Complete notes for Chapter 15 Homeostasis. Covers osmoregulation, excretion, thermoregulation, kidney structure and function, and solved exercises for FSc Part 2 Biology.
Definition: Every organism is facing two types of environment:
Homeostasis: The protection of internal environment from the harms of fluctuations in external environment is called homeostasis.
The changes always take place in external environment but the organisms make sure to keep the fluctuations within a narrow range.
The different components of internal environment which are affected by fluctuations of external environment are:
These components are balanced in the body through following processes:
The control system has been introduced for the variety of homeostatic regulations.
It has three components:
In a physical control system like a temperature control system, there is a sensor (thermometer) that monitors temperature change from a set point and signals to control center to take action by switching on heater or cooling units in response to drop or rise in the temperature compared to set point.
The living control system works exactly on the mechanism of physical control system. This system has three components:
In living system there is a set point temperature in regulated (endothermic) animals. The sensor (receptor) detects temperature change, e.g., if there is an increase, it signals to the control center for action of cooling system and vice versa.
Detection of change and signaling for effector's response to control system is called a feedback mechanism. In these processes there is an inverse effector's response to the change in internal environment. For example, there is generally a cooling effector's response to warmth sensing in external environment. This is termed as negative feedback mechanism.
Osmoregulation: The process in which the water and ion concentration in the body is regulated is called osmoregulation.
Water is the most important solvent in the cell which is to be maintained. Every cell has a definite amount of water in relation to salts. Through the process of osmoregulation, the quantity of water is maintained in the body.
According to the availability of water, the cell has to face three types of environment:
To avoid such situations, cells osmoregulate themselves to keep water and salts balanced in plants and animals. Osmoregulation has enabled the animals and plants to distribute themselves in wide range of habitats.
According to the availability of water, plants are divided into three groups:
These are those plants which live in aquatic environment. They have a lot of water supply therefore they have adapted themselves to reduce the flooding of their cells in fresh water.
Adaptations:
Example: Water lily, Lotus
These plants have moderate supply of water. In sufficient supply of water, stomata are kept open to promote loss of excess water, but in less supply, stomata are closed to prevent the loss.
Example: Brassica, Rose, Mango etc.
These plants have to face severely terrestrial conditions. They show the following adaptations:
Example: Cactus
In reference to the balancing of the water and solutes in the body, the animals are divided into two categories:
The animals which do not maintain their internal solute concentration are called osmoconformers. Such animals maintain the ionic concentrations of their body fluid at the same level as that of medium in which they are living and change the ionic concentration of their body fluid when the ionic concentration of the external environment changes.
Such animals maintain an internal solute concentration which does not vary regardless of the environment in which they are living. When the body fluid concentration differs from the outside environment, these animals actively regulate to discharge excess water in hypotonic and excrete salts in hypertonic conditions.
There are different methods found in marine animals to osmoregulate their bodies:
(i) Invertebrates: Many of the invertebrates living in the marine environment are osmoconformers. Thus they do not maintain their internal solute concentration.
(ii) Vertebrates:
Fresh water has very low salt concentration thus the animals living in this environment are in danger of loss of salts from their body and entry of large amount of water in their body. The following adaptations are found:
In terrestrial environment, the water supply to the animals is very moderate or even less. So there is a danger of loss of water through evaporation. This may lead to dehydration which can bring serious results in terrestrial animals. To live in such environment, the animals show the following adaptations:
Excretion: The elimination of the wasteful metabolites mainly of nitrogenous nature outside the body is called excretion.
It is a process in which the nitrogenous wastes are removed from the body.
Animals use the food which contains nutrients such as carbohydrates, proteins, lipids and nucleoproteins etc. When the metabolism of carbohydrates and lipids occurs, then CO2 and water are produced. On the other hand, proteins and nucleoproteins on metabolism give nitrogenous wastes which are highly toxic and therefore should be removed from the body. Therefore the process of excretion is needed.
Excretion in plants is entirely different from animals. The waste products of plants are CO2 and H2O. They also produce some organic and inorganic compounds which can be stored for various purposes and removed when necessary.
During the process of photosynthesis, plants produce oxygen which is considered a waste product at that time. When the plants respire, they produce CO2 and water as waste products. Water is eliminated from the plant body through transpiration or it may be used for maintaining turgor in cells.
