Introduction to Physiology and Homeostasis

The very first lecture for medical school was an introduction to physiology, which I majored in in undergrad. Hence a lot of these initial posts will likely consist mainly of links to previous posts that I've written about physiology.

Gain an understanding of the field of Physiology

Physiology is basically the study of how the body works. Physiology often views the body as being like a machine, mostly responding to the world around it to keep the internal environment constant (though there are feed-forward mechanisms as discussed here).

Describe the level of organisation in the body

As multicellular organisms, we are made up of a lot of cells. Many of our cells are specialised; that is, they have been built to perform a few tasks and to perform those tasks well. Cells can combine to form tissues, which combine to form organs, which combine to form organ systems, which then combine to form us.

Know the constituents of the intracellular and extracellular environment

This lecture didn't really go into the intracellular environment that much- it focused more on the extracellular environment. The extracellular fluid (ECF) "bathes" the cells. It contains a variety of constituents, such as glucose and various ions (sodium, potassium, calcium, chloride, etc.). The concentration of these constituents, as well as the temperature and pH, has to be kept fairly consistent in order to keep the cells happy.

Discuss the concept of homeostasis and homeostatic control systems

Homeostasis refers to the need to keep the internal environment relatively constant. As I mentioned just above, the extracellular fluid needs to be kept constant so that the cells that are bathing in it can stay healthy. In order to keep the internal environment consistent, there are control systems in place. You can read about them in more detail than you really need to know here and here. If you don't want the detailed review, the tl;dr version is this: you need some kind of sensor to detect the level of a variable (concentration of a constituent, temperature, pH, etc.), some kind of integrating centre to decide what to do with this information, and some way to make a change if it's necessary. for instance, with blood pressure, we have baroreceptors in our carotid sinus and aortic arch to monitor blood pressure and send signals to the medulla, which then makes necessary adjustments via the sympathetic and parasympathetic nervous systems.

Know that multicellular life involves specialisation of individual cells and communication between cells

As I mentioned earlier, many of our cells are specialised in that they can perform a few tasks and perform them well. For instance, cardiac muscle is pretty good at contracting and making the heart beat, but I wouldn't count on it to help me type up a blog post. Since we have many different types of cells located all over our body, it is important that they can all work together. Cardiac muscle cells would be pretty useless if they only beat randomly: you need a good strong contraction from a bunch of them in order to make the heart beat.

Describe and understand the forms of communication between cells

There are many different types of communication between cells. They mainly differ in terms of how direct they are (some target only very specific cells while others target basically anything within firing range) and how fast they are.

Juxtacrine

Juxtacrine signalling occurs between two cells that are physically in contact with each other. It occurs via a membrane-bound signalling molecule on one cell and a receptor on the other cell. This type of signalling is pretty rare, and pretty much only occurs in Notch signalling, which occurs in neural development.

Gap Junctions

Gap junctions, just like juxtacrine signalling, can be found between two cells that are directly in contact. In gap junctions, there are channels between the cells, made up of proteins called connexons. Due to these channels, the two cells basically get to share the same intracellular fluid, allowing for the free passage of ions and other molecules. Gap junctions are very handy during contraction of the heart and some smooth muscles, as the signal can propagate between cells and create a rhythmic, coordinated contraction.

Autocrine

In autocrine signalling, a cell releases a molecule into the extracellular fluid, and then this molecule binds to a receptor on the same cell. In essence, the cell is communicating with itself. Autocrine signalling is not very common, except for in the immune system. For instance, monocytes send interleukin-1 to themselves, and macrophages send TNF-α to themselves. Smooth muscle also uses autocrine signalling in the mediation of the stretch response.

Paracrine

Similar to autocrine signalling, paracrine signalling also involves the release of molecules into the ECF. However, the target here is not the same cell, but rather other cells that are a short distance away. An example of paracrine signalling is the effect of various metabolites on blood vessels: for instance, carbon dioxide can cause vasodilation, which is dilation of blood vessels (except in the respiratory system, where it causes vasoconstriction, which is constriction of blood vessels).

Endocrine

In endocrine communication, hormones are released into the blood and are transported through the blood to their target organs. Hormones will only have an effect on cells that have a receptor for them. Endocrine signalling can affect a lot of cells at once, but is slow as it takes time for the hormones to be transported by blood.

Neural

Neural signalling literally just uses neurons to send signals. I have briefly described how neurons work here. In contrast to endocrine signalling, neural signalling is faster and more specific.

Neuroendocrine

Neuroendocrine signalling is like a hybrid of neural and endocrine signalling. Essentially, neurons release neurotransmitters into the blood, rather than to other neurons or target cells. Examples of neuroendocrine signalling include the release of ADH and oxytocin from the posterior pituitary.