Homeostasis

These are the support notes for the 4ish 1MbChB/1BDS lectures given by Pete Smith. These lectures cover aspects of cellular and whole-body physiology and share a common theme of homeostasis. This page describes the learning objectives, in very general terms, for these 4/5 lectures taken together.

  1. Barriers, Pores, pumps and Gates
  2. Body Fluid Compartments
  3. Why do we need Kidneys? A lecture in two parts
  4. Getting the message across

Homeostasis may be defined as:

The maintenance of the internal environment within physiological limits.

Or as Claude Bernard (rather more elegantly) put it: "....animals have really two environments: a milieu exterieur in which the organism is situated and a milieu interieur in which the tissue elements live. All the vital mechanisms, varied as they are, have only one object: that of preserving constant the conditions of life in the milieu interieur."

In a living organism just about everything, from Arterial blood pressure to Zymogen granules, is subject to homeostatic regulation. Homeostasis occurs in single cell organisms and at every level of organisation in a multicellular organism (such as yourself) from the single cell up to the entire body.

The most common way of exploring the principles of homeostasis is to look at how hormones regulate the function of different organs and integrate these functions to the best advantage of the organism as a whole. You can find descriptions of these aspects of homeostasis in many textbooks. These notes look at homeostasis from a slightly different perspective. A most literal definition of "internal environment" is the actual salts and water that form the basis of both the cytoplasm and the fluid space between the cells. To be able to make sense of this, most straightforward, exploration of homeostasis you need to how and why cells regulate their own intracellular environments, how salts and fluids move between different body compartments, what are the different body compartments and why do we have them and how the body as a whole defends these comfortable arrangements against the hostile environment in which we live.

Most of the time, homeostasis may be thought of as an automatic process or collection of processes that act by negative feedback to keep things "optimal". This is true, so far as it goes, but what is "optimal"? "Optimal" is not a fixed target but rather a rapidly moving one. "Optimal for a muscle at rest is not the same as for a muscle in action. "Optimal" for an empty stomach is not the same as for a full one. And so on. So, we will also explore both the ways in which cells respond to changes in their environments in order to move from one homeostatic position to another and also investigate how an organism can compensate, on a short term basis, for a homeostatic imbalance in one area by introducing a counteracting imbalance somewhere else.

A rough guide to the content and objectives of the lectures is as follows

  1. Barriers, Pores, pumps and Gates: Introduces the concept of homeostasis and the the basic processes necessary for salt and water homeostasis at a cellular level. The most basic concept is that of the plasma membrane as an effective barrier made selectively permeable by the presence of protein channels and pores. Layered on top of this simple model are the entropy defying active transport processes that make homeostasis possible. These same transport processes underlie the movement of fluid in and out of the body and understanding how they work is a prerequisite for appreciating the problems associated with travellers' diarrhoea.
  2. Body Fluid Compartments: Introduces body fluid compartments and covers hormonal control of salt and water balance in the body as a whole. Here we take homeostasis from the level of single cells up to the level of the whole organism. Physiologically speaking, every organism has more than one fluid compartment and each compartment has a different function. We will identify these compartments and determine their function.
  3. Why do we need Kidneys?: Presents the kidney as the guardian of salt and water balance (and shows how useful it can be for all sorts of other things). This lecture is just too exhausting to experience in a single sitting, so it has been split into two parts. Nine times out of ten, if you are thirsty then the kidney is somehow responsible. The other time, you just fancy going to the pub. To understand why someone may have a "raging thirst" you have got to know what the kidney is up to.
  4. Getting the message across: Looks at how cells respond to changes in their environment using 2nd messengers. Both the normal and pathological changes in airway epithelia and muscle are mediated by 2nd messengers....

Lectures or any large-group teaching session in support of a PBL course are going to be different from traditional lectures. Not least because they are not the primary source for you to achieve your Learning Objectives. That should be your own research. By and large, my lectures are concept-led rather than fact-led. There are all manner of "facts" in my lectures, but that is not the point of them. My objective is to explore the ways in which these "facts" fit together. None of the factual information in these lectures (well, at least in the 1st 3) should be new to anyone with 'A' level Biology. Most of it should be at least familiar to anyone with 'GCSE' Biology (or their equivalents from far-away places...like Scotland). Nonetheless, you are unlikely to have seen this information assembled or focussed in quite this way before.......

One consequence of being concept- rather than fact- led is that I will cover the "facts" at speed and dwell on the concepts. These support notes are intended to give you advance knowledge of the "facts" so that you won't get left behind on the day.

Please feel free to offer feedback, positive or negative, to help me improve these plenaries.