Constructing a Functional Unit

Constructing a Functional Unit

“The significant problems we have cannot be solved at the same level of thinking with which we created them.”
– Albert Einstein –

Functional units are the key to developing a practical understanding of the body. They are also essential in diagnosing disease. The more you use them, the better you will become at clinical thinking, improving your knowledge of physiology and anatomy along the way. To begin our first functional unit we will start where all medicine should start – with a patient:
Your next patient, a 60-year-old Chinese female presents complaining that she is ‘swelling up’. It started in the ankles, but now is involving most of her legs.
On examination you find that indeed her legs are very swollen. You decide she has ‘oedema’.
You think it might be heart failure…
But what else could it be??
So we have a problem to solve: what are the causes of oedema? We will need to work out a differential diagnosis… so let us use a functional unit. To do this, we put together all the relevant anatomical and physiological details we know from medical school:
1.The reason the body has a circulatory system at all is to get nutrition to cells. So let us begin our functional unit with a few cells. To make them happy, we should bathe them in a bit of extracellular fluid.

2. Next, we’ll need some arteries and arterioles to bring fluid to the extracellular fluid… and veins and venules to take the fluid away. We’ll also need some capillaries to link up our vascular network.
3. Now we need to explain how the fluid gets out and around the cells, and then returns to the capillary network. Ernst Starling explained this, through the balance of two important pressures. As you will remember from your physiology textbook, these are the hydrostatic pressure (P), and the oncotic pressure (π).

4. Actually, we know that not all the tissue fluid returns to the capillaries. A significant amount also returns to the intravascular space through the pumping of lymph vessels. Therefore we should add a final piece to our functional unit – some lymph (L).

And that is it – a basic functional unit of oedema. If you can easily construct this diagram, determining a differential diagnosis for oedema will be easy!

Ernst Starling (1866-1927)
Starling was born to a poor and religious family. He was educated in medicine at Guy’s Hospital, where he set a record for his scholarship. He became interested in physiology, and besides his famous work on the circulating fluids, he also coined the word ‘hormone’ in relation to his novel work on secretin. He died while on a Caribbean cruise near Kingston, Jamaica. He said, “Science has only one language, quantity, and only one argument, the experiment.”