(i) Leaves (Excretophores): Plants produce various waste organic and inorganic compounds which are stored in vacuoles of the leaves. The leaves having such wastes fall off on the ground in the autumn season. Thus in the autumn season, gardeners find rotted autumn leaves as good source of humus. Such leaves are therefore called excretophores as they get rid of accumulated wastes. The change of color of these leaves is due to presence of excess pigmented compounds and toxic materials.
(ii) Bulbs: Some organic and inorganic wastes also accumulate in certain bulbs. Blue bell is one of the examples which leaves the bulb underground.
Some trees deposit strange chemicals in their branches and trunks especially in old xylem which is no longer used for water transport. This takes place in ebony which produces very black wood in the center due to accumulation of these strange chemicals. These are considered to be waste material.
Sometimes these waste materials are considered as chemical weapons by secreting them in the soil to compete with other plants. The example is of some conifers.
Animals produce different kinds of excretory products which are as follows:
Water is considered an excretory product in hypotonic environment, where water potential is very high in the body cells.
Salts are removed by animals of hypertonic environment.
Amino group (NH2) is produced during the catabolism of amino acids or transferred to another molecule for removal or reuse. If it remains in the body, it may cause convulsions, coma and may lead to death. Therefore, it may be removed from the body as soon as possible.
Excess amount of nitrogen is excreted from the body of animals in the form of ammonia, urea, and uric acid.
Less amount of nitrogen is excreted from the body of animals in the form of creatinine, which is trimethyl oxide, very small quantities of amino acids, purine and pyrimidine. Metabolism of purine and pyrimidine bases produces wastes of hypoxanthine, xanthine, uric acid, allantoin, urea and ammonia.
Different nitrogenous products in animals are ammonia, urea and uric acid. Animals excreting ammonia, urea and uric acid are called ammonotelic, ureotelic and uricotelic respectively. Ureotely and uricotely are evolutionary adaptations of nitrogenous wastes in their habitats.
Ammonia is a highly toxic compound. It can easily dissolve in body fluids. As it is very toxic, so its concentration is kept low in the body. For this purpose, large amount of water is required so that it is readily excreted out through urine as it is produced. As the excretion of ammonia requires a large amount of water, so it is the excretory product of those animals that live in hypotonic environment i.e., fresh water.
Water Requirement: To excrete 1 gm ammonia, 500 ml of water is required.
Urea is the excretory product of those animals that live in moderate supply of water. Urea is less toxic as compared to ammonia which requires only 50 ml of water to remove its 1 gm. The excretory nitrogen produced during metabolism undergoes urea cycle to convert into urea.
Water Requirement: To excrete 1 gm urea, 50 ml of water is required.
Uric acid is a waste product of those animals which have acute shortage of water supply. 1 gm of uric acid requires only 1 ml of water. Reptiles and birds excrete uric acid as they live in such environment.
Water Requirement: To excrete 1 gm uric acid, 1 ml of water is required.
There are no specialized excretory structures in hydra. Waste products are taken out from the body simply by diffusion in the isosmotic surroundings.
Planaria belongs to the group of flat worms. It has a very simple tubular excretory system which is called protonephridium.
Protonephridium: It is a network of closed tubules which has no opening. This tubular system is spread on the whole body. This system also has very special cells called flame cells.
Every flame cell has a tuft of cilia whose beating moves the interstitial fluid in the whole tubular system. These cells are termed as flame cells because when their cilia moves, they look like flame.
The tubular system opens into the ducts called excretory ducts which open to the exterior through several nephridiopores.
Excretory Material: The excretory material in flat worms is nitrogenous waste. As some of them live in fresh water, so they excrete dilute urine while some of the parasitic flatworms live in isotonic environment.
Earthworm is an annelid and it lives in soil. It also consists of a system of excretory tubules called metanephridium. Each segment of earthworm has a pair of metanephridia.
This system consists of internal ciliated opening called nephrostome which is immersed in coelomic fluid. Nephrostome is surrounded by a network of capillaries.
The nephrostome collects that fluid which is present in coelom. Nephrostome has a large network of capillaries. As fluid moves along the tubule, epithelium reabsorbs the salt from lumen and sends it to blood vessels surrounding the nephridium. The left over fluid appears as urine containing nitrogenous waste.
Urine is excreted through nephridiopores.
Arthropods especially the insects eliminate metabolic waste by a unique system called Malpighian tubules that extend from the digestive system into the haemolymph.
