Homeostasis: Coordination and Response

SABRINA PACE-HUMPHRES

Homeostasis, Co-ordination and Control and the Excretory system

What is homeostasis?

Homeostasis is the condition of equilibrium (balance) in the body’s internal environment due to the constant interaction of many of the body’s regulatory processes. The body strives to maintain balance and therefore is constantly adjusting. Homeostasis is dynamic in this respect and is controlled through the nervous and endocrine systems, running through a path which includes three elements: receptor, control centre and effectors. In response to changing environmental conditions, equilibrium can shift among points in a narrow range thus making life maintenance compatible. Each bodily structure – from cells to systems – contributes to keeping the internal environment within normal limits.

How is body temperature maintained? Why this is necessary for the organism?

Humans must maintain constant core temperature of 37°C. If exceeded (above 45°C) cellular protein becomes inactive/unable to function. Below 0°C cellular water freezes and ice crystal growth affects membranes/kills the organism.

Shell temperature of peripheral systems e.g. near skin, arms and feet and Core temperature at which core organs (heart/lings etc) work efficiently is constantly monitored.

The hypothalamus deals with temperature regulation. It compares ideal temperature to what the rest of the body is experiencing. Its preoptic control centre evaluates information from receptors in the mouth, skin, spinal cord and brain which detect temperature changes. If change occurs the control centre sends signals to effectors who commence response processes – vasodilatation or vasoconstriction – that change the situation reinstating normal levels.

A negative feedback loop ensures that changes are reversed and set back to normal. Receptors  control centre  effectors work in a constant cycle for temperature maintenance.

If temperature is too high vasodilatation occurs – blood vessels leading to the skin capillaries dilate to reduce temperature. Core blood is sent near skin surface and cooled by outside temperature via heat radiation. On return to the core its cooler and temperature lowered. Sweating causes core temperature decrease through water evaporation, although water loss in the wrong environment can lead to fatal dehydration.

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Vasoconstriction – the narrowing of blood vessels leading the capillaries – occurs when the core temperature low. Blood flow to peripheral parts is reduced. Vessel constriction maintains core temperature by reducing cold blood from periphery areas. To combat low core temperatures a person may start to shiver, producing a muscular waste product – heat.

A vasoconstriction negative is that muscles can’t function under a certain shell temperature. If immersed in extremely cold water. and vasoconstriction activated, muscular ability is restricted due to reduced blood flow causing paralysis. Homeostasis which aims to preserve core temperature can be fatal.

Behavioural mechanisms like looking for a warmer environment/adding clothing are voluntary but not automatic as they are still regulated by the hypothalamus.

Water is important for homeostasis, how are water levels maintained within the body and why is it important to do so?

Water makes up tissues and blood. It’s used to distribute nutrients and oxygen around our systems and carry away waste products. Water equilibrium is necessary for survival and is ingested from food, drink and respiration and lost through sweat, urine and breathing.

Kidneys are responsible for maintain water equilibrium. As blood flows through them, water content is measured. If blood contains the correct amount, kidneys expel water (evidenced by pale urine). If they detect not enough water, they reabsorb it from circulating blood (evidenced by dark – concentrated – urine).

Intracellular cell dehydration is monitored by Osmoreceptors located in Hypothalamus. They monitor the body’s water balance and, when they sense cellular dehydration, these receptors becoming dehydrated too (shrivel and deform).

The control centre – hypothalamus – detects osmoreceptor dehydration and makes adjustments by sending a message to the pituitary gland to produce/release Anti-Diuretic Hormone (ADH).

The function of ADH is to lessen excretion of water by causing kidney nephrons to increase their water permeable ability. When a signal is received from hypothalamus the Pituitary Gland releases ADH which travels to the kidneys via blood. Hypothalamus  Pituitary Gland  Kidneys  Osmoreceptors form the homeostatic negative feedback loop controlling water balance.

Increased cell dehydration leads to increased ADH released into blood passing the kidneys. In parallel, if osmoreceptors indicate the body’s water balance is normal (by swelling), the hypothalamus communicates to the pituitary gland that little/no ADH is needed, the effect being that kidneys expel more dilute urine.

Extracellular dehydration relates to overall blood volume and is monitored by sensors in the kidneys. When they detect a volume change they secrete rennin into blood which causes vessel constriction and, by reacting with angiotensin, this leads to a thirst sensation/need for salt. For example if a person is injured and looses blood they will feel thirsty even through cells are not dehydrated. Satiety sensors determine whether there’s enough water in the system to replenish cells/blood even when water has not reached all parts.

Insulin and Glucagon are homeostatic hormones: how is this the case and why are they necessary?

The endocrine system controls and coordinates biological systems through homeostasis via the production and release of chemical messengers called hormones.

A hormone is a chemical signal to re-prioritise cell activities. It’s a mediator molecule that’s released by one part of the body but regulates cells in other parts of the body. Endocrine Glands secrete hormones into interstitial fluid surrounding secretory cells. From this fluid, hormones diffuse into blood capillaries to be carried by blood to target cells throughout the body where said hormones exert their effects by binding to specific cell receptors.

