Homeostasis

Homeostasis : Homeostasis refers to maintenance of a constant internal environment, despite external changes. It is the process by which the conditions of the internal environment are maintained within very narrow limits.

This allows organisms to survive and remain active in a range of environments as optimum conditions for enzyme activity are always provided and respiration is not affected. The body’s metabolic rate is kept constant. Negative feedback occurs as a result of homeostasis.

KIDNEY

Functions :

·                Excretion

·                Osmoregulation (homeostasis)

Excretion : Excretion refers to removal of waste products of metabolism. If they build up in concentration, it would be toxic. They must be got rid of before they upset themselves. For example –

·                urea

·                carbon dioxide

·                uric acid

·                water

·                excess salts

·                hormones

Animals lose water by

·                sweating

·                urination

·                exhalation

·                egestion / defecation

·                crying / spitting / bleeding

Amount of water in blood is kept constant by osmoregulation and homeostasis. Water input varies with

·                diet

·                the amount of water drunk

Output varies with

·                sweating

·                level of activity of the individual

·                external temperature

·                illness, for example, vomiting and diarrhoea

Note : Excretory products, like urea, are removed from the body in large amounts.

Structure and Functions of the parts of the human urinary system : The human urinary system consists of two kidneys. Each kidney consists of many kidney tubules called nephrons. Ureters carry urine from kidneys to the bladder. The bladder stores urine. There are sphincter muscles at the exit of the bladder, that relax to expel urine from the body, when the bladder contracts. Urethra carries urine from the bladder to the exterior.

The kidneys are attached to the abdomen, below the diaphragm to the back of the abdominal wall.

Note : Smooth, involuntary, unstriped, unstriated muscles are found in the bladder wall.

Composition of urine :

·                water

·                urea

·                minerals or salts

·                uric acid

·                creatinine

·                hormones

·                alcohol / toxins

·                bilirubin

·                amino acids

·                vitamins

·                ions

Urine formation :

Wastes, like urea, dissolved in blood plasma are carried to the knot of capillaries, called glomerulus. High blood pressure is generated by the heart. Blood vessels become narrow to increase blood pressure in the glomerulus, causing ultrafiltration to occur. Blood plasma is forced out under pressure through the porous walls of the glomerulus and the Bowman’s Capsule.

Small molecules, like glucose, water and urea pass through, and larger molecules, like, red blood cells, white blood cells, platelets and blood proteins, like fibrinogen, are not present in glomerular filtrate. These molecules are too large to pass through the porous walls of the capillary even under pressure during ultrafiltration. They do not pass out of blood, and so, are neither present in the filtrate or in urine. The filtrate passes along the nephron.

Most of the salts, such as, sodium chloride and all the glucose are absorbed back to blood by selective reabsorption from the proximal convoluted tubule. Water reabsorption is adjusted by the secretion of ADH by the pituitary. Water is reabsorbed until normal blood concentration is detected.

There is more urea in filtrate. Bulk of water in filtrate and some salts are reabsorbed, but urea, creatinine and ammonium salts are not. These stay in nephron to form urine to be expelled from the body. So, percentage of urea in urine increases.

Urea passes into the collecting duct into the pelvis and then via the ureters to the bladder. Urea is excreted from the body in urine via urethra.

Note : Renal artery has higher concentration of urea than renal vein as urea passes into filtrate in nephrons and is not reabsorbed. Urea is lost in urine. Water is reabsorbed.

There is an overall decrease in concentration of glucose as cells use glucose in respiration. Selective reabsorption is an active process. Concentration of glucose rises as it is reabsorbed as the filtrate passes through the proximal convoluted tubule of the nephron. Water is reabsorbed.

Oxygen concentration is higher in renal artery than in renal vein as oxygen is used by respiring kidney cells. Carbon dioxide is produced and is higher in concentration in renal vein than in renal artery.

Urea :

·                Proteins are absorbed as amino acids.

·                Liver removes excess amino acids from blood in hepatic portal vein by deamination.

·                Amino group from amino acids is removed to form toxic ammonia, which cannot be stored.

·                Hence, it is changed to the less toxic urea as ammonia and carbon dioxide combine / react.

·                The rest of the amino acid is converted to glucose.

·                Urea is transported in blood plasma to the kidneys as it is very soluble, to be excreted in urine.

·                Concentration of urea in urine rises. 

·                Ammonia + Carbon Dioxide → Urea + Water

Urea travels in solution in blood plasma via the hepatic portal vein to the vena cava and drains into the right atrium of the heart. The blood, rich in urea travels via the pulmonary arteries to the lungs and returns to the left atrium of the heart via the pulmonary vein. The blood containing urea leaves the heart via the aorta to the renal arteries of the kidneys.

