Homeostasis and Excretion

Topic Index
Human Physiology Index
5.6.1 Homeostasis: controlling the internal environment within narrow limits:


  • Changes in pH, temperature, solute concentration and water are all factors that affect the efficiency of cellular processes
  • In section 2.3.3 the various factors that affect enzyme pathways such as pH and temperature are considered.
  • In addition the concentration of blood and tissue fluid must be controlled as they will have affect osmotic effects on solutions.
  • The exchange of gases carbon dioxide and oxygen are also important for efficient cell processes.
  • Carbon dioxide if it is allowed to accumulate creates acidic conditions unsuitable for enzyme function.
  • Oxygen depletion reduces the rate of respiration and undermines important processes such as active transport.
  • Homeostasis is the name given to the combination of process that controls the internal environment.
  • The various factors are controlled within very narrow limits.
  • Failing to control such conditions leads to inefficiencies, system failures and in some instances can be fatal.
5.6.2 Monitoring & Responding

Homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms.

feedback loop
  • This model represents the main features of a negative feedback model .
  • Specialised receptors detect changes within the internal conditions
  • This information is relayed to a central coordinator that determines the level of response
  • The coordinator in turn relays such a decision to the effector that is specialised to produce the response behaviour
  • Notice that this response will modify the internal environment and that these new conditions will in turn become the new stimuli.
  • The cycle will continue until conditions are reduced back to within narrow acceptable limits (fixed regulation point).
  • Notice that system works responding to conditions lower that and higher than the fixed regulation point.
  • Very efficient systems allow very little in the way of undershoot and overshoot.



Model: regulation of the internal environment between narrow limits

  internal regulation  
5.6.3 Endocrine and Nervous System Control


  • Both the Nervous system and the endocrine system are involved in homeostasis
  • Communication between the the receptors and the effectors can be either neurons (nerve cells) or by hormones
  • Examine the model above notice that the connections between receptors, coordinators and effectors could be either by hormone or by neurons.
5.6.4 Homeostasis under control of he nervous system

Feedback loop CNS

Click to see diagram of nervous system

The nervous system consists of the:

  • Central Nervous System (CNS) which co-ordinates responses and includes the brain and spine.
  • *Peripheral nerves which includes sensory neurons and motor neurons.
  • Special cells called neurons that can carry electrical impulses rapidly.


  • Stimuli are the internal changes in pH: temperature, glucose , carbon dioxide and oxygen .
  • Receptors are the specialised cells that can detect internal changes and convert this information into nerve impulses.
  • Sensory neurons are nerve cells that relay the nerve impulses to the coordinators of the central nervous system
  • Coordinators are regions of the central nervous system (brain, spine) that determine the appropriate response.
  • Motor nerves relay the response nerve impulses to the effectors
  • Effectors are organs, tissues or cells that react to the motor nerve input producing a response that will bring about control of the changing conditions
5.6.5 Temperature Control
  • Control of body temperature including the transfer of heat in blood, the role of sweat glands and skin arterioles, and shivering.


Model of temperature control:
  • The sensors are found in the hypothalamus.
  • Effectors are found in the skin and in muscles.
  • The fixed point for regulation is around 37.8 degrees centigrade.

Note the particular features of skin which are involved in temperature regulation:

  • Hairs with the erector pili muscle
  • Sweat glands
  • Blood arterioles
  • The skin is an effector in the control of body temperature.
  • It is particularly important to prevent cooling or overheating of the core (essential organs and brain)

When body temperature falls:

  • Blood flow to the surface via arterioles in reduced.
  1. Notice that (a) is dilated but (b) & (c) are vasoconstricted.
  1. Less heat is lost to the environment by radiation
  1. Heat is retained in the core.

When body temperature rises:

  • Blood flow to the surface is increased.
  • (a) is closed and (b) and (c) arterioles are open
  • Blood flows closer to the surface
  • More heat is lost to the external environment
  • Core temperature is reduced
  • Called vasodilation


Sweating and heat loss:
  • Sweat is secreted onto the surface of skin when body temperature is high
  • Sweat is largely composed of water which has a high specific heat capacity (absorbs a heat easily)
  • Body heat is transferred from skin and blood to the sweat
  • The sweat evaporates transferring heat away and in doing so cools the body


Shivering and heat gain:
  • When core temperature falls an uncontrolled phase of rapid muscular contraction occurs
  • The contractions generate heat which is used to raise the core temperature
  • Shivering stops when the core temperature is high enough again.
Hair and temperature control :
  • In warm weather the erector-pilli muscle are relaxed and the hairs lie flat.
  • This prevents a build up of a 'boundary layer' of warm air.
  • AIr movement will further accelerate the loss of heat.
  • In cold weather the erector-pilli muscle contracts and the hair moves vertical. This traps a 'boundary layer' of warm air that reduces the temperature gradient and in turn reduces heat loss.
5.6.6. Endocrine control of homeostasis:
endocrine system  

Hormones (small proteins/ steroid) can be synthesised by endocrine glands.(Click image)

The gland secretes these hormones into the blood stream

The hormone travels in blood to the target tissue (effector) that brings about a response.

The response modifies the internal environment and this becomes feedback stimuli

  • This systems is slower than that of the nervous system but the response tends to be longer lasting.
  • It should be noted that one hormone can have a number of target tissues and the responses can be quite different from each of the targets.
5.6.7 Control of blood glucose concentration
blood sugar regulation  

Blood sugar concentration is regulated for a number of reason amongst which:

Osmosis. content of a tissue is determined by the concentration of the surrounding tissues.

Respiration: Some tissues are entirely dependent on blood sugar as a respiratory substrate being unable to either store glucose of metabolise fat.


a) Low glucose concentration is detected by the pancreas

b) Alpha cells in the pancreatic islets secret glucagon

c)Glucagon flows through the blood to the liver.

d)Liver responds by adding glucose to blood stream.

h) High blood glucose levels stimulate the beta pancreatic cells

a) Beta pancreatic cells secrete insulin

f)Insulin flows through the blood to the liver

g)Insulin stimulates the liver to remove blood glucose and store this as glycogen (insoluble)

Note from the second diagram that the glucose levels remain within a set of narrow limits

The response and change in blood glucose levels becomes the new stimuli for receptors

This is a typical feedback control.

  blood sugar  regulation
5.6.8 Excretion
  • The removal from the body of metabolic waste from the cells, tissue fluid and blood
  • Metabolic waste is molecules from chemical reactions within the body
  • These waste molecules are toxic in some form and must not be allowed to accumulate in tissues
  • Excretion is not limited to the kidneys and urine production. carbon dioxide is a toxic waste that is excreted by the lungs when we exhale.


5.6.9 Role of the kidney in excretion and the maintenance of water balance

The blood content of water, urea and salts is determined by the filtering mechanism of the kidney.

Blood enters the kidney via the renal artery and its contents are adjusted by removing combinations fob the following molecules:

  • Water
  • Urea
  • Salts

The filtered blood is returned to the main circulation via the renal eatery.

The compounds removed known as urine are collected in the bladder and periodically released to outside the body

If the blood is hypotonic then more water is removed from the blood and the urine produced will be dilute.

If the blood is hypertonic then water is retained in blood and increased levels of salts released. This urine is more concentrated