Importance of homeostasisTo maintain internal environment of organisms especially higher vertebrates in a steady and balanced state.To establish optimum condition of organisms
Benefitsa. Life of organism become less dependent on the external environmentb. The organism can live in a wider range of habitats and the species can live in areas with variable conditionsc. The organism can increase and decrease its metabolic rate according to its requirementsd. A controlled internal environment enables more efficient and economical metabolic reactionse. Enzyme can function at the optimum level ensuring metabolic balance.
Homeostasis refers to the physiological processes by which organisms maintain constant and balanced physical and chemical factors in their internal environment. Internal environment – environment surrounding cell (tissue fluid) Physical factor – temperature, blood pressure and osmotic pressure Chemical factor – pH value, concentration of sugar and salt in blood and tissue fluid. Any variation from normal conditions will initiate homeostatic reaction A series of correction mechanisms will act to restore the physical and chemical composition of the tissue fluids to normal conditions.
Homeostatic control processes include: - negative feedback - positive feedbackAny changes in the physical or chemical factors of the internal environment will be detected by receptors.Part of nervous systemSend information about the changes to the central control system (brain)
Parts of homeostatic control system:a. Receptors – able to detect any changeb. A control centre – define changesc. A mechanism that triggers appropriate corrective actionsd. Effectors – execute the corrective actions
Homeostatic Control System
Negative FeedbackFrom the control centre, action signals are transmitted through the nerve impulses or hormones to the target organ.This will trigger correction mechanism to return the physical and chemical factors to their normal conditions.Conversely, a reduction in the value of the physical or chemical factor will trigger a mechanism to increase the amount of that factor.
Positive FeedbackOccurs when a change in a factor causes the value of the factor to rise or fall even further.Might be harmful or useful.
Example of negative feedback1. Control of blood glucose level2. Body temperature control in Mammals
Control of blood glucose level
1. Control of blood glucose levelNormal concentration of sugar/glucose in human blood is about 90mg per 100cm3 blood.Excess sugar in blood will result in the loss of water from tissues through osmosisLow blood sugar level in blood will cause low blood pressure, fatigue or coma (extreme cases)
Pancreas and liver regulates blood glucose levelGlusose (simple sugar) is carried from the intestine to the liver through the hepatic portal veinIn the liver, glucose maybe: - converted to CO2 and water through cell respiration - converted into glycogen for storage - converted into fat to be stored as diapose tissue - released into the bloodstream
When the blood sugar level arises: - glucose to glycogenWhen the blood sugar level falls: - glycogen to glucose
Changes in blood glucose level is detected by pancreasFunctions as endocrine gland to secrete the hormones insulin and glucagonBoth hormones are secreted by the islets of Langerhans
Islets of Langerhans comprise of 2 types of cells: a. alpha cells (glucagon) b. beta cells (insulin)
When the blood sugar level arises, hormone insulin is secreted into the blood and taken to the liverInsulin in liver causes :a. the speed up the oxidation of glucose to CO2 and water through cell respiration (aerobic respiration) C6H12O6 6CO2 + 6H2O + energy
b. speed up the conversion of glucose to glycogen and fat for storage insulin nC6H12O6 + 6O2 (C6H10O5)n + nH2Oc. to prevent the formation of glucose from glycogen and non- carbohydrate sources The overall effect is that the blood sugar level is lowered to normal
When the blood sugar level drops, the hormone glucagon is secreted into the blood and transported to the liver.Glucagon in liver causes:- Slow down the oxidation of glucose and prevent the conversion of glucose to glycogen- Promote the conversion of stored glycogen into glucose.The overall effect is that the blood sugar level is raised to normal.
2. Body temperature control in mammalsTo enable metabolic cell activities at normal ratesBirds and mammals are homoiotherms (endoterms)Body temperature constant, for mammals is at 370C while birds it is between 40-430CAnimal obtain heat from within the body through physiological metabolic processes.They are called endothermic animals.
When the external temperature rises, the metabolic rate falls and physiological processes such as blood circulation increase to get rid of excess heat from the bodyThus the body temperature does not rise.When the external temperature drops, the reverse happens to prevent the loss of the heat from the body
Heat is loss from the bodies of humans and these animals throughi. conduction, convection and radiation from the skinii. Evaporation of sweat produced by the skiniii.Urination and defecationiv. Exhalation
Conversely, they obtain heat throughi. Heat absorption from the environmentii. Heat production through cell respiration
There are receptors in the skin that are sensitive to temperature changes in the external environmentThese receptors are called thermoreceptors.Thermoreceptors comprise of 2 primary types:i. The bulb of Krause (cold stimuli)ii. The organ of Ruffini (heat stimuli)
Stimuli received by these thermoreceptors are sent in the form of impulses through afferent nerves to the hypothalamus of the brain (act as the heat regulatory centre)The hypothalamus also senses temperature changes in the body.After interpretation, message are sent to effector organs such as skin arterioles, hair/fur erector muscles, sweat glands, skeletal muscles and specific endocrine glands through efferent nervesThese effector organs then respond to regulate the body temperature.
