Homeostasis

Homeostasis

• The maintenance of the internal environment in a relatively stable state in the face of changes in the external or internal environment.

• In multicellular organisms when we talk about the internal environment we are considering the interstitial (extracellular) fluid between its cells.

• In unicellular organisms we are considering the internal environment of the cell itself.

Intracellular and Extracellular Fluids• Both tissue fluid and plasma are located outside cells and can be

labelled as extracellular fluids (extra = outside).

• In contrast, the fluid, called cytosol, that is located within all cells can be labelled as intracellular fluid (intra = within).

• Tissue fluid and plasma are separated from each other by the thin flat cells that form the walls of the capillaries, the smallest vessels of the blood circulatory system.

• Extracellular fluids are separated from intracellular fluids by the partially permeable cell membranes. We can think of the various intra- and extracellular fluids as ‘compartments’.

• It is the extracellular fluids, and in particular the tissue fluid, that forms the internal environment for cells of the body.

Changing composition of intracellular and extracellular fluids• Although the tissue fluid and

plasma that make up the internal environment are separated, continuous exchange occurs between them.

• Nutrients (such as glucose) and gases (such as oxygen) pass from the blood to the tissue fluid.

• Waste products (such as carbon dioxide and urea) pass from tissue fluid into the blood.

• In addition, many substances can move between cells and the fluids of their environment.

Purpose of blood tests• Two-way exchanges of many substances can occur between cells,

tissue fluid and plasma.

• This means that the make-up of one fluid can be affected by that of another fluid.

• So, the composition of tissue fluid is affected by the make-up of blood plasma. Similarly, the composition of cell cytosol is affected by the makeup of tissue fluid.

• Blood is a readily accessible body tissue, and the exchange of fluids between compartments means that blood tests can provide information about the state of cells in other parts of the body.

• Alarm is raised if abnormally high (or low) levels of certain substances are detected in a person’s blood. Why?

• These levels will not just be present in the blood cells themselves, but also in the tissue fluid and the other cells of the body which are bathed in tissue fluid.

Why maintain a stable internal environment?

Why maintain a stable internal environment?• Biochemical reactions in living cells can only occur

when pH, various salts and nutrients, and physical conditions are within certain limits.

• Concentrations of substances such as glucose, gases such as oxygen and carbon dioxide, and hydrogen ions in cells and tissue fluids all have an impact on the way cells function.

• Movement of materials across the membrane is affected by concentration of water, nutrient particles and ions on either side of the membrane.

• Activity of enzymes responsible for metabolic activities of cells is affected by temperature.

Homeostasis in different organisms

• The complexity of the mechanisms involved in homeostasis varies according to the complexity of the organism.

• All homeostatic mechanisms require the ability to detect changes in the external or internal environment and respond to them.

Homeostasis in different organisms

• Simple unicellular organisms are able to sense and respond to environmental conditions – generally moving away from heat and light, and towards food.

• Multicellular organisms such as sponges rely on the cellular membrane to regulate concentration of cytoplasmic contents while being bathed directly in the fluids of the external environment.

• Homeostatic mechanisms in multicellular organisms such as plants and animals are more complex, and the basic physiology of multicellular organisms has evolved in order to provide and appropriate internal environment. For example:– Gas exchange: leaves, stomata, root hairs, lungs, gills, skin of

amphibians– Nutrients: roots of plants, digestive systems of animals– Transport within organisms: xylem and phloem in plants, circulatory

systems in animals– Waste: excretory cells and organs

Conformity vs Regulation

• When an organism is faced with a change in its environment, it has two broad categories of response:

1. conform to the change (because it is unable to maintain homeostasis for internal conditions)

2. regulate their internal environment (i.e. maintain homeostasis) over a broad range of external environmental changes by using biochemical, physiological behavioral and other mechanisms (generally there is a maximum ability to do this)

• The ability to regulate their internal environment enables organisms to live in a wider range of conditions.

• Organisms with narrow tolerance limits are often those with little or no ability to regulate their internal environment.

