Hospitals are all about flow. As new patients arrive, others must move on through the system and, eventually, leave.
But what has happened in the past month is that the system has stalled. Or - as one doctor put it to me - there has been "gridlock". At least in some places.
Let's take it step-by-step. Each year more than 14m patients arrive at A&E - effectively the front-door of the hospital.
Most of those are dealt with by the doctors and nurses in A&E. But one in four need more complicated care than can be given so are admitted into the hospital. These tend to be the frailest and most vulnerable patients.
Once the decision has been made to admit, those patients need to be found a bed. When that has happened, they can receive their treatment and, once they are ready, leave the hospital.
But this steady flow can be disrupted at any point - as this graphic below shows.
- 1. Patients: Most patients arrive themselves, either after referrals by GPs or the NHS 111 helpline or under their own steam.
- 2. Ambulances: At busy times crews face delays handing over patients. Queues can develop keeping them from returning to the road.
- 3. Trolley waits: Patients who need to be admitted can face long waits - more than four hours - before a hospital bed can be found.
- 4. Delayed discharge: Some need support in the community before they can be released. If this is not in place, they cannot leave hospital and so continue to take up a bed.
From the figures published by NHS England on Friday, the BBC has been able to analyse data for the last four weeks to see what is happening at all these pressure points. We have then compared the results to the same period the year before.
The findings are startling: at every juncture, the problems are getting worse.
With A&E units full, ambulances have found it difficult to drop off patients. When they turn up at hospital, it is only meant to take 15 minutes to handover a patients.
But over the last four weeks there were over 37,000 occasions when ambulances were left waiting for over 30 minutes. That is more than double the number the year before.
Once inside A&E the delays have continued - as we know from the extensive coverage given to how the NHS has been struggling to meet its four-hour target.
A growing number of those that are then admitted into hospital are facing long waits (over four hours) for a bed to be found. These are sometimes known as trolley waits. During the last four weeks the number of these has trebled from last year to over 47,000.
But it doesn't stop there. When they are ready to be discharged, delays are once again being experienced.
The frailest and most vulnerable patients need care to be in place in the community either via council-run social care teams or local NHS services before they can be released.
If it isn't available, they have to stay in hospital. The NHS measures this as bed days lost due to delayed discharge. There were over 62,000 of these over the past four weeks - up by nearly a third.
This creates a scarcity of beds. The effect of this is two-fold. Firstly, hospitals are forced to put emergency patients in whichever ward they can find a bed, which means patients do not always end up in the place that is most appropriate for them.
This is not good for patients or for the effective running of the hospital. Doctors and nurses waste time going on what are known as "safari rounds" looking for their patients that can be spread across many different areas. In short, the hospital stops running smoothly.
The problems also have an impact on the non-emergency side of the system. One step that is taken is that routine operations, such as knee and hip replacements, are cancelled. Again, the figures the BBC looked at showed this had happened too - nearly 5,500 have been postponed, up by 62%.
It is this catalogue of problems that prompts hospitals to raise the alarm. In recent weeks a number of trusts have even been forced to declare major incidents - something normally reserved for major accidents, like train crashes.
The most common reason put forward for the problems has been the rise in attendances. But what seems remarkable when you look at the figures is that the increase in those seems extremely small by comparison.
They have only risen by 7% on last winter. But that is significant because it equates to more than 100,000 patients.
"That is the equivalent to two new A&E departments," says Dr Cliff Mann, president of the College of Emergency Medicine. "The problem is that we just don't have the capacity. The NHS has reached a tipping point and that is why we are seeing the problems we are."