option for kidney failure. When confronted with the seriousness of renal or kidney failure, most patients find it difficult to understand all of the various aspects of their treatment. There are many questions that go unasked and unanswered. For example: "How does the artificial kidney or dialyzer work?" Another question commonly asked is: "I see blood going through a series of tubes. What is happening to my blood?" Read on to find the answers to these commonly asked questions.
Defining Hemodialysis
In defining HD, let's break the word down into a simpler form. The word "hemo" means blood, and dialysis refers to the removal of wastes and fluid from the blood using a special filter known as a semi-permeable membrane.
A semi-permeable membrane is a thin sheet of protein material that has microscopic holes or pores. These holes allow for the movement of small particles to pass from the blood into a chemical solution known as dialysate (to be discussed later). Large particles or objects such as blood cells are too large to pass through these pores.
Blood is taken from the patient's blood stream by a tube and brought into the "artificial kidney" or dialyzer by a pump. Clean filtered blood is brought back to the patient through another tube. The process of HD may take between 2-5 hours. During this time, blood is continuously flowing through the tubing and artificial kidney.
Cleaning the Blood
All the cleaning of the blood occurs inside the hemodialyzer. It acts just like the kidneys. As blood passes through the hemodialyzer, wastes and fluid will be removed. In addition, salts in the blood are balanced and acids are regulated.
The kidney's function is replaced by the hemodialyzer, but not all the kidneys functions are replaced! In addition to filtering waste products and fluid removal, the kidneys act as an endocrine organ. In other words, an organ that secretes hormones (special chemicals in the body used for specific body functions). HD cannot replace these hormones. Instead, nurses and doctors will provide medications during the dialysis treatment that will substitute for these hormonal functions.
The HD Circuit
To provide HD, the patient first must have a vascular access. This is a surgically created device placed in the patient's arm or other location that allows the nursing staff to attach the HD circuit to the patient. The circuit consists of these basic parts:

  •     A patient in renal failure with a working access;
  •     Arterial line;
  •     Hemodialyzer;
  •     Venous line;
  •     Dialysis machine with delivery system; and
  •     Water and electricity

The Working Access
Depending on the specific needs of the renal failure patient, one of the following accesses will be placed:

  •     A central venous catheter;
  •     An arteriovenous fistula; or
  •     A graft.

The purpose of each access is to provide a means to extract and return blood rapidly from the blood stream and bring it to the hemodialyzer. Without a functioning access, HD cannot occur. A central venous catheter is a long slender tube placed in a vein, either in the chest or neck. A fistula is the surgical connection of an artery to a vein, allowing for the vein to enlarge for use as a continuous site for HD needle insertion. A graft is the connecting of an artery to a vein using a surgically placed graft material. The HD needles are inserted into the graft material instead of the vein. The physician and the patient make a choice of one of these accesses.
Arterial Line
A tube known as the arterial line is attached to the access to take blood away from the body and bring it to the hemodialyzer. The blood is "milked" out of the blood stream by a pump containing two rollers that glide along the arterial line tubing on the HD machine.
As these rollers rotate around in a circle, they gently compress on the tubing, creating a pulling force. This pulling force brings blood into the line, and then pushes it into the hemodialyzer that is attached to the end of the arterial line.
In the US, hollow-fiber dialyzers are primarily used. Hollow-fiber dialyzers consist of a plastic casing filled with approximately 10-20 thousand "straws," each the size of human hair. Blood is directed downward as it travels inside these straws.
These straws or fibers are the semi-permeable membrane. They act as thousands of filters allowing for blood to stay inside the fibers while wastes and fluid seep out. While the blood travels inside the fibers, a cleaning solution (dialysate) is brought through a side port known as the dialysate port. This cleaning solution surrounds the fibers, but blood and dialysate do not mix!
A special plastic resin acts as a cap on the top and bottom of the dialyzer, keeping blood inside the fibers and dialysate out. The dialysate travels upward as it fills the spaces around the outside of the fibers, but remains within the casing of the dialyzer. A counter-current flow is created, as dialysate is brought in going up, and blood is pushed into the dialyzer going down.
Diffusion, Ultrafiltration, and Solvent Drag
Blood is cleaned inside the dialyzer by the processes of diffusion, ultrafiltration (UF), and solvent drag. Particles are removed from the blood using diffusion into the dialysate.