Osmoconformers and Osmoregulators

Hyas araneus – the intertidal decorator crab• Is an osmoconformer• Solute concentration of its body fluids is always

about the same as that of the surrounding water.

Carcinus maenas – common shore crab• Is an osmoregulator• Expends energy to maintain relatively constant

levels of ions and solutes in its body fluids.

Osmoconformers and Osmoregulators

• Animals that osmoregulate are more able to survive the fluctuations of salinity encountered in seashore and estuarine habitats.

• There is a cost in energy in terms of active pumping of salts.

• In contrast, osmoconformers do not have this cost, however, they suffer considerable stress if the concentration of their body solutes decreases due to environmental factors such as increased rain in their rock pool.

• The tolerance limits of osmoconformers are set by the ability of their tissues to function in dilute salt solutions.

Detecting and Responding

• Ability to detect and respond to changes is fundamental to all organisms.

• Changes serve as stimuli.

• Stimuli are detected by receptors.

• Intensity of stimulus must be sufficient to reach the threshold of the receptor (weakest stimulus to which receptor can respond).

• Receptors then stimulate effectors to produce a response.

• Response of the effector influences the magnitude of the stimulus and returns the variable to homeostasis.

• This is known the stimulus-response model.

Stimulus-Response Model

Stimuli

• Both internal and external factors act as stimuli for animals in the maintenance of homeostasis.

• Changes in any of these factors may stimulate a response in the animal that tends towards returning the internal environment to its stable state.

• Internal conditions include:– Chemical factors e.g. levels of oxygen, carbon dioxide, glucose,

ions, wastes and water.– Physical factors such as temperature, blood pressure and

balance.

• External conditions include:– Physical factors such as light, temperature, gravity, sound and

day length.– Chemical factors such as food, oxygen, carbon dioxide, water

and specific chemicals.

Detecting and Responding

• Information comes from detectors functioning at one of two different levels:– Misalignment detectors – monitor a precise factor of

the internal environment that is being controlled and detect when a particular factor is out of line.

– Disturbance detectors – detect the presence of external or internal changes that are likely to cause a change in the particular factor of the internal environment being controlled. They warn of problems before they actually arise.

• Regulation of body temperature involves both misalignment and disturbance detectors.

Regulation of body temperature

Types of Receptors• Chemoreceptors - detect chemicals

– Olfactory lining in nose; taste buds; oxygen concentration receptor in aorta; osmoreceptors in hypothalamus; glucose level receptors in pancreas; pH/CO2 receptors in medulla, aorta and carotid arteries

• Mechanoreceptors – detect pressure and movement– Ear; touch & pressure receptors in skin muscles, joints and

connective tissue; muscle length receptors in skeletal muscle; muscle tension receptors in tendons; joint receptors; venous pressure receptors; arterial pressure receptors; lung inflation receptors; lung deflation receptors; lung irritant receptors.

• Photoreceptors – detect light– Eye.

• Thermoreceptors - detect temperature– Heat receptors and cold receptors in skin; body temperature

receptor in hypothalamus.

Feedback systems

• Homeostasis or regulation therefore involves fluctuations around a set-point.

• The size of the fluctuations depends on the sensitivity and location of the sensory receptors, the tolerance of the control centre to variation from the set-point, and efficiency of the response mechanism.

• Most biological feedback systems are negative feedback systems which operate as proportional control systems – the size of the response is proportional to the size of the stimulus.

Feedback System

Negative Feedback

• Most common type of biological feedback system.• Activity of effector opposes stimulus.• Effector produces opposite effect of stimulus.

• Example: – Home heating system. The temperature of the home is

monitored and heating will be turned off until the temperature returns to set level.

• Biological examples: body temperature and blood glucose levels

Positive Feedback

• Not particularly common in biological systems.

• Activity of effector reinforces stimulus.• Effector produces response in same

direction as stimulus• Must have a mechanism to halt the cycle

• Biological examples: childbirth and blood clotting

Detecting and Responding to Change

Body systems involved in regulation of homeostasis• Nervous system

– Receives and transmits information about both the external and internal environment. Transmits electrical impulses to body cells that respond in various ways.