Structure: These are slender projections that are blind at one end. These are attached at the junction of midgut and hindgut. These are the only excretory structures in animal kingdom that are associated with the digestive tract.
Functions: Malpighian tubules remove nitrogenous waste from haemolymph (as cockroach does not contain red blood cells, so the fluid present in the sinuses is called haemolymph).
The epithelium lining of the tubules transports solutes including salt and nitrogenous waste from haemolymph into the lumen of the tubules. Fluid then passes to hindgut into the rectum.
Rectum reabsorbs most of the salts and water. In this way, nitrogenous wastes are excreted in the form of uric acid crystals along with the faeces. In this way, terrestrial insects can live with high shortage of water.
Metabolic Wastes: The wastes which are produced during the metabolism are called metabolic wastes. If these wastes are not removed, they can cause serious results.
Liver is an important organ for the metabolic reactions. Liver performs the following major homeostatic functions:
(i) Liver produces many poisonous chemicals such as ammonia, urea and uric acids from the nitrogen of amino acids. The major homeostatic effect of this function is to support kidney in waste disposal.
(ii) Liver synthesizes plasma proteins like prothrombin, fibrinogen and albumin etc. Prothrombin and fibrinogen help in blood clotting, while albumin maintains osmotic balance of blood.
(iii) Lipids, cholesterol, and lipoproteins are formed in liver. These compounds regulate blood chemistry, store energy and help to maintain cell membranes.
(iv) Liver plays an important role in bile production. Bile emulsifies fats in small intestine.
Urea is a principal excretory product. One of the major nitrogenous wastes, urea is formed in the liver through the urea cycle.
Two ammonia and one carbon dioxide molecules combine in cycle to produce one molecule of urea:
2NH3 + CO2 → NH2-CO-NH2 + H2O
Liver can store iron and glycogen. Iron is an important element of haemoglobin which is involved in oxygenation of tissues, while glycogen is an important energy reserve.
Liver converts glucose of the blood into glycogen. Glycogen is stored in the form of glycogen which can be used when needed.
Liver is responsible for recycling of contents of red blood cells which help in oxygenation of tissues.
Liver can detoxify many harmful chemicals, such as food additives, pesticides, drugs etc. It thus assists kidney in toxin disposal.
Urinary system consists of the following organs:
Kidney is an important organ of urinary system. Two kidneys are located asymmetrically, one on each side of the vertebral column.
Structure: Each kidney is a bean-shaped organ. The convex side of the kidney is present outward while the concave side faces the vertebral column. Each kidney is about 12 cm long, 6 cm wide and 3 cm thick. Right kidney is present anteriorly than the left kidney.
Urine is collected in a central cavity of the kidney which is called pelvis.
It is a tube which leads from the concave side of each kidney. Urine leaves the kidney through ureter.
The ureters of both the kidneys join into the urinary bladder through the urethral orifice.
Urine leaves the body during urination through a tube called urethra which empties near the vagina in females or through the penis in males.
There are special muscles called sphincter muscles which are present near the junction of urethra and the bladder which control the urine in bladder.
Nephron: It is the structural and functional unit of kidney. A nephron is arranged along two distinct regions: cortex and medulla.
Nephron is divided into two main portions:
Renal corpuscle is further divided into two parts:
The second part of the nephron is a long and narrow tube called loop of Henle. Bowman's capsule continues as convoluted tubules known as:
Distal tubule finally empties into collecting tubule. Loop of Henle has three parts: first part is coiled, second is U-shaped and third is also coiled.
In juxtamedullary nephrons, additional capillaries extend down to form a loop of vessels called vasa recta.
The kidney performs three functions:
All these functions are performed in the nephron.
A large number of liquids is filtered from the blood in glomerulus and goes to the Bowman's capsule. The glomerular walls are excellent for filtration processes as they have very small pores.
Blood cells and most of the protein molecules being larger in size are unable to pass through the walls of the glomerulus and the blood which reaches there creates the filtration pressure.
The filtrate appearing in glomerulus is called glomerular filtrate. The glomerular filtrate contains numerous useful substances such as glucose, amino acids, salts etc. in aqueous solution.
All the useful constituents of the glomerular filtrate are reabsorbed. When filtrate reaches the first part of the renal tubules (PCT), 2/3 of the filtrate containing useful materials like salts and amino acids is reabsorbed in the blood. The waste materials are not absorbed from the filtrate which becomes dense.
