Homeostasis

• Homeostasis is the process by which animals and plants maintain an internal environment that promotes proper cellular function.

• The body’s internal environment is made up of the interstitial fluid that surrounds all cells and tissues, and the plasma in the blood.

• A typcial adult has 15 L of extracellular fluid.

• In homeostasis, the intracellular fluid (inside the cells) isn’t considered.

• Homeostasis is an ongoing dynamic process that acts in response to both internal and external conditions.

• For example maintaining an optimal internal temperature even though the external temperature varies greatly.

• Other conditions that must be maintained include:– Hormone levels, blood pressure, blood flow, pH,

glucose concentration and other solutes in the blood.– For some factors the tolerable range is very narrow.

Negative Feedback Mechanisms

• Negative feedback is the response of a system that acts to maintain equilibrium by compensating for any changes made to the system.

• Homeostatic mechanisms include three elements: a sensor, and integrator, and an effector.

• The sensor consists of tissues or organs that detect any change – or stimulus in external or internal factors, such pH, termperature, or concentration of molecules (glucose)

• Once the sensore gathers information, it is transmitted to the integratro which acts as processing or control centre.

• If the environmental condition is outside the set point,the integrator activates the effector, which is the system that returns the measured condition back to within the set point.

Thermostat Example

• Thermostat that maintains a constant temperature in most homes is an example of negative feedback using antagonistic effectors.

• Antagonistic effectors means they act to produce the opposite effect of the change recorded by the sensor.

Thermoregulation

• Mammals and bird maintain body temperature within a narrow range using a homeostatic mechanism.

• Groups of neurons in a region of the anterior hypothalamus receive information from thermoreceptors in various locations – skin, spinal cord, hypothalamus

• The range in set point is 35 to 37.8 °C

If temperature is below set point…

• The hypothalamus activates effectors that induce vasoconstriction in the skin.

• Less blood flow near skin, les thermal energy lost to environment.

• Additional effectors may include shivering, which raises body temperature.

If temperature is above set point…

• Hypothalamus triggers effectors that induce vasodilation in the skin, increasing blood flow and allowing thermal energy to be dissipated to the environment.

• Other effectors cause our body to sweat, which causes loss of thermal energy as the sweat evaporates.

• New set point for times when a person has an infection – fever – helps the body to fight off bacteria or viruses.

Regulation of body temperature• Thermoregulation

• 4 physical processes:

• Conduction~transfer of heat between molecules of body and environment

• Convection~transfer of heat as water/air move across body surface

• Radiation~transfer of heat produced by organisms

• Evaporation~loss of heat from liquid to gas

• Sources of body heat:

• Ectothermic: determined by environment

• Endothermic: high metabolic rate generates high body heat

Regulation during environmental extremes

• Torpor~ low activity; decrease in metabolic rate

• 1- Hibernation long term or winter torpor

(winter cold and food scarcity); bears, squirrels

• 2- Estivationshort term or summer torpor (high temperatures and water scarcity); fish, amphibians, reptiles

• Both often triggered by length of daylight

Positive Feedback Systems

• Mechanism which increase the change in environmental condition.

• Usually does not result in homeostasis, instead system becomes unstable.

• Example: an animal is attacked, the body releases adrenaline and other hormones into the blood to prepare organ systems for “fight or flight” response. The release of chemicals stimulates further release.

Positive Feedback Example

Water Balance

• Animals need to regulate their extracellular fluid.

• Freshwater animals

– Show adaptations that reduce water uptake and conserve solutes

• Desert and marine animals face environments that dry them out

– Potential to quickly deplete the body water

Review Osmosis

• Osmosis is the movement of water molecules from a region of higher concentration to a region of lower concentration.

• The different concentrations of water are a result of the different numbers of solute molecules (and a selectively permeable membrane)

• Hypertonic (hyperosmotic), hypotonic (hypoosmotic), isotonic (isoosmotic) solutions and direction of water…

Osmoregulation

• Regulates the solute concentrations and balances the gain and loss of water.

• Excretion – gets rid of metabolic wastes

• Osmoregulation balances the uptake and loss of water and solutes

• Is largely based on controlled movement of solutes between internal fluids and the external environment

• Cells require a balance between osmotic gain and loss of water.

• Water uptake and loss are balanced by various mechanisms of osmoregulation in different environments.

• Osmoconformers (marine animals) are isoosmotic to their environement

• Osmoregulators expend energy to control water uptake and loss in a hyperosmotic or hypoosmotic environment.

• An animals nitrogenous waste products may have a large impact on its water balance.

• The nitrogenous wastes that make up the largest portion are the breakdown products of proteins and nucleic acids.

• Depending on the organism, the nitrogenous wastes are released as ammonia, urea, or uric acid.

• Animals that excrete waste as ammonia:

– Need access to lots of water – because ammonia is fairly toxic

– Includes fish who release it across whole body surface or even through gills.

• The liver of mammals convert ammonia to less toxic urea, which is carried to the kidneys, concentrated and excreted with minimal loss of water.

• Uric acid is excreted by insects, reptiles and birds

• Uric acid is largely insoluble in water and can be secreted as a paste with little water loss

Excretory Processes• Most excretory systems produce urine by refining a

filtrate derived from body fluids.• There are excretory tubules and blood vessels.

Filtration. The excretory tubule collects a filtrate from the blood.Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule.

Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids.

Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule.

Excretion. The filtrate leaves the system and the body.

Capillary

Excretorytubule

FiltrateU

rine

1

2

3

4

1. Filtration: the excretory tubule collects a filtrate from the blood. Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule.