An example of endocrine system homeostasis is shown in the maintenance of blood sugar levels. The pancreas – and endocrinal gland – is responsible for this as it produces and releases Insulin and glucagon which increases/reduces glucose levels. As can be seen below, each hormone opposes the action of the other, producing antagonistic effects.

Glucagon increases blood glucose levels when it falls below normal limits (hypoglycaemia) while insulin lowers the levels when it’s too high (hyperglycaemia). The level of blood glucose controls secretion of glucagon and insulin via negative feedback as shown below.

Hypoglycaemia stimulates secretion of glucagon from alpha cells of pancreatic islets  Glucagon acts on liver cells, accelerating the conversion of glycogen into glucose and promoting formation of glucose from lactic acid/certain amino acids  hepatocytes release glucose into blood more rapidly, blood glucose rises. If blood glucose continues to rise, the high level inhibits the release of glucagon (negative feedback).

High blood glucose (hyperglycaemia) stimulates secretion of insulin by beta cells of pancreatic islets  Insulin acts on various cells to accelerate facilitated diffusion of glucose, speeds conversion of glucose into glycogen, increase cellular uptake of amino acids, speeds synthesis of fatty acids, slows conversion of glycogen into glucose and slows formation of glucose from lactic/amino acids. Blood glucose levels then fall. If glucose levels drop below normal it inhibits release of insulin (negative feedback) and stimulates glucagon release.

The endocrine system – via the Pituitary Gland, Thyroid Gland, Adrenal Gland, Pancreas, Kidneys and the Reproductive Organs Ovaries (F) and Testes (M) – helps to regulate and maintain various body functions including responses to stress and injury, growth and development and birth and lactation by producing and releasing hormones to maintain homeostasis mainly by using negative feedback mechanisms such as that shown above.

Neural control centres in the brain control endocrine glands. The main neural control centre being the hypothalamus. This control centre sends messages to the Pituitary Gland (master gland) which, in turn, releases hormones that regulate body functions.

The functions of the subdivisions within the nervous system and, in the case of certain answers, their relationships to each other

Brain receives information, interprets it and guides the response.

• Cerebrum controls memory, personality and conscious thought. Sensors receive information from receptors/pass to other brain regions for analysis/action. Motor area consists of motor neurons which, when stimulated, send impulses for skeletal muscle movement.

• Cerebellum receives information from the sensory systems, spinal cord and brain regions for motor movement regulation. Co-ordinates posture and balance. Important for learning motor behaviours.

Spinal cord nerves provide ‘command and control’ routes for information transmission to limbs/organs. Provides route for sensory signals to the brain/way out for motor signals from it.

Somatic system is responsible for movement of voluntary muscles and the reflex arc process. Carries nerve impulses back/forth between the CNS, skeletal muscles, skin and sensory organs.

• Sensory neurons convey information to CNS from receptors in head, body wall and limbs and from vision, hearing, taste and smell receptors.

• Motor neurons conduct impulses from the CNS to skeletal muscles only. Under conscious control.

Autonomic system sensory neurons convey information from receptors located in visceral organs to the CNS. Motor neurons conduct nerve impulses from the CNS to smooth muscle, cardiac muscle and glands. Involuntary.

Sympathetic system neurons increase heart rate. Helps support exercise emergency actions, ‘fight or flight’ responses

Parasympathetic system neurons reduce heart rate. Take care of ‘rest or digest’ activities.

The similarities between the endocrine system and the nervous system as well as the differences.

What organs are involved in the excretory system and what structures do they have to carry out this role, use annotated diagrams as part of the answer

Key excretory system organs are the liver and kidneys.

The liver fulfills a range of processes including metabolism of proteins, fats, carbohydrates and bilirubin (produced when blood cells die) which is excreted through bile. During protein metabolism for ATP production, nitrogen removal from amino/nucleic acids produces amonia. The liver changes toxic amonia to a less toxic chemical called urea. It’s then transported to the bloodstream and removed by the kidneys.

The kidneys are sophisticated machines. Waste and water excreted from them forms urine which flows along the ureter to the bladder for storage/secretion. Kidneys contains 1-2 million nephrons – functional units that perform excretion and reabsorotion.

Nephron structure

Blood travels down the renal aorta to the kidneys and enters the Glomerulus – a network of capillaries responsible for blood filtration – where it’s squeezed through capillaries at high pressure allowing filtrate (including small ions like sodium and glucose) to pass into Bowman’s Capsule. Filtrate then travels through the Proximal Convoluted Tube and materials the body doesn’t want to lose – such as glucose, sodium, acids and some water – is reabsorbed using an active transport mechanism. Filtrate then enters the Loop of Henle. This hair-pin shaped structure uses active transport as a mechanism in its descending and ascending parts to remove sodium ions out of the filtrate, allowing for more effective reabsorption at the collecting duct. Filtrate then enters the Distal Convoluted Tubule where more calcium, sodium etc is reabsorbed. Waste products and excess water which is left travels to collecting ducts that send, what is now urine, down the ureter to the bladder for storage and secretion.

The lungs through diffusion of carbon dioxide excreted through breathing and the skin, through excretion of sweat which includes waste materials (urea, water) are also considered part of the excretory system.

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