Homeostasis notes

Homeostasis and Feedback Mechanism

Homeostasis keeps internal conditions within suitable limits. These conditions include body temperature, water content, salt concentration, blood glucose level, pH and removal of metabolic wastes.

Most homeostatic processes use negative feedback. A change is detected, corrective action occurs, and the condition moves back towards its normal range. The three basic components are receptors, control centres and effectors.

• Receptors detect a change in the internal or external environment.
• The control centre receives information from receptors and decides the suitable response. The brain and endocrine glands may act as control centres.
• Effectors carry out the response. Muscles and glands are common effectors.
• Negative feedback reduces or reverses the original change.
• Homeostasis does not mean that conditions remain absolutely constant. It means that they remain within a narrow suitable limit.
• Immunity is a biological defence of the body that protects it against disease-causing organisms and foreign substances.

Excretion and the Urinary System

Excretion is the removal of metabolic waste products from the body. It is different from egestion, which is the removal of undigested food from the alimentary canal. In humans, the kidneys are the main organs of excretion and osmoregulation.

The urinary system consists of a pair of kidneys, a pair of ureters, the urinary bladder and the urethra. Blood enters each kidney through the renal artery, which is a branch of the aorta. Filtered blood leaves through the renal vein.

• Main nitrogenous waste in human urine: urea.
• Urea is formed in the liver from excess amino acids and ammonia.
• Kidneys remove urea, excess salts and excess water from the blood.
• Ureters carry urine from the kidneys to the urinary bladder.
• The urinary bladder temporarily stores urine.
• The urethra carries urine from the bladder to the outside.
• Anuria is the failure of the kidneys to form urine.
• Complete kidney failure may be treated by dialysis or kidney transplantation.

Structure of the Kidney and Nephron

Each kidney contains numerous microscopic functional units called nephrons. A nephron consists of a renal corpuscle and a renal tubule. The renal corpuscle includes the glomerulus and Bowman's capsule.

The glomerulus is a network of capillaries inside Bowman's capsule. Blood pressure causes ultrafiltration from the glomerulus into Bowman's capsule. The filtrate then passes through the proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.

• Renal cortex: outer region of the kidney containing renal corpuscles and parts of tubules.
• Renal medulla: inner region containing loops of Henle and collecting ducts.
• Glomerulus: capillary network where ultrafiltration occurs.
• Bowman's capsule: cup-shaped structure that receives glomerular filtrate.
• Proximal convoluted tubule: site of selective reabsorption of useful substances.
• Loop of Henle: helps establish the concentration gradient in the medulla.
• Collecting duct: receives filtrate from several nephrons and carries urine towards the renal pelvis.
• Kidney blood flow: renal artery to arterioles and glomerulus, then capillaries and renal vein.

Urine Formation and Selective Reabsorption

Urine formation occurs through ultrafiltration, selective reabsorption and tubular secretion. During ultrafiltration, small molecules pass from the glomerular blood into Bowman's capsule. Blood cells and most plasma proteins normally remain in the blood because they are too large to pass through the filtration barrier.

Selective reabsorption returns useful materials from the filtrate to the blood. Nearly all glucose and amino acids, much of the water, and suitable amounts of salts are reabsorbed. Glucose is taken back from glomerular filtrate mainly by active transport in the proximal convoluted tubule.

Tubular secretion adds some substances from the blood into the tubule. This helps remove excess ions, drugs and other wastes. The final urine contains water, urea and variable amounts of mineral salts.

• Ultrafiltration is driven mainly by high blood pressure in the glomerulus.
• Glomerular filtrate contains water, glucose, amino acids, urea and mineral ions.
• Large plasma proteins and blood cells are normally not present in glomerular filtrate.
• Glucose reabsorption occurs through active transport.
• The proximal convoluted tubule reabsorbs most useful solutes and a large amount of water.
• A mammalian kidney can reabsorb approximately 99.5% of glomerular filtrate, especially under restricted water supply.
• Free haemoglobin in urine with lysis of red blood cells can indicate malfunction of the proximal convoluted tubule in the given renal context.
• Urine volume and concentration change according to the body's water requirement.

Osmoregulation, ADH and Counter-Current Systems

Osmoregulation is the control of water and dissolved salt concentration in body fluids. The kidneys perform this function by changing the amount of water and salts reabsorbed from the filtrate. The hormone antidiuretic hormone, or ADH, has a central role in water conservation.

When water is scarce, more ADH is released. ADH increases the permeability of the distal nephron and collecting ducts to water. More water returns to the blood, so a smaller volume of concentrated urine is produced. When water is plentiful, less ADH is released and a larger volume of dilute urine is formed.

The loop of Henle and vasa recta form two counter-current systems. Their arrangement helps produce and maintain a concentration gradient in the kidney medulla, allowing the kidney to conserve water.

• ADH is also called vasopressin.
• High ADH level: increased water reabsorption, decreased urine volume and hypertonic urine.
• Low ADH level: decreased water reabsorption, increased urine volume and more dilute urine.
• The production of urine of different concentrations depends on water availability.
• The loop of Henle and vasa recta form the two counter-current systems of the kidney.
• The descending limb of the loop of Henle is more permeable to water than the ascending limb.
• The ascending limb helps move salts into the medulla and is relatively impermeable to water.
• Osmoregulatory activity helps maintain the water potential of blood and tissue fluids.