Diffusion is the movement of particles from an area of high concentration (an area of lots of particles) to an area of lesser concentration (an area with few or no particles) through a semi-permeable membrane. Diffusion allows for waste particles to be taken out of the blood, which is then transferred into the dialysate through the pores of the fibers. Water is both pushed and pulled off the blood inside the fibers by the process of UF.

UF is the removal of fluid using a pushing and pulling pressure at the same time. The blood pushes down from the top of the dialyzer through the fibers. Similar to pushing down on an orange on an orange grinder, water is "squeezed" off the blood. The dialysate is pulled through the dialyzer by pumps on the machine. Combined, these two forces push and pull fluid through the holes of the semi-permeable membrane. Particles are also removed with the fluid using the forces of UF in a process known as solvent drag.
Venous Line
At the bottom of the dialyzer is the venous line, which returns cleaner blood to the patient.
Blood that was pushed through the fibers by the blood pump on the machine is now brought back the patient via a tube that is connected to either the central venous catheter, or another dialysis needle of a graft or fistula.
Dialysis Machine
The dialysis machine contains alarms for safety and is the vehicle to deliver blood and dialysate to the hemodialyzer. Along the blood tubing are pressure monitors, clamps, and an air detector that monitors the presence of air in the blood. If air is present in the blood, the dialysis machine clamps the venous bloodline shut, the blood pump automatically shuts, and the machine gives an audible and visual alarm to alert the nursing staff.
The machine also carries dialysate (once it has been specially mixed and prepared) to the dialyzer. This solution is proportioned out by the machine in precise amounts to ensure safety and to effectively remove wastes and water from the blood. Machine monitors protect the patient from dialysate that is not properly mixed or prepared. If an alarm sounds, the problem is corrected by the nursing staff that is monitoring the entire dialysis treatment.
Water and Electricity
Dialysate consists of salts and specially cleaned water. Tap water that is used for HD must be purified to prevent harmful contaminants such as chlorine, chloramines, and minerals from entering the patient's bloodstream.
Water treatment facilities exist inside dialysis units to provide purified water to each dialysis machine at that facility. Using semipermeable membranes, dionizing tanks, and carbon filters, tap water is passed through a series of water treatment components before it is brought to the dialysis machine. Electricity is needed to run the machine and the water treatment facility. When a power failure occurs, the patient's blood must be manually cranked back by the patient or nursing staff. Dialysis will either resume when the power returns, or it will be rescheduled.
Blood travels in a loop. It comes from the patient, passes through the dialyzer, and returns by means of the HD access.
While inside the dialyzer or "artificial kidney," wastes and fluid are removed from the blood (similar to the function of the human kidney). Wastes found in the blood are removed by diffusion, and fluid is removed by UF. Once the process is complete, the blood is returned to the patient through the venous line.
It is important that all dialysis appointments are kept to ensure the well-being and health of the patient. Remember, the 24 hour-a-day job the kidneys once provided are now being replaced in hemodialysis for only hours a day, three times a week.
HD replaces some of the functions of the kidneys, and is the treatment option for many patients in renal failure.
About the Author
Sherri Bresn, BS, BSN, RN, CNN, has worked in the dialysis field since 1989, and currently works as a Nurse Educator and Staff Developer for Everest Healthcare Services, now part of Fresenius Medical Care. For nine years, Sherri has worked as a nurse educator, instructing nurses and technicians on hemodialysis and renal failure. Sherri has had her Certification in Nephrology Nursing since 1993. She holds a Bachelor's degree in Psychology with an emphasis on learning behavior science from the University of Illinois, Urbana-Champaign, and received a Bachelor's in Nursing Science from Rush Presbyterian St. Luke's Medical Center, Chicago, IL. This article was originally posted on the iKidney.com website in November 2001.