Chapter 15

• Homeostasis 


  maintenance of a constant internal environment, despite external changes
• Negative feedback

  • process that reverse changes in internal conditions to ensure optimal steady state is maintained & internal environment is returned to original set of conditions
  • change detected by sensory receptors & as a result, effectors work to reverse the change & restore conditions to their base level

  
• Positive feedback 


  • sensory receptors detect a change in the internal environment & effectors are stimulated to reinforce that change and increase the response
  • e.g. in the blood clotting cascade

  
• Difference between receptors & effectors

  • receptors detect stimuli
  • effectors produce changes required
• What is an ectotherm?


  an animal that depends on external sources of heat to determine body temperature
• Behavioural responses of ectotherms to warm up

  • basking (orientating bodies so max surface area is exposed to sun)
  • pressing bodies against warm ground to gain heat through conduction
  • changing body shape to increase surface area to gain heat
• Behavioural responses of ectotherms to cool down 


  • shelter from sun by seeking shade or burrowing
  • press bodies against cool stones to lose heat through conduction
  • orientate bodies so minimum surface area is exposed to sun
  • minimise movements to reduce metabolic heat generated
• Physiological responses of ectotherms to warming

  • dark pigments to absorb radiation
  • alter heart rate to increase or decreases metabolic rate & to affect warming or cooling across the body surfaces
• Advantages of ectothermy

  • use less energy regulating their temperatures so have lower food requirements
  • can survive in some difficult habitats where food is in short supply
  • a greater proportion of energy intake can be used for growth
• Disadvantages of ectothermy

  • lower activity levels in cold temperature can lead to greater risk of predation
• What is an endotherm?

  an animal that can use internal sources of heat to control body temperature
• How does the body detect temperature changes in endotherms?

  
  

  • peripheral temperature receptors in the skin detect changes in the surface temperature
  • temperature receptors in the hypothalamus detect the temperature of the blood deep in the body
  • combo of the two gives body great sensitivity & allows it to respond to actual changes in the temp of the blood as well as to pre-empt possible problems that might result from changes in the external environment
• What do temp receptors in the hypothalamus act as?

  • the thermostat of the body
  • control responses that maintain the core temp in a dynamic equilibrium to within about 1°C of 37°C
• Physiological adaptations endotherms use to cool down

  • vasodilation - arterioles near surface of skin dilate, forces more blood to flow through capillaries close to surface of skin, more heat is radiated from the body
  • increased sweating - as sweat evaporates from the surface of skin heat is lost, cooling the blood below the surface
  • erector muscles relax so hairs/ feathers lie flat on the skin - avoids trapping an insulating layer of air
• Why do some animals pant to cool down?

  • sweat glands are restricted to the less hairy areas of the body such as the paws
  • panting results in losing heat as the water evaporates
• Anatomical adaptations of endotherms to cool down

  • large SA:V ratio to maximise cooling (e.g. large ears & wrinkly skin)
  • pale fur or feathers to reflect radiation
• How do endotherms warm up?

  • vasoconstriction - arterioles near surface of skin constrict, little blood flows through capillaries near skin surface, less heat is radiated from the body
  • decreased sweating - less heat lost by evaporation of water from surface of skin
  • erector muscles contract, pulling hair or feathers upright - traps an insulating layer of air & so reduces cooling through the skin
  • shivering - involuntary contracting & relaxing of large muscles - metabolic heat from the exothermic reactions warm up the body
• Anatomical adaptations of endotherms to warm up

  • adaptations that minimise SA:V ratio to reduce cooling e.g. small ears
  • thick layer of insulating fat underneath skin
• Behavioural adaptations of endotherms in cold climates to keep warm

  • hibernate - build up fat stores, build well-insulated shelters & lower their metabolic rate
  • female polar bears dig dens in the snow and remain in them warm & insulated while they give birth to their cubs
• Importance of homeostasis

  • enzyme activity - conditions need to be optimum
  • cell size - changes in water potential of blood could cause cells to desiccate or swell & burst
  • independence from external conditions - so animals can maintain constant level of activity regardless of environment
• Main metabolic waste products in mammals

  • Carbon dioxide - produced by respiration, excreted from lungs
  • Bile pigments - excreted in bile from liver into the small intestine
  • Nitrogenous waste products (urea) - produced by deamination (breakdown of excess amino acids), excreted by kidneys in urine
• The liver

  • one of main organs involved in homeostasis
  • makes up about 5% of total body mass
  • lies just below diaphragm
  • is made up of several lobes
  • has a rich blood supply - about 1 dm3 blood flows through every minute
• Blood supply to liver

  • oxygenated blood is supplied to liver by the hepatic artery & returned to heart in the hepatic vein
  • blood also supplied by the hepatic portal vein (HPV) - carries blood w the products of digestion straight from intestines to liver - this is starting point for many metabolic activities of the liver
  • up to 75% of blood in liver comes via the hepatic portal vein
• Structure of liver cells (hepatocytes)


