How does haemodialysis work?

Blood is drawn from your body through a needle or catheter, pumped through a machine and a filter (the dialyser) that removes waste and excess fluid, then returned to you. The dialyser acts as an artificial kidney, and your blood is cleaned continuously through the session.

What happens during a session?

You are usually weighed before and after to check fluid removed and work towards your target (dry) weight, and your blood pressure and the machine are monitored. Once connected, you can read, rest, use a device or sleep, with staff watching over you throughout.

Why do I feel dizzy or crampy during dialysis?

A drop in blood pressure and muscle cramps are common, often related to fluid being removed. Tell the staff at once so they can act, for example by slowing the fluid removal or adjusting the machine. These are routine issues the unit manages during the session.

Haemodialysis: What to Expect in Singapore

Q: How often and how long is haemodialysis?

Commonly about three times a week, with each session usually lasting a few hours, but the exact frequency and length are set by your renal team based on your size, remaining kidney function, fluid gain and blood results. Some people later do it at home.

Q: Why do fluid limits matter between sessions?

Failing kidneys make little urine, so fluid builds up between sessions. Large fluid gains mean more must be removed in one session, which is harder on the body and more likely to cause cramps and low blood pressure. Steady, moderate gains make each session smoother.

Written & reviewed by the BestDoctors.sg editorial team · Last updated 28 Aug 2026 · General information, not medical advice

Haemodialysis is the most familiar form of dialysis: your blood is filtered through a machine and returned to you, usually at a dialysis centre several times a week. Knowing how it works and what a session is like takes much of the worry out of starting. This guide walks through the process, what to expect during and after a session, and the small number of warning signs worth knowing.

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In a medical emergency, or for severe or sudden symptoms such as breathing difficulty, chest pain, fainting or stroke signs, call 995 immediately.

This is general information and does not replace your renal team’s advice. Tell the unit staff at once if you feel dizzy, sick or crampy during a session. Never miss or shorten a session without contacting your unit. Apply firm pressure to any bleeding from your access, and call 995 if it will not stop.

How haemodialysis works

In haemodialysis, blood is drawn from your body through a needle or a catheter, pumped through a machine and a filter called a dialyser, where waste products and excess fluid are removed, and then returned to you. The dialyser acts as an artificial kidney, and the machine controls how fast the blood flows and how much fluid is taken off. Your blood is being cleaned continuously throughout the session, a little at a time, before being returned. Only a small amount of your blood is outside the body at any moment, which is one reason the treatment is safe when carefully monitored.

In-centre sessions

Most people have haemodialysis at a dialysis centre, which may be in a hospital or a community satellite unit. As the NHS describes, it is commonly done about three times a week, with each session usually lasting a few hours, though the exact frequency and length are set by your renal team based on your size, any remaining kidney function, your fluid gain and your blood results. Some people are later trained to do haemodialysis at home, covered in our home haemodialysis guide.

What a session is like

A session follows a routine that quickly becomes familiar. You are usually weighed before and after, so the team can check how much fluid was removed and work towards your target, or “dry”, weight. Your blood pressure and the machine are monitored throughout. Once you are connected, the time is yours: many people read, rest, watch something, use a phone or tablet, chat, or sleep. The staff are there throughout to keep an eye on you and the machine. Many people get to know the staff and other patients at their unit well, and the regular routine becomes a familiar part of the week.

The access you need

Before haemodialysis can start, you need a way for blood to be taken and returned, called your access. This is usually an arteriovenous (AV) fistula in the arm, sometimes a graft, or a tunnelled catheter (line) when a fistula is not ready or possible. A fistula needs to be created ahead of time and takes weeks to months to mature before it can be used. Looking after your access is one of the most important parts of dialysis life, covered in our access care guide.

Common during-session effects

Some effects during a session are common and are managed by the unit on the spot. The most frequent are a drop in blood pressure, which can make you feel dizzy, sick or faint, and muscle cramps, both often related to fluid being removed. Tell the staff at once if you feel unwell, so they can act, for example by slowing the fluid removal or adjusting the machine. These are routine issues the team handles during dialysis, not something to manage alone.

Fluid between sessions

Because failing kidneys make little or no urine, fluid builds up in your body between sessions. Too large a fluid gain means more has to be removed in one session, which is harder on the body and more likely to cause cramps and low blood pressure. This is why keeping to the fluid, and often diet, limits agreed with your team matters so much, and it is covered in our fluid and diet guide. Steady, moderate fluid gains make each session smoother.

After a session, and red flags

It is common to feel washed out or tired for a while after dialysis, and many people plan a rest around their dialysis days. Contact your unit for bleeding from your access that will not stop with firm pressure (call 995 if it is heavy or will not stop), for a hot, red, swollen or painful access, or if you lose the usual buzz over a fistula. Also seek help for severe breathlessness or chest pain, which can signal too much fluid. Keeping this simple split clear in your mind, and sharing it with your family, means you can act quickly and correctly in a worrying moment. In Singapore, keep your unit’s number to hand, and 995 is the emergency number.

Sources

NHS – Dialysis: nhs.uk
NHS – Dialysis, how it is done: nhs.uk
Kidney Care UK – Haemodialysis: kidneycareuk.org
National Kidney Foundation – Hemodialysis: kidney.org

General information only. This guide supports, but does not replace, the advice of your kidney (renal) team, who set your dialysis, diet, fluid limits and medicines. Never miss or shorten a dialysis session without contacting your unit, and protect your fistula arm (no blood-pressure cuffs, blood tests or tight clothing on it). Contact your dialysis unit at once if your drained peritoneal-dialysis fluid looks cloudy or you have tummy pain and fever (a possible peritonitis), or for a hot, red, painful or leaking access site. Call 995 for bleeding from your access that will not stop with firm pressure, or for severe breathlessness (a possible dangerous fluid overload). If you are struggling emotionally, you are not alone: the Samaritans of Singapore (SOS) are on 1767 and mindline.sg on 1771.

Haemodialysis: What to Expect (Singapore)