• Endocrine (hormonal) system– Produces hormones that are secreted directly into the bloodstream and

transported throughout the body where they regulate cell activities.

• Respiratory system– Obtains oxygen from air and eliminates carbon dioxide which is a waste

product of metabolism of cells. Assists in regulation of pH through removal of carbon dioxide.

• Circulatory system– Transports O2 to cells, CO2 away from cells, and hormones, wastes and

nutrients such as glucose, amino acids and fatty acids throughout the body. Has a central role in maintaining the environment of cells within desirable limits.

Body systems involved in regulation of homeostasis• Digestive system

– Obtains nutrients, water and salts from the food we eat. These are transferred to blood and lymph vessels in the intestine wall. Undigested residue is eliminated.

• Excretory system– Removes waste such as urea, excess

water, salt and other ions from the blood and eliminates them from the body in the form of urine. Important in regulating pH.

• Integumentary system– Barrier between body and external

environment. Evaporation of sweat is important in temperature regulation. Also inhibits entry of micro-organisms.

Some systems controlled by homeostasis

Control of Requires regulation of

nutrient levels

(e.g. glucose)

•nutrient intake

•digestive and circulatory system functions

•storage and mobilisation of nutrients

•behaviour

body temperature

•general metabolism

•blood flow to tissues

•muscle activity and sweating

•behaviour

water and salt balance

•excretion of water and salts to maintain correct osmotic concentration of internal body fluids

•behaviour

metabolic rate

•lung ventilation and circulation to deliver adequate oxygen to tissues

•nutrient intake and storage

•behaviour

Homeostasis in the Newborn

• Research has shown that the regulatory systems of newborn babies mature at different rates.

• Premature babies need particular care as they have poor temperature control and brain and breathing functions are underdeveloped.

Regulatory role of the liver

• Eating a meal results in a considerable disturbance to the internal environment.

• Blood leaving the gut suddenly contains very high concentrations of nutrients such as simple sugars and amino acids. If this blood simply passed into the circulation, blood composition would be drastically altered and impair normal function.

• Instead the blood passes via the hepatic portal vein to a second network of capillaries in the liver, which is one of the most important homeostatic organs in the body.

• The composition of blood is regulated by the liver before it passes into the hepatic vein and then back to the heart.

• The liver plays and important role in storing and mobilising nutrients in a regulated manner so that blood levels are maintained within relatively narrow limits.

Regulatory functions of the liver

Controlling Blood Glucose • The pancreas produces two hormones, insulin and glucagon — each

produced in special cells in the pancreas.

• These hormones are involved in the control of glucose in the blood. The hormone insulin controls the uptake by cells of glucose from the blood. The hormone glucagon acts on the liver to release more glucose into the blood.

• If the blood glucose level falls below normal, the pancreas usually responds in two ways:

1. Some cells, called alpha cells, increase their production of the hormone glucagon which acts on the liver to convert stored glycogen to glucose. Glucose passes from the liver into the bloodstream.

2. Other cells, called beta cells, decrease their production of insulin. Less insulin in the blood results in less glucose being taken from the blood by cells of the body.

• These events cause the blood glucose level to rise. This is detected by the pancreas which then responds by increasing the insulin and decreasing the glucagon it produces.

• A steady state is achieved, although there are small fluctuations. The steady state of glucose in the blood is produced as a result of negative feedback mechanisms involving both insulin and glucagon.

Controlling Blood Pressure

Challenges to Homeostasis

• Homeostatic mechanisms must operate to coordinate appropriate integrated responses to different situations.

• Examples of challenges for this integrated coordination include:– Exercise: results in transient changes in oxygen demand, blood

glucose levels, blood pressure and distribution, body temperature and water and salt. It can also produce an oxygen debt.

– Pregnancy: particularly during the latter stages, increases nutritional requirements, the physical workload associated with weight gain, and the demands on kidney function.

– Space: The lack of gravity experienced by astronauts reduces the work done by the cardiovascular and musculoskeletal systems. This weakens bones and muscles and reduces cardiovascular fitness.