Adjustment and Control: Homeostasis + Hormonal Control+ thermoregulation + the liver + osmoregulation

Homeostasis

The maintenance of constant conditions in the internal environment

Homeostatic mechanisms

These makes organisms independent of the environment, in which it is changing continuously. Homeostatic mechanisms can be: anatomical, physiological and behavioral

Control systems

organisms make use of these to achieve homeostasis. The basic components include:
Input→ Detector→ Regulator→ Set point→ Effect→ Output

Input

A change in the environment that can be detected by an organism

Detector

Detects a change in some variable of the animal’s internal environment

Regulator

processes information it receives from the detector. It compares the information with the set point

Set point

the desired value, amount or concentration

Effector

brings about an appropriate response according to the detected change

Output

the actual response observed in the body i.e. a physiological or behavioural change

Feedback

refers to the interdependence of input and output into the system. There are two types: the negative and the positive

Negative Feedback

here a change in the internal environment leads to a response, which counteracts further change in the same direction. Therefore the response restores the system to its original state. Example compare to a thermostat regulating temperature.

Positive feedback

Here a change produces a response that intensifies the original change. Thus, a disturbance leads to events that amplify (increase) the disturbance even further. This response leads the system to an extreme state. Example: During childbirth, the pressure of the baby’s head against sensors near the opening of the uterus stimulates uterine contractions. These cause greater pressure against the uterine opening, heightening the contractions, which causes still greater pressure. Positive feedback brings childbirth to completion

Thyroxine

a hormone released by the thyroid gland, which regulates the basal metabolic rate, growth and development of an organism

Nervous Control

• Fast responses

• impulses pass along neurons

• Responses precise and localised

• rapid, short-lived responses

• Make use of chemical transmitters

Hormonal Control

• Slower responses

• hormones carries by the blood

• responses diffuse and widespread

• slower responses which continue over a long period

• make use of chemical transmitters

Dynamic Equilibrium

The internal state of an animal body. This implies that internal conditions are never absolutely constant, but they fluctuate continuously within narrow limits. However, the body compensates for them through its control mechanisms. The net results of this activity is that physical and chemical parameters are kept within the narrow range that cells need to function

The endocrine system

This consists of a number of glands that secrete hormones. It is adapted to carry information from one source to another to bring about long lasting responses.

Gland

It is a structure, which secretes a specific chemical substance or substances.

Exocrine glands

produce secretions that are not hormones, into a duct

Sweat glands

secretes sweat into sweat ducts leading to the surface of the skin.

Salivary glands

release saliva into the mouth

Endocrine glands

secretes chemicals called hormones. Has no duct and so the hormone is secreted directly into the bloodstream. Has a rich supply of blood.

Hormones

It is a regulatory chemical messenger, secreted by a gland (endocrine gland) that is released into the blood plasma, and is transported to a target organ on which it exerts its effects.

Characteristics of hormones

• A small soluble organic molecule

• Produces in a gland but acts at another site (target). Have no effect on the glans itself.

• Transported in blood

• Specific to the target. Hormone fits to its receptors by means of a lock and key arrangement.

• Effective in low concentrations

Mechanisms controlling hormone release

1.       Presence of a metabolite in the blood example glucose stimulates release of insulin

2.       Nervous stimulation example adrenaline secretion

3.       Presence of another hormone example many glands release hormones if pituitary hormones are present.

Chemical nature of hormones

Molecular structure must be sufficiently complex to convey regulatory information. Simple molecules are not good enough. They must also be stable enough to resist destruction prior to reaching target cells. However, they must not persist in blood for too long.

Amines

simple molecules derived from single amino acid molecules. Example adrenaline and noradrenaline are derived from tyrosine.

Peptides

example insulin, glucagon, antidiuretic hormone and growth hormone

Steroids

example sex hormones such as oestrogen, progesterone and testosterone

Fatty acids

consists of 2 fatty acid carbon chains attached to a five-carbon ring. Example prostaglandins

Glycoproteins

example follicle stimulating hormone (FSH), luteinizing hormone (LH) and thyroid stimulating hormone

The Steroid hormone

• Diffuses through the plasma membrane.

• Binds to a receptor protein present only in the target cells.

• Hormone receptor complex enters the nucleus.

• Binds to specific regulatory sites

• Stimulates the transcription of specific genes to mRNA. The hormone-receptor complex may also suppress the expression of other genes.

• mRNA is translated to specific proteins.

• The proteins that result often have a regulatory function in pathway of hormone release. For example, when thyroid hormone binds to a receptor in the anterior pituitary gland, it inhibits the expression of the gene for thyrotropin. This is a negative feedback mechanism

Control of hormones: Negative feedback

When blood glucose level rises, the pancreas produces insulin, which causes the liver to store glucose. The stimulus for the production of insulin has thus been dampened, and insulin production stops.

Control of hormones: Antagonism

contrary hormones oppose each other’s actions. Example: Thyroid gland releases calcitonin to lower the blood calcium level, whilst the parathyroid glands release parathyroid hormone (PTH) to raise the blood calcium level. Insulin and glucagon are also antagonistic hormones

The Cascade effect

it is a mechanism that enables the effect of the release of a small amount of the initial hormone to become amplified at each stage of a pathway involving several hormones.

The Anterior lobe-adenohypophysis

It has a glandular origin and is functionally connected to the brain by the circulatory system. It consists of endocrine cells that synthesize and secrete several hormones directly into blood. Neurosecretory cells in the hypothalamus control the anterior pituitary by secreting 2 kinds of hormones into the blood the releasing hormones and the inhibiting hormones

Releasing hormones

induce the anterior pituitary to secrete its hormones

Inhibiting hormones

prevent the anterior pituitary from secreting its hormones

The Posterior lobe- neurohypophysis

As the posterior lobe receives ends of neurosecretory cells from the hypothalamus, it is considered as an extension of the hypothalamus itself. It releases only two hormones: antidiuretic hormone and oxytocin

Antidiuretic hormone (ADH)

prevents dehydration due to excessive loss of water with urine

Oxytocin

causes contraction of uterus during childbirth and ejection of milk from mammary glands

Insulin

compensates for high levels of glucose

·       Small protein consisting of 2-polypeptide chains

·       Secreted by beta cells in the Islets of Langerhans in the pancreas

·       Insulin release increases when blood glucose concentration rises above set point

·       Transported in blood plasma bound to beta globulin.

Glucagon

Compensates for low levels of glucose

• Protein molecule consisting of a single polypeptide chain.

• Alpha cells in the Islets of Langerhans

• Secreted when blood glucose concentration falls below set point.

Insulin’ s mode of action

It binds to a glycoprotein receptor on the cell surface. This leads to changes in the cell membrane permeability and the activity of enzyme systems within the cell.It reduces in the amount of glucose in blood due to: An increase in the uptake of glucose into cells especially skeletal muscles, An increase in the rate of cellular respiration and the use of glucose as a respiratory substrate, An increase in the conversion of glucose to fat in adipose cells, An increase in the rate of uptake of amino acids into cells and the rate of protein synthesis, An increase in the rate of conversion of glucose to glycogen in liver and muscle cells (glycogenesis), Decrease in the formation of glucose (gluconeogenesis).
Insulin appears to affect all cells, but muscle, fat and liver cells are its main targets.

Regulation of Insulin Production

It is regulated by a negative feedback mechanism. An increase in blood glucose level is detected by the β cells in the pancreas, which produce more insulin. As insulin levels rise, glucose is removed from the blood. As blood glucose level decreases, the β cells reduce output of insulin

Effects of Insulin Deficiency

• Blood glucose level above set point (Hyperglycaemia)

• Breakdown of muscle tissue

• loss of wieght

• tiredness

• damage to blood vessels, kidney and nerve supply to lower limbs

Effect of Insulin Excess

• Blood glucose level below set point (hyperglycaemia)

• Hunger

• sweating

• irritability

• palpitations

Mode of action of Glucagon

When glucagon binds to receptors in liver cells:

• It activates adenyl cyclase to form cyclic AMP. This activates phosphorylase enzymes that catalyse the breakdown of glycogen to glucose (glycogenolysis).

• It increases the conversion of amino acids and glycerol into glucose - 6 - phosphate.

• It increases gluconeogenesis i.e. The synthesis of glucose from non-carbohydrate sources

Diabetes Mellitus

A metabolic disorder caused by a lack of insulin or a loss of responsiveness to insulin. May result from incomplete development of, damage to, or disease of the Islets of Langerhans.

Type I (Insulin dependnet)

The pancreas does not produce insulin. This is possibly due to an autoimmune disorder that destroys the Islets of Langerhans. Generally, appears before the age of 40, often diagnosed in childhood. If condition is untreated it may result in a lethal diabetic coma. Generally, type I diabetes is treated through a combination of planned diet, exercise, glucose monitoring and a daily insulin injection. The main injection sites are the thighs (green), arms (yellow), abdomen (orange) and buttocks (blue).

Type II (non-insulin dependent)

This condition arises when the pancreas produces a low amount of insulin, or the body cells do not respond to insulin. Symptoms often show above the age of 40. Best controlled through a low-fat diet and regular exercise. If this fails, oral drugs may be used to make cells more sensitive to the effects of insulin or stimulate pancreas to secrete more insulin

Thermoregulation

The processes organisms use to maintain their internal body temperature within an optimal range despite variations in the external environment. It is crucial for the survival and proper functioning of living organisms, as enzymatic and cellular activities are temperature-sensitive.

Ectothermy

Ectothermic animals rely primarily on external heat sources to regulate their body temperature. Their internal physiological mechanisms for temperature regulation are minimal, and their body temperature often fluctuates with the environment. Example: Reptiles (e.g., lizards, snakes), amphibians (e.g., frogs), and most fish (e.g., goldfish).

Endodermy

Endothermic animals generate and regulate body heat internally through metabolic processes, allowing them to maintain a relatively constant body temperature regardless of external conditions. Example: Mammals (e.g., humans, lions) and birds (e.g., pigeons, owls).

Behavioural mechanisms of thermoregulation in Reptiles

1. Basking

2. Seeking shade

3. Burrowing

4. Body Orientation

5. Nocturnal Activity

6. Water Utilization

Basking

Reptiles, such as lizards and snakes, bask in the sun to absorb heat and raise their body temperature, especially during the morning or after periods of inactivity.

Seeking Shade

During excessively hot conditions, reptiles retreat to shaded or cooler areas to avoid overheating

Burrowing

Some reptiles dig into the ground to escape extreme heat or cold, taking advantage of the soil’s stable temperature

Body orientation

Adjusting their body position relative to the sun (e.g., flattening their body to increase surface area exposed to sunlight or turning sideways to reduce it)

Nocturnal Activity

Some desert reptiles, like geckos, are active at night to avoid the daytime heat

Water Utilization

Crocodiles and other reptiles may immerse themselves in water to cool down

Role of Mammalian skin as a thermoregulatory organ

Mammalian skin plays a critical role in thermoregulation through its structure and associated physiological mechanisms:

1. Structural Features

2. mechanisms

3. Behavioural aspects

Structural features of the skin

• epidermis

• dermis

• hypodermis

Epidermis of the skin

Acts as a barrier to prevent water loss and excessive heat exchange.

Dermis of the skin

Contains blood vessels, sweat glands, and hair follicles, all essential for thermoregulation

Hypodermis of the skin

Stores fat, providing insulation against heat loss

Mechanisms of the skin

• sweating

• vasodilation

• vasoconstriction

• piloerection

Sweating

Eccrine sweat glands release sweat, which evaporates on the skin surface, removing heat and cooling the body.

Vasodilation

Blood vessels in the dermis widen, increasing blood flow to the skin surface and allowing heat to dissipate.

Vasoconstriction

Blood vessels narrow to reduce heat loss in cold conditions by retaining heat in the body core

Piloerection

Hair stands upright to trap a layer of air, providing insulation in cold environments

Behavioral Aspects of the skin

Mammals may seek shade, water, or change posture to either minimize or maximize heat absorption

Thermoregulation in Mammals

Mammals regulate their body temperature through a combination of:

• Structural mechanisms

• behavioral mechanisms

• physiological mechanisms

Structural mechanisms in mammals

• fur/hair

• fat layer

Fur/hair in mammals

Acts as an insulating layer to minimize heat loss. Thickness and density vary with species and climate.

Fat layer in mammals

Found in animals like seals and whales, providing excellent insulation in cold environments

Behavioural Mechanisms in Mammals

1. Seeking shelter or shade during extreme temperatures.

2. Burrowing or huddling in groups to conserve heat.

3. Migrating to warmer or cooler regions based on seasonal changes

Physiological mechanisms

1. shivering

2. non-shivering

3. evaporation

4. panting

5. metabolic adjustment

Shivering in mammals

Rapid muscle contractions generate heat in cold conditions

Non-shivering thermogenesis in mammals

Brown adipose tissue burns fat to produce heat

Evaporation in mammals

Sweat evaporation cools the body

Panting in mammals

In animals like dogs, rapid breathing increases heat dissipation

Metabolic adjustment in mammals

Increasing or decreasing metabolic rate to generate more or less heat

Hibernation

A state of prolonged dormancy during winter, characterized by a significant drop in metabolic rate, heart rate, and body temperature to conserve energy. Examples: Bears, ground squirrels, and bats

The purpose of hibernation

To survive periods of food scarcity and harsh cold conditions

Aestivation

A state of dormancy during extreme heat or drought conditions, marked by reduced metabolic activity. Examples: Lungfish, some amphibians (e.g., spadefoot toads).

Purpose of aestivation

To avoid dehydration and excessive heat during dry season

The liver

The liver is a vital organ responsible for numerous essential functions, including metabolism, detoxification, synthesis of important biomolecules, and storage of nutrients. It is the largest internal organ and possesses remarkable regenerative capacity.

Histology of the liver

The liver is composed of repeating structural and functional units called liver lobules. The histology of the liver reveals its intricate architecture designed for efficient metabolism and filtration

What is the shape of a liver lobule?

Hexagonal units organized around a central vein.

What are hepatocytes?

The main functional cells of the liver, polygonal in shape, responsible for most metabolic and synthetic processes.

What are sinusoids?

Capillary-like blood vessels with fenestrated walls, allowing substance exchange between hepatocytes and blood.

What are Kupffer cells?

Specialized macrophages in the sinusoids that phagocytose old red blood cells and pathogens

What are bile canaliculi?

Small ducts between hepatocytes that collect bile and transport it to bile ducts

What is the portal traid?

A structure located at the corners of the lover lobule, consisting of hepatic artery, portal vein and bile duct

Hepatic artery

Supplies oxygen-rich blood

Portal vein

Brings nutrient-rich blood from the digestive tract

Bile duct

Drains bile synthesized by hepatocytes