Homeostatic Organs Kidney Liver
LiverLiver Structure- Food is processed and stored in liver before distributed to other parts of the body- Metabolic centre of the body- Received blood supply through 2 main blood vessels: a) hepatic artery – bring oxygenated blood from the dorsal aorta b) hepatic portal vein – bring nutrient-rich blood from the small interstine
Liver is made up of many cylindrical lobules (each 1mm diameter)Each lobule made up of rows of liver cells , branch radially from the centre to the peripheryLiver cell (hepatocyte) is undifferentiated cell, have identical structure.
• interlobular blood vessels – branches of the hepatic artery andhepatic portal veins• intralobular vein – centre of each lobule, branches of hepaticvein merged
Blood from interlobular blood vessels passes through sinusoids in the liver tissue to the intralobular vein.The bile duct branches into a network of fine vessels called canaculli which pass between the cells of the lobule.
Oxygenated blood from hepatic artery and nutrient-rich blood from hepatic portal vein pass from the interlobular blood vessels through the sinusoids into the intralobular vein.The liver cells extract the required substances from the following blood and release their processed products into the blood.Bile also produced by liver cells, passes directly into the canaliculi to be taken to the bile duct.Kupffer cells – specialised phagocytic cells - found on the wall of sinusoids - function in destroying old red blood corpuscle
Structure of Liver Lobules
Homeostatic Functions of Livera) Regulation of blood glucose level in the metabolism of carbohydrateb) Regulation of the amino acids and proteins (the urea/ornithine cycle)c) The formation of uread) Regulation of fatty acidse) Detoxification
a) Regulation of blood glucose level in the metabolism of carbohydrate - excess glucose in the blood is converted into glycogen by insulin for storage in the liver - this process is called glycogenesis
- when the level of blood glucose falls below a certain cruciallevel, glycogen is broken down into glucose in the liverthrough a process called glycogenolysis.- requires activation of the enzyme by glucagon
b) Regulation of the amino acids and proteins (the urea/ornithine cycle) - excess amino acids and proteins cannot be stored in the body - must be returned to the liver and broken down into non- nitrogenous (keto acid) and nitrogenous parts (amino group, -NH2) - through the process of deamination
In the liver, non-nitrogenous keto acids are either: - converted into glycogen - broken down in cellular respiration to release heatThe nitrogenous part, the amino group (-NH2) undergoes: - used to synthesise organic bases (guanine, cytosine, thymine, uracil) – component of nucleotides - combined with a keto acid to synthesise another amino acid through process transamination - converted and excreted as urea – less toxic than ammonia through process detoxification
c) The formation of urea - urea is less soluble than ammonia - much less toxic than ammonia - less water is needed for safe elimination of urea from the body compared to ammonia - urea is synthesised from ammonia and carbon dioxide by utilising amino acid, ornithine - Arginine, an intermediate amino acid formed during the cycle is split by enzyme action into urea and ornithine - this process is called ornithine cycle
d) Regulation of fatty acids - liver processes fatty acid and glycerol to produce lipid - transport them to fat storage area to form adipose tissues - liver cell extract cholesterol and phospholipids from blood to be added to bile – used for digestion lipid - liver cells also involved in making chemical changes to lipids and mobilisation of lipids as energy substrates
e) Detoxification - drugs, toxins and poisons taken in with diet/ produced by bacteria in the body are transported in the bloodstream to the liver - harmful if accumulated in the body - the harmful substances are biochemical broken down in the liver before delivered elsewhere in the body
Kidney- Primary organ for maintaining homeostatic balance in mammals- Play importance role in osmoregulation and excretion- Osmoregulation- - control the vol of water, ion concentration, osmotic pressure and pH of blood- Excretory Organ- - excrete toxic substances, urea
Kidney and nephron structure - pair of kidney, one on each side of the vetebral column below the liver and behind the lining of the abdominal activity
- bean shaped, 10cm long, 6cmwide, 3cm thick, weigh 150g- each kidney supplied by renalartery and drained by renal vein- form afferent and efferentarterioles and network of capillaries- each kidney has two section :cortex and medula- internal structure is made up ofnephrons- nephrons is 3cm length anddivided into 6 section with specificfunctions
- glomerulus consists of a networkof capillaries between the afferentarterioles and efferent arterioles- enclosed by cup-like capsulecalled Bowman’s capsule- the bowman’s capsule surroundsthe glomerulus form the firstregion of the nephrons calledMalphigian corpuscle (body)- Malphigian corpuscle leads toproximal convulated tubule,loop of Henle, followed bydistal convulated tubule andcollecting duct.
