Animal Physiology - Homeostasis

• what is physiology?

  • study of processes and functions in living organisms,
• what is physiology ?

  ·       Study of integrated functions of the body and the functions of all its parts (systems, organs, tissues, cells, and cell components), including biophysical and biochemical processes
• what is homeostasis?

  • property of a system in which variables are regulated so that internal conditions remain stable and relatively constant
• key components of homeostatic circuits:

  • Controlled variable
  • Regulated variable
  • Sensor
  • Set point
  • Error detector
  • Controller
  • Effector
• controlled variable:

  Physiological variable that is manipulated in order to maintain the regulated variable within normal values (i.e. set point) example: Heart rate
• Regulated Variable:

  physiological variable for which sensors are present in the homeostatic circuit and is maintained at a stable level. Example: blood pressure
• Sensor:

  a ‘device’ that measures the magnitude of a physiological variable by generating an output signal that is proportional to the magnitude of the stimulus.
• Sensors:

  can be specialised sensory cells, or cellular components
• set point:

  the range of values of the regulated variable that the homeostatic circuit attempts to maintain. These are changeable
• error detector:

  Determines the difference between the set point value and the actual value of the regulated variable: error signal
• Controller:

  it receives information (error signal) from the error detector and send output signals (instructions or commands) to increase or decrease the activity of effectors
• effector:

  A component of the homeostatic circuit that is activated by the controller to change the value of the regulated variable
• Homeostatic regulation is a constant, continuous process and does not ordinarily operate as an on/off switch that results in an all-or-none response
• negative feedback:

  a process in which the body senses a change and activates mechanisms that negate or reverse it
• example of negative feedback:

  an increase in blood glucose concentration above its homeostatic range (hyperglycaemia) sets into motion processes that reduces it
• an example of negative feedback

  a decrease in blood glucose concentration below its homeostatic range (hypoglycaemia) promotes processes that will increase it
• positive feedback

  a self-amplifying cycle in which a physiological change leads to even greater change in the same direction, rather than producing the corrective effects of negative feedback
• Regulated variable

  • Can have different dynamic range in different tissues
  • Brain has low tolerance for deviations in many physiological variables (e.g. oxygen, temperature)
  • Adipose tissue is usually less demanding
• Regulated variable

  Can also be a controlled variable under some circumstances
• Carbon dioxide partial pressure (Pco2) in the extracellular fluid (ECF)

  Maintained within defined limits by a regulator system that senses Pco2 and operates by negative feedback
• During compensatory adjustments in the acid-base balance (i.e. hydrogen ion homeostasis)

  Pco2 becomes the controlled variable that is manipulated to maintain a normal pH (the regulated variable in this case)
• Cell membranes

  Composed of a double layer of phospholipids (phospholipid bilayer) and proteins
• Cell membrane

  • Important function in determining what enters and leaves the cell
• Simple diffusion

  Movement of particles from high concentration to low concentration, down concentration gradient. Passive mechanism of membrane transport – no ATP required
• Osmosis

  Type of passive transport that refers exclusively to movement of water across cell membrane by way of channel proteins
• Filtration

  Water and solutes are driven through the cell membrane by hydrostatic pressure (i.e. water/solutes move from high pressure areas to low pressure areas). It is a passive mechanism of membrane transport. Depending o the size of the membrane pores (i.e. channel proteins) only solutes of a certain size may pass through
• Facilitated diffusion

  Carrier-mediated transport of a solute through the cell membrane down its concentration gradient. It's a passive mechanism of membrane transport. Carrier mediated transport uses a transmembrane protein to move substances from one side of the membrane to the other
• Active transport

  Carrier-mediated transport of a solute through the cell membrane UP its concentration gradient using energy provided by ATP
• Vesicular Transport:

  : Movement of large particles, droplets of fluid, or numerous molecules at once through the cell membrane, contained in bubble-like vesicles of membrane
• Types of endocytosis: Phagocytosis

  is the process of engulfing particles such as bacteria, dust, and cellular debris (“cell eating”)
• types of endocytosis: Pinocytosis

  : the process of taking in droplets of ECF containing molecules of some use to the cell (“cell drinking”)
• types of endocytosis: Receptor mediated endocytosis

  phagocytosis or pinocytosis in which specific molecules bind to specific receptors on the cell membrane, and are taken into the cell in clathrin-coated vesicles with a nominal amount of ECF
• Autocrine / paracrine signalling:

  A chemical messenger diffuses a short distance through the ECF and binds to receptor on the same cell or a nearby cell
• Endocrine signalling

  a hormone is releases into the bloodtstream and binds to a specific target cell receptor
• nervous signalling

  involves the rapid transmission of action potentials, often over long distances, and the release of a neurotransmitter at a synapse
• neuroendocrine signalling

  involves release of a hormone from a nerve cell and the transport of the hormone by the blood to a distant target cell
• Channels across plasma membrane of cells:

  • ion channels = allow specific charged particles to cross in response to an existing concentration gradient
• Chemically-gated channels:

  open in response to specific chemical messengers that binds the extracellular region of the channel
• voltage gated channels

  open in response to changes in membrane potential
• mechanically-gated channels:

  open in response to physical distortion of membrane surface