The amount of urea produced by the liver varies as the amount of protein in diet varies and the amount of protein used by the body varies. If amino acids are in excess, more urea is formed. If protein intake is less, less urea is formed and less passes out in urine.

Osmoregulation : The kidney carries out osmoregulation – that is, it regulates the water content of blood. It removes toxic and unwanted materials, like urea, from blood. Blood is filtered at high pressure in the Bowman’s capsule by ultrafiltration. Most of the salts and all the glucose are absorbed back to blood by selective reabsorption. Water reabsorption is adjusted by the secretion of ADH by the pituitary.

A fall in water content of blood :

• On a hot day, people sweat as body temperature rises. Water is lost from the body by evaporation. Sweat, thus, cools the body by carrying body heat away as latent heat of vapourisation.
• So, water content of blood / carotid sinus falls.
• Information is sent to the hypothalamus.
• Hypothalamus detects the fall from the blood being pumped to the brain via arteries.  
• More ADH is secreted by the pituitary as water content of blood plasma is less.
• ADH is carried in blood to the collecting duct of the nephrons of the kidney.
• ADH stimulates reabsorption of more water into blood by making collecting ducts more permeable to water. The more ADH is secreted – more is the volume of water reabsorbed.
• The thirst control centre of the brain is stimulated.
• Water is reabsorbed until the normal level is restored.
• Negative feedback stops the secretion of ADH.
• Urine becomes more concentrated.

A rise in the water content of blood :

This may be due to :

• Vigorous exercise, where more water is released by respiration.
• Drinking a lot of fluid, so that more water is lost to maintain osmotic potential of blood
• Visiting a cold place, so that less water is lost as sweat

A rise in the water content of blood is detected by the osmoreceptors and the information is sent to hypothalamus of the brain. This causes the pituitary gland to secrete less ADH. This reduces permeability of the collecting duct. Less water is reabsorbed. More water is excreted in urine and the urine produced is less concentrated.

An accident where a lot of blood is lost can lead to the kidneys to stop working.

·                A fall in blood pressure as ultrafiltration in glomerulus requires high blood pressure.

·                Blood pressure forces substances out of blood into Bowman’s Capsule.

·                Filtration will stop / slow down.

·                Reabsorption will stop.

·                Urine production stops / slows down.

Oral Rehydration Therapy :

• Severe diarrhoea and severe vomiting can lead to loss of salts / electrolytes and water from the gut in faeces.
• Body fluids become more concentrated after vomiting.
• So, this causes dehydration of body cells.
• Oral Rehydration Therapy is a drink which contains water and salts.
• It also has sugar for energy release by respiration.
• It allows body fluids to regain normal concentration / returns osmotic concentration of blood by replacing water, lost salts and sugar.
• It is easy, cheap and can be done at home.

Less water is reabsorbed into blood. Less water is available for reabsorption. So ADH secretion has little effect.

Changes to blood occurring during passage of blood through the kidneys :

• Oxygen level falls
• Glucose level falls as it is used by cell respiration and for active transport
• Carbon dioxide level rises as it is produced by cell respiration
• Salt level falls, but not all is reabsorbed.
• Urea level falls as urea is forced out of blood during ultrafiltration and is no longer reabsorbed.
• Blood leaving the kidneys has constant water level
• Blood entering the kidneys has variable water content
• Alcohol, drugs and hormones are excreted and are not reabsorbed

Osmoregulation in Amoeba :

• Water enters the cell by osmosis in contractile vacuole
• Water moves out.
• This requires energy

SKIN

Functions :

·                The skin prevents water loss from the body, thus preventing dehydration as the outer layer of the skin is impermeable and water proof. (Being waterproof gives protection against harmful chemicals and entry of water, thus protecting against rainfall)

·                The cornified layer of the skin has dead, keratinized cells, protecting organs against mechanical damage, such as, knocks and bangs. It is thicker in places frequently damaged.

·                The cornified layer cells are constantly replaced by the Malpighian Layer, dividing by mitosis to form new cells to replace those worn off from the epidermis.

·                The cornified layer has dead, keratinized cells in it. The layer is continuous – there are no gaps between the cells. This acts as a barrier to the entry of pathogens, reducing the risk of infection.

·                The skin has a dark pigment called melanin, produced by the Malpighian layer, that absorbs ultra violet light. Melanin reduces UV radiation entering the skin. This prevents skin cancer. Melanin also prevents damage to liver cells. (If melanin is not there, sunburn is caused as UV light penetrates, damages cells and causes skin cancer.)

·                The skin contains sense organs, like touch receptors, temperature receptors and pressure receptors (Pacinian Corpuscle / Meisner’s Corpuscle).

·                It has a layer of sub cutaneous fat – the hypodermis that acts as a store of energy for periods of low food intake, winter or pregnancy. It also acts as an insulating layer against excessive heat loss.

Structure :

·                Fat layer to provide insulation.

