Homeostasis

HOMEOSTASIS

Importance of homeostasis

To 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 conditions

c. The organism can increase and decrease its metabolic rate according to its requirements

d. A controlled internal environment enables more efficient and economical metabolic reactions

e. 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 feedback

Any 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 feedback

1. Control of blood glucose level2. Body temperature control in Mammals

Control of blood glucose level

1. Control of blood glucose level

Normal 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 osmosis Low blood sugar level in blood will cause

low blood pressure, fatigue or coma (extreme cases)

Pancreas and liver regulates blood glucose level

Glusose (simple sugar) is carried from the intestine to the liver through the hepatic portal vein

In 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 glycogen

When the blood sugar level falls:- glycogen to glucose

Changes in blood glucose level is detected by pancreas

Functions as endocrine gland to secrete the hormones insulin and glucagon

Both 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 liver

Insulin 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

nC6H12O6 + 6O2 (C6H10O5)n + nH2O

c. 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

insulin

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 mammals

To enable metabolic cell activities at normal rates

Birds and mammals are homoiotherms (endoterms)

Body temperature constant, for mammals is at 370C while birds it is between 40-430C

Animal 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 body

Thus 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 through

i. conduction, convection and radiation from the skin

ii.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 environment

These 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 nerves

These effector organs then respond to regulate the body temperature.

Homeostatic Organs

Kidney

Liver

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 aortab) 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 periphery

Liver cell (hepatocyte) is undifferentiated cell, have identical structure.

• interlobular blood vessels – branches of the hepatic artery and hepatic portal veins

• intralobular vein – centre of each lobule, branches of hepatic vein 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 Liver

a) Regulation of blood glucose level in the metabolism of carbohydrate

b) 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 crucial level, glycogen is broken down into glucose in the liver through 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 heat

The 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, 6cm wide, 3cm thick, weigh 150g- each kidney supplied by renal artery and drained by renal vein- form afferent and efferent arterioles and network of capillaries- each kidney has two section : cortex and medula- internal structure is made up of nephrons- nephrons is 3cm length and divided into 6 section with specific functions

- glomerulus consists of a network of capillaries between the afferent arterioles and efferent arterioles - enclosed by cup-like capsule called Bowman’s capsule- the bowman’s capsule surrounds the glomerulus form the first region of the nephrons called Malphigian corpuscle (body)- Malphigian corpuscle leads to proximal convulated tubule, loop of Henle, followed by distal convulated tubule and collecting duct.

Nephrons

- afferent arterioles supplies blood to glomerulus- from glomerulus, blood is carried by the efferent arterioles forms networks of capillaries to serve the proximal convulated tubules, distal convulated tubule - one single straight capillary network called vasa recta served the limbs of the loop of Henle

Urine Formationa) Ultrafiltrationb) Selective reabsorptionc) Secretion of toxic substancesd) Differential permeability

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 tubule

d) 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 urine concentration

- 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 limbb) 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

Osmoregulation

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 potassiumii) in the presence of excess aldosterone, potassium ion content in both body and plasma are decreasediii) an increased potassium content in plasma stimulates aldosterone secretion and a decreased content suppresses it