The thing that surprises most patients about the spleen is how small it is. It is between 3-5 inches long, 2-3 inches wide and about an inch thick. There is this thing about what units are used to measure things. Although schoolchildren in the UK have been taught in centimetres for decades, many people still think in feet and inches. An inch is the length of the top joint of the thumb and a foot is the length of a man’s foot. A yard is a man’s pace; a metre is more than I can comfortably pace. So, a spleen is 8-13 cm long, 4.5-7 cm wide and 2-3 cm thick; I can’t visualise that.
I once had an amusing correspondence with a medical journal over units of measurement. I had described a red cell as having a volume of 105 femtolitres. A femtolitre is ten to the power of minus 15 litres. In other words extremely small. The abbreviation for femtolitres is fl. Obviously the editor had never heard of such a unit, so she chided me for this unknown abbreviation. “Do you mean fluid ounces?” she asked, “The correct abbreviation for fluid ounces is fl oz.” I was dumfounded. Try to imagine a red blood cell with a volume of 105 fluid ounces. That’s more than five pints! You want a pint of blood? That will contain one fifth of a red cell.
You can’t normally feel a spleen. It is tucked up next to the diaphragm under the ribs on the left, far beyond the reach of even a finger probing under the ribs. When you take a deep breath the diaphragm descends and the spleen moves nearer to the edge of the ribs (sometimes called the costal margin) but it still cannot be felt unless it is enlarged. What constitutes an enlarged spleen? It is said that a spleen has to be three times its size to be felt. Another textbook says spleens longer than 14 cm can be felt. What is the truth?
The truth is that spleens are different sizes in different bodies. In a small woman the spleen is normally 8 cm long. If it gets to 14 cm long, its volume will certainly be more than 3 times normal, and it will be felt coming down under the ribs as the patient takes a deep breath. For bigger people bigger numbers apply.
Patients are often confused because their spleen measurements are given in centimetres as measured on a CT scan. Whereas previously they were given according to how many centimetres the spleen protruded below the costal margin. So a 16 cm spleen sounds horrific if it’s measured below the costal margin (that’s six and a half inches!), but in a man it would mean a spleen that’s about an inch larger than normal.
Normally 5-10% of the total blood supply travels through the spleen every minute. Passage through the spleen takes about a minute if the blood takes the fast route, but there is also a slow transit route that takes about an hour. When the spleen enlarges more blood takes the slow route.
The spleen has a complicated anatomy. It is divided into the red pulp and the white pulp and my next post will describe the different functions of the two.