Ch. 48: Osmoregulation

body fluids

make up 45-75% of our body

luid amount affected by

age (infant > young > elderly)

tissue ration (adipose to skeletal muscle)

skeletal muscle

~75% water

Loss of skeletal muscle mass often occurs with aging

some diseases - vitamin D

adipose

~20% water

more adipose = more susceptivity to fluid imbalance

intracellular fluid

~60-65% water

Majority of body fluid

extracellular fluid

majority is interstitial fluid and blood plasma (variable)

synovial fluid, eye fluids, CSF, inner ear, serous fluids (stable)

interstitial fluid

~32%

Forms from blood plasma

plasma

~8%

transports nutrients, gases, waste products

Electrolytes form ions in solution

Na, Cl, Mg

Osmosis

water movement specifically

H20 diffuse through a selectivity permeable membrane

osmotic pressure

Pressure exerted on hypersonic side of membrane to prevent net movement of water from hypotonic side

Goal = maintain equal osmolarity

osmolarity

Defined as number of osmoles of solute per liter of solution

Description: number of particles produced when a solute dissolves

Example: mole of NaCl in solution produces a mole of Na and a mole of C (both ions affect osmotic pressure of solution)

Mole of NaCl = two osmoles

osmoregulation

requires active regulation of osmotic pressure

Most control level of H2O and what is in the water

ion, minerals, and metabolites in solution to maintain homeostasis

examples of metabolites: glucose vitamins, amino acids

Excretory systems help maintain homeostasis

Excrete metabolic wastes

Primarily H2O CO2 and nitrogenous compound

Involved in other processes

pH regulation: metabolic regulation

RBC production: erythropoietin

Blood pressure regulation: renin-aldosterone-angiotensin pathway

nitrogenous wastes

result of protein breakdown and are excreted or reabsorbed

ammonia

much cycles back to liver and enters urea cycle (mammals)

Habitat: Animals living in or having access to H2O

Aquatic or terrestrial dwellers

Toxicity: Highly toxic,

Disrupts pH and chemical reactions, Terrestrial

urea

helps establish osmotic gradient in interstitial fluid

Habitat: aquatic or terrestrial

Toxicity: Less than ammonia, Still requires H2O for excretions, Serves as an osmolyte to maintain cell volume

uric acid

gout

Habitat: limited or no access to water [dry environments]

Terrestrial dwellers and shelled embryos

Toxicity: Very little,

Excreted as semi solid paste, almost no H2O required,

Advantage to embryos developing inside shell

Animals: Reptiles birds insects (limited in mammals), some species of arboreal frogs (arid)

cockroaches

recycle as nitrogen source when low protein diet

creatinine

produced from creatine metabolism in muscle [nonprotein source]

only secreted

Osmoconformer

Isosmotic with environment (salt water)

Animals [marine]:

Mainly invertebrates (starfish, crab, jellyfish, lobsters, sharks, skates, and hagfish)

Osmoregulator

Maintain H2O and/or ionic balance independent of environment

Animals: vertebrates,

marine invertebrates living in coastal lagoons and estuaries

protists

contractile vacuole

nematodes

Renette glands (one or two)

Like antennal gland in crayfish

Role Osmoregulation and excretion [invertebrates]

Structure: simple or branching tubes

Nephridiopores: surface opening of tubes

Protonephridia

Animals: acoelomates and coelomates, Platyhelminthes, some annelids

Tubule structure: Interstitial flame cells on end of tubules

Internal cilia propels interstitial fluid into tubules

Flame cell structure:

Slit like openings cilia hang down in center

Excess fluid: Enters flame cell and exits via nephridiopores

Urine: often hypo-osmotic (can be modified), Most nitrogenous waste leaves across body surface

Metanephridia

Animals: Most annelids, mollusks and crustaceans, coelomates

Tubule structure: open at both ends

Funnel cell structure: opens into interstitial (funnel) on end of tubule

Cilia extend out from internal organ

Excess fluid: Drawn into tubule, Solutes from coelom move into tubule and exit through nephridiopores