·                Cornified layer is waterproof and is made of dead cells.

·                Malpighian layer produces new cells and melanin that gives protection against UV light by absorbing it and preventing skin cancer.

·                Hair follicles produce hairs.

·                Sebaceous gland alongside shaft of hair secretes sebum, which is waterproof and antiseptic, thus acting as a barrier to the entry of pathogenic microorganisms.

·                Sweat glands secrete sweat that evaporates carrying body heat away as latent heat of vapourisation and reduces body temperature if it rises above normal.

·                Nerve endings are sensitive to touch and temperature.

A decrease in internal body temperature :

·                Reduced sweating occurs. Less evaporation occurs. So, less heat is lost from the body as latent heat of vapourisation.

·                Vasoconstriction occurs. Less blood flows through the skin capillaries and more blood flows through the shunt vessels. Less heat is brought to the outer layers of the skin. Less heat is brought from the body by radiation and convection.

·                Goose pimples : Hair erector muscles contract. Hair becomes erect – they are pulled upright. This traps more hair. Air is a good insulator.

·                Shivering generates heat.

A rise in internal temperature :

Sensory temperature receptors detect the rise in temperature. Nerves transmit the impulse to the hypothalamus of the brain. The nerves send a message to the skin.

·                Increased sweating occurs. Sweat passes to the surface of the skin. Sweat evaporates. Latent heat is removed. This cools the body by removing excess heat.

·                Vasodilation : Arterioles to the upper layer of the skin dilate and carry more blood. More blood is diverted to arterioles and thus through the skin capillaries. More blood flows through the skin. More heat is brought to the skin. More heat is lost via convection / radiation. The skin becomes bright red.

·                Hairs lie flat.

Role of defence mechanisms in protecting against harmful chemicals :

·                The outer layer of dead cells on the skin forms a barrier to the entry of chemicals.

·                The skin is waterproof and cannot be penetrated by gases.

·                Living cells are not in contact with air.

·                Mucous membranes trap some gases and dust particles, that are swept away by cilia.

Role of skin in protecting against pathogens :

·                The outer epidermis is made of dead cells – pathogens cannot flourish on dead layer

·                It is waterproof as it is keratinized – most chemicals are kept away from live cells

·                The cornified layer is replaced as it is worn away.

·                Sebum acts as an antiseptic.

·                Nerve endings in the dermis (living layer) limit damage.

This forms a barrier to the entry of pathogenic bacteria.

Core temperature: Core temperature is higher than skin temperature as heat is generated within the body, whereas, skin loses heat to the environment.

If cold water is drunk, it chills the throat and the stomach. Temperature of blood leaving these organs falls. This affects the rest of the circulatory system.

Notes :

·                Mean daily energy intake varies with mean daytime temperature. There is a negative correlation between the two. Greater the difference between body temperature and external temperature, greater is the rate of heat loss to the environment. Body temperature must be maintained at a constant level in order to remain active. Hence, more energy input is required for this to happen.

·                Wearing several thin layers of clothes is more effective than wearing one thick layer of clothes. Each layer traps air. Air acts a good insulator. More layers result in more trapped air, resulting in greater insulation. Less heat is lost from the body. Thus, body temperature is maintained.

·                In case of severe burns, tissue fluid leaks out. Plasma is used to replace the lost tissue fluid. Loss of fluid can cause dehydration of body cells and damage the cells. Blood pressure falls. The kidney fails as ultrafiltration does not work. Dizziness occurs.

·                Smaller mammals need a greater supply of food per gram of body mass than larger mammals as smaller mammals have a greater surface area to volume ratio. Hence, they lose more heat. Thus, more heat needs to be generated by respiration.

·                If a person damages a large area of the epidermis, a clear fluid (plasma / tissue fluid / serum) leaks out. Tissue fluid / plasma is formed from the capillaries by blood pressure, keeping the outer layer waterproof, preventing loss of fluid from the body. The outer layer is scraped off, exposing the cells.

·                In mammals only, sweat glands, hair, pili, external ears and fur are features involved in temperature regulation.

·                Water, urea, mineral salts, such as, sodium chloride and ammonium salts are excreted.

·                In hot temperatures, it is advised to remain in the shade at daytime as rise in body temperature is less, so less water is lost as sweat.

·                If a protein rich food is eaten, more urea is produced and this requires more production to get rid of it.

·                Humans can keep their body temperature higher than that of the surrounding air as heat loss is balanced by heat gain. Heat is produced by respiration / muscle activity / shivering / high metabolic rate. Insulation is provided by fat layer. Heat loss is regulated by vasoconstriction, minimal sweating and use of clothing. These are examples of homeostasis and negative feedback.

·                The tips of the finger and toes are cooler than central body parts due to more loss of heat to the environment as blood travels through arm and legs / close to air. Fingers and toes have little fat layer.