  • large nuclei
  • prominent Golgi apparatus
  • lots of mitochondria for generating required ATP
  • able to divide & regenerate to repair damaged parts of liver
• Blood supply in sinusoids 


  • blood from hepatic artery & hepatic portal vein is mixed in spaces called sinusoids which are surrounded by hepatocytes
  • mixing increases oxygen content of blood from hepatic portal vein, supplying hepatocytes with enough oxygen for their needs
• Structure of sinusoids 


  • contain Kupffer cells which act as the resident macrophages of the liver, ingesting foreign particles and helping protect against disease
  • hepatocytes, lining sinusoids, secrete bile from the breakdown of blood into spaces called canaliculi, and from these the bile drains into the bile ductules which take it to the gallbladder
• 4 functions of the liver

  • carbohydrate metabolism
  • transamination
  • deamination of excess amino acids
  • detoxification
• Carbohydrate metabolism 


  • when blood glucose levels rise, insulin levels rise & stimulate hepatocytes to convert glucose to glycogen
  • when blood glucose levels fall, glycogen stores are converted to glucose
• Transamination 


  • conversion of one amino acid into another
  • needed as our diet doesn't always contain required balance of amino acids so this balances it all out
• Deamination of excess amino acids 


  • removal of an amine group from a molecule, converting it to ammonia - ammonia then converted to urea in the ornithine cycle by reacting w CO2
  • Happens because our body can’t store excess amino acids - if not for action of hepatocytes excess would be excreted & wasted
  • the remainder of the amino acid molecule can be used in respiration or converted to lipids for storage 
• Detoxification 


  • Levels of toxins in body always tend to increase
  • Urea & many other metabolic pathways produce potentially poisonous substances 
  • We alo take in wide variety of toxins by choice e.g. alcohol & other drugs 
  • Liver detoxifies these substances - 2 examples:
  • Breakdown of hydrogen peroxide, a metabolic waste product - split into water & oxygen using catalase in hepatocytes
  • Hepatocytes contain alcohol dehydrogenase which breaks down ethanol into ethanoate which can be used in respiration or used to make new fatty acids
• 3 main areas of the kidney 


  • cortex: dark outer layer - where filtering of blood takes place
  • medulla: lighter inner region - contains tubules of the nephrons
  • pelvis: central region - where urine collects before leading out down the ureter
• Label the gross structure of the kidney

  A) cortex

  B) medulla

  C) pelvis

  D) renal artery

  E) renal vein

  F) ureter

  
• Nephrons 


  • the functional units of the kidney
  • where blood is filtered
  • kidney contains around 1.5 million nephrons
• Name the 5 main structures of the nephron

  • bowman's capsule
  • proximal convoluted tubule
  • loop of henle
  • distal convoluted tubule
  • collecting duct
• How does ultrafiltration take place? part 1

  • high blood pressure in the glomerulus (caused my narrowing capillaries) creates high hydrostatic pressure in capillaries compared to bowman's capsule which brings about filtration
  • the capillaries have endothelium pores to allow substances through
  • fluid then passes through basement membrane - network of collagen & glycoproteins - acts as filtration membrane
• How does ultrafiltration take place? part 2 


  • podocytes in wall of bowman's capsule give support & act as an additional filter
  • podocytes have extensions called pedicels that wrap around the capillaries to form slits, preventing large plasma proteins & other cells, that may have passed through the basement membrane, from entering the tubule itself
  • The filtrate will contain glucose, salt, urea, water, & other substances in the same concentration as they present in the blood plasma
  • Up to 20% of the plasma contents leave the blood during this process
•  What takes place in the proximal convoluted tubule?

  • selective reabsorption
  • All of glucose, amino acids, vitamins & hormones are moved from filtrate back into blood by active transport
  • 85% of sodium chloride & water is reabsorbed as well - sodium ions are moved by active transport while chloride ions & water follow passively down concentration gradients
•  Selective reabsorption in the PCT?

  • sodium ions are actively transported out of cells lining PCT into tissue fluid - lowers concentration of sodium ions inside cell - creating a concration gradient
  • causes sodium ions to diffuse passively into PCT - also drags along glucose or amino acids via facilitated diffusion using sodium-glucose co-transporter proteins - allows glucose or amino acids to move into PCT simultaneously
  • glucose & amino acids diffuse into blood capillary
• Adaptations of the PCT 


  • covered with microvilli - increase surface area over which substances are absorbed
  • have many mitochondria to provide ATP needed in active transport systems
  • plasma membranes have many pumps & transporter proteins for active transport & facilitated diffusion
  • tight junctions between cells to ensure transcellular movement - prevent waste products leaking out of PCT into blood
  • Infoldings of basal membrane - inc surface area to allow movement of substances into blood