- afferent arterioles supplies blood to glomerulus- from glomerulus, blood is carried by the efferent arteriolesforms networks of capillaries to serve the proximal convulatedtubules, distal convulated tubule- one single straight capillary network called vasa recta servedthe limbs of the loop of Henle
a) Ultrafiltration - in glomerulus, high hydrostatic pressure because of diameters of the efferent arterioles and capillaries are narrower than the diameter of afferent arterioles - filtration pressure – hydrostatic pressure - forces almost all contents of the blood through the pores of capillaries and seive-like inner epethelial wall of Bowman’s capsule into the lumen of capsule - glomerulus filtrate – same as blood plasma (but no plasma protein and blood cells) - they cannot pass through the pores in the capillaries and inner wall of Bowman’s capsule (big size)
b) Selective reabsorption - reabsorption of almost all the glucose and free amino acids from the glomerulus filtrate occurs in the proximal convulated tubule through active transport - against concentration gradient – requires ATP - tubule wall is one cell thick, brush like-border of microvili and lots of mitochondria in epithelial cells lining the wall. - pinocytotic process – to remove any blood protein left in the filtrate - result: reduced vol. of filtrate, isotonic to body fluid passes into decending loop oh Henle (urine concentration)
c) Secretion of toxic substance - poisonous by-product of metabolism - added into the filtrate by active transport from the cell in the proximal convulated tubuled) Differential permeability - certain substance such as water, inorganic ions and urea cannot be diffused or transported back into the blood - Antidiuretic hormone (ADH) control impermeability of collecting duct and distal convoluted tubule wall
Water balance and urineconcentration- Water is loss during excretion through formation of urine- Mammals and humans are capable of producing urine that is more concentrated (hypertonic) than blood plasma- Loop of Henle plays important role in the conservation of water and structurally modified for the production of hypertonic urine
- Each loop of Henle consists of 2 parallel limbs: a) descending limb b) ascending limb
- The parallel descending and ascending limbs of the loop of Henle are very close together- They have different permeability to water and solutes- Descending limb (thin segment) permeable to water, impermeable to solutes/ions- High osmotic concentration- Ascending limb (thick segment) impermeable to both water and ions
- In thick segment of ascending limb, sodium and chloride ions actively transported/pumped out from the filtrate into the peritubular fluid (tissue fluid surrounding nephrons)- Osmotic gradient occurs between the filtrate and peritubular fluid- In the thin segment of the ascending limb, some sodium and chloride ions diffuse into the peritubular fluid - contribute to osmotic gradient
Countercurrent multiplier mechanism- Countercurrent: fluid in each limb moving in opposite direction- Multiplier: effect increases as fluid movement continues- Highest osmotic concentration is at the hairpin bend of loop- Osmotic concentration is reduced as the filrate reach the end of ascending limb- Result: hypotonic filtrate
- Urine is further concentrated in the distal convulated tubule and collecting duct- Wall of collecting duct is permeable to water but not to salt- More water is drawn out by osmosis as the fluid flows through the osmotic gradient- Osmotic gradient provide forces but ADH determine the rate of urine concentration High ADH - collecting duct becomes more permeable to water - increase in water removal - Result: Less but more concentrated urine excreted Low ADH - decrease water removal - result: larger vol of more dilute urine exreted
Role of hormones in water regulation and osmoregulation of mineral ions Antidiuretic hormones (ADH) - secreted by posterior lobe of pituitary gland - influence water absorption in kidney tubules - increase in osmotic pressure of blood will stimulate specialised sensory receptors called osmoreceptors in the hypothalamus in the brain - osmoreceptors send impulses through nerves to the posterior lobe of the pituitary gland - ADH is secreted into the blood and carried to the kidney - influences the reabsorption of water from the glomerular filtrate - diabetes insipidus : excessive and frequent production of urine
Regulation of sodium absorption and potassium secretion by aldosterone - regulated by hormone aldosterone secreted by adrenal cortex - occur when the vol of body fluid drops because of dehydration - total sodium ion content also drop - adrenal gland is stimulated to secrete aldosterone - reabsorption of sodium ions and water - reduce production of urine - maintain the water balance of body fluids - deficiency of aldosterone reduces the reabsorption of sodium - sodium loss through urine
Aldosterone -stimulated the secretion of potassium in the distal convulated tubule - also has the ability to stimulate sodium potassium ATPase activity in the cell of the distal tubule - also has the ability to alter the apical (urinary) membrane conductance of potassium in these cells i) in the absence of aldosterone, potassium ion content in the body and plasma are increased due to a decrease in the renal excretion of potassium ii) in the presence of excess aldosterone, potassium ion content in both body and plasma are decreased iii) an increased potassium content in plasma stimulates aldosterone secretion and a decreased content suppresses it