When filtrate leaves proximal tubules, it mostly contains nitrogenous wastes.
The tubular epithelium also secretes substances into the lumen. This secretion is very selective and is mainly of hydrogen ions to balance pH value of the filtrate passing through tubule.
Passing through the middle part, the filtrate is diluted or concentrated according to the need. When the filtrate is passed through the last part of the duct, it takes the form of urine.
The production of varied concentration of urine depending on availability of water shows that kidney functions as an osmoregulatory organ.
If the supply of water is less, then the water is conserved. This is done by counter current and hormonal mechanisms.
Counter Current Mechanism:
The interstitial fluid of the kidney is gradually concentrated from cortical to medullary part. Thus inner medulla is highly concentrated with the presence of urea and a mechanism called counter current multiplier.
The mechanism which causes gradual osmotic outflow of water from the filtrate back to kidney as it passes downward in the descending loop of Henle is called counter current multiplier.
Moreover, ascending loop of Henle does not allow outflow of water from its filtrate. Instead of it, Na+ ions are transported actively into the kidney interstitium to sustain its high concentration.
Hormonal Control:
The adrenal cortex secretes a hormone called aldosterone. The function of the aldosterone is the active uptake of sodium in ascending limb of loop of Henle.
Antidiuretic Hormone (ADH): Water function to transport water from filtrate in collecting tubules back to the kidney.
In excess supply of water, reabsorption of water from the filtrate is reduced, specifically due to inhibition of release of antidiuretic hormone. The reduction in reabsorption causes large volumes of diluted urine.
Mammalian kidney including human is adapted to conserve water by 99.5% reabsorption of glomerular filtrate.
The major kidney problems are:
Diagnosis: The kidney stones are diagnosed with X-rays, sonography and ultrasound machines.
What are Kidney Stones? Stony materials which are found in the kidney are said to be kidney stones. These stones cause "Urinary Obstruction" and are generally complicated by infections. Kidney stones have specified chemical nature. In this problem, stone materials are deposited in the kidney which cause difficulties in urination. This disease becomes more complicated during infections.
Causes: The stony material is formed during metabolic disease i.e., hypercalcemia and hyperoxaluria.
Cure:
Due to some pathological and chemical problems, the nephron can be destroyed especially its glomerular part. This destruction leads to increase in plasma level of urea and other nitrogenous wastes which may increase blood pressure and anemia etc.
Cure:
Plants show great adaptation in low and high temperatures.
Problems:
Adaptations:
Problems:
Adaptations:
Animals change the rate of their body heat and produce heat through metabolic processes. This transfer of heat is done in different ways.
With reference to the thermal characteristics, animals are divided as following:
The animals with body temperature that fluctuates with that of environment are called poikilotherms.
Examples: All invertebrates, amphibians and reptiles fall in this group.
The animals which maintain their stable body temperature and do not change as the temperature of environment changes are called homeotherms.
Examples: Birds and mammals.
This classification is based on the "source of heat production":
The animals that generate their own body heat through heat production as a by-product of metabolism are called endotherms.
Examples: Birds, most mammals, flying insects.
The animals which produce very low metabolic heat which can be exchanged with the environment, however absorb heat from their surroundings, are called ectotherms.
Examples: Many invertebrates, fishes, amphibians and reptiles.
The animals which have the capability of producing varying degree of endothermic heat but do not regulate their body temperature within a narrow range are called heterotherms.
Examples: Bat, Hummingbird.
Different adaptations for regulation of heat exchange are:
Mammals maintain their body temperature between 36-38°C. Mammals maintain their body temperature; thus this endothermy is much helpful to keep high metabolic rate and availability of energy every time. All these things provide greater adaptations.
Thermoregulation is a homeostatic feedback mechanism. The homeostatic thermostat is present in a part of brain called hypothalamus which responds to change in temperature from set point which is 37°C.
When temperature is high above 37°C, then the thermoreceptors in skin, hypothalamus and other parts of nervous system send signals to the system that increases blood flow to skin and causes sweat gland activation.
At low temperature, cold receptors send impulses to hypothalamus to inhibit heat loss mechanism and activate heat conservation mechanism.
This is also called pyrexia. When any infection is caused due to some pathogen, then the number of leukocytes is increased. Pathogens and leukocytes produce chemicals called pyrogens. Pyrogens displace the set point of hypothalamus above normal point of 37°C. Fever helps in stimulating protective mechanism against pathogens.