Excretion and Water Balance in Animals

Animals have different water problems depending on their habitats. Freshwater animals tend to gain water by osmosis, while marine animals tend to lose water to their surroundings. Their kidneys, gills and behaviour are adapted to maintain water and salt balance.

A freshwater fish usually does not drink water. It gains water through its body surface and gills, produces a large volume of dilute urine, and actively takes up salts. A marine bony fish drinks seawater, gains excess salts, and removes salts through its gills and kidneys while producing a smaller volume of urine.

Some terrestrial animals conserve water through behaviour and special body adaptations. The kangaroo rat can survive without drinking water because it obtains water from food and metabolic reactions and greatly reduces water loss.

• Freshwater fish do not drink water under normal conditions.
• Freshwater fish gain water by osmosis and excrete a large volume of dilute urine.
• Marine bony fish drink seawater because they lose water by osmosis.
• Marine bony fish remove excess salts through their gills and kidneys.
• Kangaroo rat: an animal that can survive without drinking water.
• Kangaroo rats produce highly concentrated urine and very dry faeces, reducing water loss.
• Fish in marine water face water loss and salt gain, while freshwater fish face water gain and salt loss.
• Osmoregulation in animals depends on the salinity of the habitat.

Kidney Disorders and Treatment

Kidney stones are solid deposits formed from substances present in urine. They may block the urinary tract and cause severe pain. The composition of a stone depends on the substance that crystallises.

Kidney failure reduces the ability to remove wastes and regulate water and salts. Dialysis removes selected wastes and excess water from the blood when the kidneys cannot perform this function adequately. A kidney transplant can provide a functioning kidney from a suitable donor.

Medical treatment depends on the cause and severity of the condition. Persistent changes in urine, blood in urine, severe pain or reduced urine production require medical assessment.

• Hyperoxaluria is associated with kidney stones formed from excessive oxalate; such stones are about 15% of all kidney stones in the stated syllabus context.
• Anuria means absence or failure of urine formation.
• Dialysis uses a selectively permeable membrane and dialysis fluid to remove wastes from blood.
• Dialysis can help keep a patient alive when the kidneys have failed completely, but it does not replace all kidney functions permanently.
• Kidney transplantation places a healthy kidney into a patient with severe kidney failure.
• Kidney stones can obstruct the ureter and interfere with urine flow.
• Failure of kidney function can disturb urea removal, salt balance, water balance and blood pH.

Thermoregulation and Adaptations

Thermoregulation is the maintenance of body temperature within a narrow range. Humans are endothermic. They generate much of their body heat through metabolic reactions and regulate heat loss using nervous and hormonal responses.

When body temperature rises, the hypothalamus promotes heat loss by increasing sweating and vasodilation of skin arterioles. When body temperature falls, vasoconstriction, shivering and increased metabolic activity help conserve or produce heat.

Animals and plants also show structural adaptations to their habitats. Xerophytes live in dry environments and reduce water loss. Hydrophytes live in water or very wet environments and commonly contain aerenchyma, which provides air spaces for gas exchange and buoyancy.

• Endothermic animals maintain body temperature mainly through internally produced metabolic heat.
• Humans maintain body temperature within a narrow limit because they are endothermic.
• Sweating cools the body when sweat evaporates from the skin.
• Vasodilation increases blood flow near the skin surface and promotes heat loss.
• Vasoconstriction reduces blood flow near the skin surface and conserves heat.
• Shivering produces heat through rapid, involuntary muscle contractions.
• Blubber is a thick layer of fat found in marine mammals and provides insulation and energy storage.
• Xerophytes have adaptations such as a thick waxy cuticle, reduced needle-like leaves and sunken stomata.
• A thick waxy cuticle reduces the rate of transpiration in xerophytes.
• Aerenchyma is common in hydrophytes and contains large air spaces.

Selected Biological Defence and Osmotic Facts

Some body processes included with homeostasis involve protection from harmful agents. Immunity is the ability of the body to resist disease. Antibodies are proteins produced by B-lymphocytes that bind specifically to antigens.

The variable part of an antibody recognizes and binds to the antigen. Passive immunization supplies ready-made antibodies, so protection develops quickly but usually does not last as long as active immunization.

Osmosis also affects cells and organisms. A plant cell placed in a hypertonic solution loses water, but its cell wall gives mechanical support and prevents the cell from bursting. The cell membrane may pull away from the wall during plasmolysis.

• Antigen: a foreign substance that can stimulate an immune response.
• Antibody: a specific protein produced by B-lymphocytes against an antigen.
• The variable part of an antibody recognizes the antigen.
• Anti-tetanus serum, or ATS, provides passive immunization.
• Passive immunization gives preformed antibodies and does not usually produce long-lasting immunological memory.
• Hypertonic solution: a solution with a higher solute concentration than the cell sap.
• Plant cells do not burst in a hypertonic solution because the cell wall provides support.
• Cell wall and turgor pressure help plant cells maintain their shape.