2. Reabsorption: The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids.

3. Secretion: Other substances, such as toxins and excess ion, are extracted from body fluids and added to the contents of the excretory tubule.

4. Excretion: The filtrate leaves the system and the body.

• Kidneys are the excretory organs of vertebrates, and the also function in osmoregulation – water balance and salt regulation.

• The functional unit of the kidney is the nephron and its associated blood vessels.

• Each kidney

– Is supplied with blood by a renal artery and drained by a renal vein

Figure 44.13a

Posterior vena cava

Renal artery and vein

Aorta

Ureter

Urinary bladder

Urethra

(a) Excretory organs and majorassociated blood vessels

Kidney

• Urine exits each kidney through a duct called the ureter.

• Both ureters drain into a common urinary bladder.

(b) Kidney structure

UreterSection of kidney from a rat

Renalmedulla

Renalcortex

Renalpelvis

Figure 44.13b

• The mammalian kidney has two distinct regions

– An outer renal cortex and an inner renal medulla

• The nephron, the functional unit of the vertebrate kidney– Consists of a single long tubule and a ball of capillaries

called the glomerulus

Figure 44.13c, d

Juxta-medullarynephron

Corticalnephron

Collectingduct

To renalpelvis

Renalcortex

Renalmedulla

20 µm

Afferentarteriolefrom renalartery

Glomerulus

Bowman’s capsule

Proximal tubule

Peritubularcapillaries

SEM

Efferentarteriole fromglomerulus

Branch ofrenal vein

Descendinglimb

Ascendinglimb

Loopof

Henle

Distal tubule

Collectingduct

(c) Nephron

Vasarecta(d) Filtrate and

blood flow

Renalcortex

Renalmedulla

20 µm

Afferentarteriolefrom renalartery

Glomerulus

Bowman’s capsule

Proximal tubule

Peritubularcapillaries

SEM

Efferentarteriole fromglomerulus

Branch ofrenal vein

Descendinglimb

Ascendinglimb

Loopof

Henle

Distal tubule

Collectingduct

Vasarecta

(d) Filtrate and blood flow

• Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule.

• Filtration of small molecules is nonselective, and the filtrate in Bowman’s capsule is a mixture that mirrors the concentration of various solutes in the blood plasma.

• From Bowman’s capsule, the filtrate passes through three regions of the nephron:

– The proximal convoluted tubule

– The loop of Henle

– The distal convoluted tubule

• Then fluid from several nephrons flows into a collecting duct.

• Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that subdivides into the capillaries of the glomerulus.

• The capillaries converge as they leave the glomerulus into the efferent arteriole.

• The vessels subdivide again forming the peritubular capillaries surrounding the proximal and distal tubules and the vasa recta surrounding the loop of Henle.

• Following the filtrate along its’ path:

• Proximal tubule – reabsorption in the proximal tubule is critical for recapture of ions, water, and valuable nutrients.

• Na+ is actively transported to transport epithelium, and positive charge movement drives passive transport of Cl-, this causes water to follow by osmosis.

• Salt and water diffuse into the peritubularcapillaries.

Proximal tubule

• Proximal tubule actively and passively transport glucose, amino acids, potassium ions and other essential substances into the peritubularcapillaries.

• Proximal tubule helps maintain relatively constant pH by secreting H+ and making and secreting NH3.

• Also reabsorption of buffer bicarbonate balances pH.

• Location where toxins and drugs from liver are secreted

Descending limp of loop of Henle

• Reabsorption of water continues as filtrate moves into descending limp of loop of Henle.

• Water channels (aquaporin) make the epithelium permeable to water.

• Almost not channels for salt and other small solutes, low permeability.

• For water to move out of the tubule by osmosis, the interstitial fluid must be hyperosmotic to the filtrate.

Ascending limp of the loop of Henle

• Ascending limp of the loop of Henle returns filtrate back toward the cortex.

• Ascending limb has transport epithelium that contain ion channels, but not water channels, making membrane impermeable to water.

• NaCl became concentrated in descending limb diffuses out from the thin segment of ascending limb into interstitial fluid.

Loop of Henle continued…

• In thick segment of ascending limb NaCl is actively transported into interstitial fluid.

• This loss of salt makes the filtrate more dilute as it moves up the cortex.

• The loop of Henle works as counter current exchange because the vasa recta flows in opposite direction to the loop of Henle.

Afferentarteriolefrom renalartery

Glomerulus

Bowman’s capsule

Proximal tubule

SEM

Efferentarteriole fromglomerulus

Branch ofrenal vein

Descendinglimb

Ascendinglimb

Loopof

Henle

Vasarecta

(d) Filtrate and blood flow

Distal tubule

• Distal tubule regulates the K+ and NaCl concentrations.

• Varies the amount of K+ that is secreted into filtrate.

• Varies the amount of NaCl that is reabsorbed from filtrate.

• Like proximal tubule, it also regulates H+ and HCO3-.

Collecting duct

• The collecting duct carries the filtrate through the medulla into the renal pelvis.

• There is hormonal control of permeability of the transport epithelium as the filtrate passes through the collecting duct.

• In conserving water the aquaporin channels allow water to cross, but the epithelium remains impermeable to salt. The filtrate becomes increasingly concentrated.

Collecting duct

• As the collecting duct moves through the medulla it become permeable to urea allowing a small amount to diffuse out, and adding to the high osmolarity of the interstitial fluid in the medulla.

• In producing dilute urine, the kidney actively reabsorbs salts without allowing water to follow by osmosis.

• The osmolarity of urine is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys.

• Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney.