Homeostasis and Endocrine Signaling

Nidhi Ahir, Crystal Tingle, Jennie Bravo

Introduction● The endocrine system is made up of endocrine glands that

release hormones into the blood● All cells, except for red blood cells, produce

hormonelike substances called prostaglandins○ prostaglandins are modified fatty acids that usually affect only

nearby cells and tissues

● Steroid Hormones are produced from cholesterol. They can cross cell membranes of target cells, bind with the receptors, and change the expression of genes

● this often results in dramatic changes in the cell’s activity

Introduction Cont.● Nonsteroid Hormones can be proteins, small peptides, or

modified amino acids● they cannot cross cell membranes

○ the receptors are on the cell membrane

● compounds called secondary messengers carry the messages of nonsteroid hormones inside target cells

the endocrine systemA collection of glands that produce hormones that

regulate metabolism, growth and development, tissue function, sexual function, reproduction, sleep, mood, and many other bodily processes.

Overview● Anatomy: the study of the biological form of an organism

● Physiology: study of the biological functions an organism performs

hierarchical organization of animal bodiesCells are organized into:

● Tissues: groups of cells with similar appearance and common function

● Organs: different types of tissues organized into functional units

● Organ Systems: groups of organs that work together

Complex organ systems of animals● They are built from a limited set of cell and tissues

types

● Tissue types can be grouped into 4 categories: ○ epithelial○ connective tissue○ muscle○ nervous

Regulating and conforming● Animals are faced with environmental fluctuations, they

manage their internal environment by either regulating or conforming. (interstitial fluid)

● Regulating: uses internal mechanisms to control internal change despite external fluctuation

● Conforming: allows it internal condition to change in accordance with external changes

Homeostasis● An organism uses homeostasis to maintain a steady state

or internal balance regardless to the external environment

● Animals achieve homeostasis by maintaining a variable at or near a particular value = set point

● Fluctuations above or below the set point = stimulus● These are detected by a sensor and trigger a response● The response returns the variable to the set point

Homeostasis● In animals, it relies largely on a control mechanism that

reduces the stimulus = negative feedback● Homeostasis moderates, but does not eliminate changes in

the internal environment● Thermoregulation: process by which animals maintain an

internal temperature within a tolerable range.

Endothermy and Ectothermy● Endothermic: animals generate heat by metabolism, birds

and animals are an example of this

● Ectothermic: animals gain heat from outside sources, most invertebrates like fish, amphibians and nonavian reptiles are examples of this

Endothermy and Ectothermy● Endotherms can maintain a stable body temperature in

large fluctuations in environmental temperature

● Ectotherms regulate temperature by behavioral means● Ectotherms usually need to consume less food than

endotherms, but the heat source is largely environmental

Balancing heat loss and gain● Organisms exchange heat by 4 physical processes:

○ radiation○ evaporation○ convection○ conduction

Circulatory adaptations for thermoregulation● Animals can alter blood (heat) flow between their body

core and skin

● Vasodilation: the widening of the diameter of superficial blood vessels, promotes heat loss

● Vasoconstriction: the narrowing of the diameter of superficial blood, reducing heat loss

Circulatory adaptations for thermoregulation● The arrangement of blood vessels in many marine mammals

and birds allows for countercurrent exchange

● Countercurrent heat exchangers transfer heat between fluids flowing in opposite directions and reduce heat loss

Physiological thermostats and fever● Thermoregulation in mammals is controlled by a region of

the brain called the hypothalamus.

● Hypothalamus triggers heat loss/ heat generating mechanisms

● Fever is the result of a change to the set point for the biological thermostat

2 major systems for controlling & coordinating● Endocrine System

○ signalling molecules (hormones) released into the bloodstream by endocrine cells that reach all locations in the body.

● Nervous System○ neurons transmit signals along dedicated routes, connecting specific

locations in the body

simple endocrine pathways● Digestive juices in the stomach are extremely acidic and

must be neutralized.● The release of acidic stomach contents into the duodenum

stimulates endocrine cells to secrete the hormone secretin.

● This causes target cells in the pancreas to raise pH in duodenum.○ pancreas can act as an exocrine gland that secretes substances or and

endocrine gland that secreted hormones directly into interstitial fluid.

Neuroendocrine Pathways● Hormone pathways that respond to stimuli from the

external environment rely on a sensor in the nervous system

● The hypothalamus combine endocrine and nervous systems in vertebrates

● Signals from the hypothalamus travel to the pituitary gland located at its base

● Signals from the hypothalamus trigger composition and release of hormones from the anterior pituitary

● the posterior pituitary is an extension of the hypothalamus and produces oxytocin○ oxytocin regulates the release of milk during nursing in mammals

● the posterior pituitary also produces ADH (antidiuretic hormone)

Feedback Regulation in Endocrine Pathways● a feedback loop links the response back to the original

stimulus in an endocrine pathway● while negative feedback dampens a stimulus, positive

feedback supports a stimulus to increase the response

Pathways of Water-Soluble and Lipid-Soluble Hormones● the hormones previously discussed so far are proteins

that bind to cell-surface receptors and that trigger events that lead to a cellular response

● the intracellular response is called signal transduction● a signal transduction pathway has multiple steps● lipid-soluble hormones have receptors inside cells● when attached by the hormone, the hormone-receptor

complex moves into the nucleus

Effects of Hormones● many hormones draw out more than one type of response● for example,epinephrine is produced by the adrenal glands

and can raise blood glucose levels, increase blood flow to muscles, and decrease blood flow to the digestive system

● target cells vary in their response to a hormone because they differ in their receptor types or in the molecules that produce the response

Evolution of Hormone Function● over the course of evolution, the function of a given

hormone may be different between species● For example, thyroid hormone plays a role in metabolism

across many lineages, but in frogs it has taken on a unique function○ in frogs, the thyroid hormone stimulates the resorption of the

tadpole tail during metamorphosis

● Prolactin (hormone used to stimulate milk) also has a broad range of activities in vertebrates

Osmosis and Osmolarity● Osmoregulation is the term for the processes by which

animals control solute concentrations in the interstitial fluid and balance water gain and loss

● cells need a balance between uptake AND loss of water● Osmolarity is the solute concentration of a solution

○ it determines the movement of water across a selectively permeable membrane

● if two solutions are isoosmotic (have the same osmotic pressure), the net flow of water is from the hypoosmotic to hyperosmotic solution

Osmoregulatory Challenges and Mechanisms● osmoconformers: consisting of some marine animals, are

isoosmotic with their surroundings and do not regulate their osmolarity

● osmoregulators: expend energy to control water uptake and loss in a hyperosmotic or hypoosmotic environment

● Marine and freshwater organisms have opposite challenges● Marine fish drink large amounts of seawater to balance

water loss and separate and get rid of salt through their gills and kidneys

Marine Fish● Marine fish drink large amounts of seawater to balance

water loss and separate and get rid of salt through their gills and kidneys

Freshwater Fish● Freshwater fish drink almost no water and replenish salts

through eating● some also replenish salts by uptake across the gills

● land animals have mechanisms to prevent dehydration such as:○ body coverings that help reduce water loss○ drinking water○ eating moist foods○ producing water metabolically

Nitrogenous Wastes● the type and quantity of an animal’s waste products may

greatly affect its water balance● among the most significant wastes are nitrogenous

breakdown products of proteins and nucleic acids● some animals convert toxic ammonia (NH3) to less toxic

compounds prior to excretion

● ammonia excretion is most common in aquatic organisms● vertebrates excrete urea

○ a conversion product of ammonia, which is much less toxic

● insects, land snails, and many reptiles (including birds) excrete uric acid as a semisolid paste (BIRD POO POO)

● it is less toxic than ammonia and generates very little water loss○ but it is energetically more expensive to produce than urea

Excretory Processes● in most animals, osmoregulation and metabolic waste

disposal rely on transport epithelia○ a membrane transporter that transports substances across the

epithelium

● these layers of epithelial cells are specialized for moving solutes in controlled amounts in specific directions

● many animal species produce urine by removing a filtrate derived from body fluids

● key functions of the most excretory systems○ filtration: filtering of body fluids○ reabsorption: reclaiming valuable solutes

○ secretion: adding nonessential solutes and wastes from the body fluids to the filtrate

○ excretion: releasing processed filtrate containing nitrogenous wastes from the body

Invertebrates● No backbone ● Protonephridia-Excretory system of Flatworms● Flame Bulb-Smallest branches of network

Vertebrates● Has a backbone● Animals have highly developed brain● Kidney functions in both osmoregulation and excretion● Excretory System includes ducts ● Humans are vertebrates● Fish, Primates, Reptiles, Birds

Bowman's Capsule● Capillaries and specialized cells ● Permeable to water and small solutes ● Produces a filtrate

Proximal Tubule ● Reabsorption ● Molecules are transported● Toxic materials secreted into filtrate

Descending limb of loop of henle● Reabsorption continues through channels, formed by

aquaporin proteins ● Movement driven by high osmolarity of interstitial fluid● Filtrate becomes concentrated ● First part of the loop of Henle

Ascending limb of loop of henle● Transport epithelium ● Salt but not water move from tubule into interstitial

fluid● Filtrate becomes dilute

Distal tubule● Regulates the K+ and NaCl and reabsorbs ● Controlled movement of ions Contributes to pH Regulation ● Main function is Secretion ● Most distal portion of Nephron ● forms part of the juxtaglomerular apparatus● Juxtaglomerular apparatus controls of systemic blood

pressure and volume

Collecting Duct ● Carries filtrate through medulla to the renal pelvis ● Reabsorbed into blood● Urine is hyperosmotic to body fluids● participates in electrolyte and fluid balance ● network of small tubes that act as a drainage system in

the body● Here it is where large amount of urine is produced (what

you feel when you can't hold it)

Concentrating urine in mammalian kidney ● An important adaptation to life on land● Its functional unit is the nephron● Each nephron consists of a glomerulus, situated in the

cortex of the kidney● Consists of an outer cortex and inner medulla

Adaptations of vertebrate kidney to diverse environments● Animals Kidneys adapt differently● Dessert mammals have long loops of Henle● Birds have short loops of Henle ● Mammals control volume and osmolarity of urine● The South American bat can produce dilute or concentrated

urine

Homeostatic Regulation of Kidney ● The kidney is one of your most important organs of

homeostasis ● A combination of nervous and hormonal inputs ● Inputs contribute to homeostasis for blood pressure and

volume● Keeps fluid balanced● filters fluids and wastes through the body

Antidiuretic Hormone● ADH● An increase in blood osmolarity above a set point

triggers the release of ADH ● Decreased osmolarity causes a drop in ADH secretion ● can treat diabetes insipidus● can increase blood pressure in patients with vasodilatory

shock

Renin-Angiotensin-Aldosterone System ● RAAS Regulates kidney function● Regulates blood ● determinant of target-organ damage● The juxtaglomerular apparatus (JGA) releases enzyme renin● Triggers formation of peptide Angiotensin II

Angiotensin 2 ● peptide hormone● involved in the process of repair and vascular/ muscle

damage ● Raises blood pressure● important part of neurohumoral activation in heart

failure● Stimulates release of the hormone aldosterone

Similarities & differences between ADH & RAAS● Both increase water reabsorption ● ADH responds to changes in blood osmolarity ● RAAS responds to changes in blood volume & pressure● Both work together to keep homeostasis going in our body

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