Once treatment begins, the same basic circuit repeats continuously: blood leaves through your vascular access, passes through the dialyzer, and returns to you. Understanding what happens at each point in that circuit can make the machine, tubing, alarms, and treatment routine easier to follow.
Getting connected: vascular access and bloodlines
Before any filtering can happen, your blood needs a way in and out of your body. That pathway is your vascular access, and it is typically one of three types: an AV fistula, an AV graft, or a central venous catheter.
If you have a fistula or graft, staff place two needles into the access: one carries blood out to the machine, and one returns filtered blood to you. If you have a catheter, the connection is made through the catheter itself instead of needles. Each needle or catheter port attaches to soft tubing called bloodlines, which carry your blood to the dialyzer and back.
Staff tape and secure needles and connections carefully, and usually keep the access site visible throughout treatment. That is not just a comfort step. A dislodged needle or a loose connection can become a real problem quickly, so keeping the site visible and undisturbed matters for safety, not just tidiness.
What happens inside the dialyzer
The dialyzer is sometimes called an “artificial kidney,” though it only replaces part of what a working kidney does. Inside it, your blood flows through hundreds of thin, hollow fibers. A cleansing fluid called dialysate flows around the outside of those fibers in the opposite direction.
Waste products such as urea and creatinine move from your blood, across the fiber wall, and into the dialysate, a process called diffusion. At the same time, the machine can push extra fluid out of your blood and into the dialysate stream, a process called ultrafiltration. Your nephrologist prescribes the dialysate composition and the fluid-removal goal based on your blood work and clinical condition.
What the machine is doing throughout treatment
While you are connected, the dialysis machine continuously controls and monitors several things at once:
- Blood flow rate — how fast your blood moves through the circuit
- Dialysate flow rate — how fast the cleansing fluid moves through the dialyzer
- Ultrafiltration (fluid removal) — how much fluid is removed, and over what time
- Arterial and venous pressure — readings that help the machine and staff detect problems in the circuit
- Your blood pressure — checked at intervals throughout treatment
All of these settings follow your doctor’s prescription. Staff can adjust some parameters within that prescription based on how you are tolerating treatment, but the overall plan is not something that gets reinvented at the chair each day.
A technician’s-eye explanation: why the machine alarms
Dialysis machines alarm often, and an alarm does not automatically mean something dangerous is happening. Alarms are the machine’s way of flagging a change in pressure, flow, air, or another monitored value so staff can check it. Something as simple as bending your arm, shifting position, or a kink in the tubing can trigger one. Staff are trained to check the cause and respond, and most alarms are resolved in moments. As the routine becomes familiar, the sounds tend to become far less unsettling.
How long does a hemodialysis treatment take?
In-center hemodialysis treatments commonly last about 3 to 5 hours, according to the National Kidney Foundation, and most people are scheduled three times a week. Your own treatment time, frequency, and schedule are set by your doctor based on your individual needs, not a one-size-fits-all number.
What happens when treatment ends
When your treatment time is complete, staff begin the process of disconnecting you from the circuit. You may hear the term rinse-back, which describes using a small amount of saline to flush your remaining blood out of the bloodlines and dialyzer and back into your body before the lines are removed.
If you have a fistula or graft, staff remove the needles and hold pressure on the sites until bleeding stops. If you have a catheter, staff cap and secure it according to their procedures. You are usually weighed again, and staff may check your blood pressure and how you are feeling before you leave.
What can vary from patient to patient
Two people sitting a few chairs apart during the same treatment can have very different experiences, because their prescriptions and clinical conditions are different. Things that commonly vary include:
- Type of vascular access
- Blood flow rate and dialysate flow rate
- Fluid-removal goal and treatment time
- Dialysate composition
- How the body responds to fluid removal
- Medications given during treatment
Comparing your numbers or your experience with someone else in the clinic usually is not useful. Your treatment is built around your own prescription.
Questions worth asking your dialysis care team
- What type of vascular access do I have, and what should I know about caring for it?
- What is my current blood flow rate and treatment time?
- What is my fluid-removal goal for today, and how was it determined?
- What should I report to staff during treatment?
- What do the numbers on the machine mean for my particular treatment?
The main thing to remember
Hemodialysis is a continuous, monitored process: your blood leaves your body, passes through a filter that removes waste and extra fluid, and returns to you, over and over, for the length of your prescribed treatment. The equipment does the mechanical work, but the plan behind it — blood flow, fluid removal, treatment time, dialysate — comes from your doctor’s prescription and is adjusted based on how you are doing.
Understanding the basic flow can make the sounds, steps, and vocabulary around you much easier to follow.
Related Renal Care Planner Resources
- Your First Hemodialysis Treatment: What to Expect
- Why Does the Dialysis Machine Alarm?
- Free First Hemodialysis Treatment Checklist (PDF)
- What Is Dry Weight (Target Weight) in Dialysis?
- Why Am I So Tired After Dialysis?
- What to Bring to Dialysis: A Practical Treatment-Day Packing Guide
- What to Wear to Dialysis: A Practical Guide for Treatment Days
Frequently Asked Questions
Is hemodialysis painful?
Needle placement can cause a brief pinch or sting, and numbing options are often available if this bothers you. Most people do not describe the treatment itself as painful, though some experience cramping, dizziness, or other symptoms that should be reported to staff.
What is a dialyzer?
A dialyzer is the filter used during hemodialysis. It is sometimes informally called an “artificial kidney,” though it only replaces part of what healthy kidneys do.
What is the difference between diffusion and ultrafiltration?
Diffusion is how waste products move out of your blood into the dialysate. Ultrafiltration is how extra fluid is removed from your blood during the same treatment.
Can I choose my blood flow rate or fluid-removal goal?
No. These are part of your dialysis prescription, set by your nephrologist based on your clinical condition, and adjusted by your care team based on how you tolerate treatment. Tell your team if something does not feel right, but do not attempt to change these settings yourself.
Why does the machine make so much noise?
Beeps and alarms are the machine’s way of flagging changes in pressure, flow, or other monitored values so staff can check them. Most are resolved quickly and do not indicate an emergency.
Sources
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Hemodialysis.
- National Kidney Foundation. Hemodialysis: How It Works, Types, and What to Expect.
General education only. Renal Care Planner provides general educational information and planning resources and does not replace individualized medical care. Blood flow rate, fluid-removal goals, treatment time, and other settings are part of your dialysis prescription and are managed by your nephrologist and dialysis care team. Discuss questions about your specific treatment with your care team.