Urine may be hypertonic (can be modified by surrounding capillaries)

insects (invertebrates) osmoregulation

have blind ended Malpighian tubules

Location: interstitial

Structure: extensions from gut wall

Function:

Ions (k, Cl) and uric acid actively transported into lumen

Lumen become hypertonic so H2O enters

Fluid modified as it asses along tubule

Empties into gut where cells in rectum can further modify fluid

Urine:

iso, hypo, or hypertonic (conserves water)

vertebrates osmoregulation

Most vertebrates: kidney is main osmoregulatory and excretory organ

Other structures excrete waste products but not their major function

Osmoregulation: Freshwater fish

Body fluids: hypertonic to environment

Kidneys: dilute urine

Extreme ~ 10% of nitrogenous waste

Gills: Special gill cells actually transport salts from H2O into body

Excrete most nitrogenous waste

Water - in

Drinking - no excess water

osmoregulation: saltwater fish

Body fluids: hypotonic to environment

Kidneys: little urine

Small or no glomeruli

Gills: excrete ammonia

Water - out

Drinking - yes, salt water

osmoregulation: Cartilaginous fish (sharks)

Body fluids: accumulate urea, hypertonic to environment (facilitates water moving into body)

Kidneys: large volume hypotonic urine

Water - in

Drinking - some with eating

Osmoregulation: Saltwater drinking animals

Salt glands: remove excess salt from blood without moving H2O out

Animals: marine birds, sea turtles

Marine mammals:

Highly concentrated urine, High-protein diet so produce large amount of urea, Must be excreted in urine without losing much H2O

human urinary system structures

Kidneys, urinary bladder, and associated ducts

urine

produced in kidneys

flows through a ureter into bladder

urination

fluid from bladder through urethra

urethra

Males - long (part of reproductive system)

Females - short

kidney function

excretory - maintain homeostasis by regulating fluid and electrolyte balance

produce renin (enzyme) and erythropoietin

activate D3 - stimulate Ca absorption by intestine

nephron

functional unit of kidney

contains renal corpuscle that contains the renal capsule (bowmans capsule) and glomerulus, renal tubule regions, and collecting duct

renal capsule (bowmans capsule)

surrounds glomerulus

glomerulus

fenestrated capillaries

all glomeruli are in cortex

Surrounded by podocytes

Plasma fluid and small solutes pass through becoming filtrate

renal tubule regions

Proximal tubule

Proximal convoluted tubule

Loop of Henle: nephron loop

Distal convoluted tubule

Distal tubule

collecting duct

several nephrons empty into collecting duct

Cortical nephrons

(~ 85% of nephrons)

Majority

Small glomerulus

Almost entirely within cortex or outer medulla

Dilute urine

Juxtamedullary nephrons

Large glomerulus

Long loop of henle (deep into medulla)

Concentrating urine (hypertonic) and conserving H2O

afferent arteriole

deliveries blood

Efferent arteriole

carries blood away

peritubular capillaries

Surround cortical tubule

Absorption and secretion

Vasa recta

Straight capillaries

Around nephron loop and collecting duct

filtration membrane

Podocytes cover surface of capillary forming filtration slits

Foot processes of adjacent podocytes are separated by narrow gaps

Keeps proteins and large molecules out of filtrate

Protein appears in urine when barrier malfunctions

filtration pressures

Pressures involved in urine formation

decreases glomerulare filtration

afferent arteriole vasoconstriction

efferent arteriole vasodilation

increases glomerular filtration

afferent arteriole vasodilation

efferent arteriole vasoconstriction

Proximal Tubule Reabsorption and Secretion

Bulk return

Reabsorption: needed substances such as water (65%),

large amount of electrolytes (e.g., K+)

nutrients 100% (e.g., glucose and amino acids)

bicarbonate (acid-base balance)

Tubular transport maximum (Tm): Maximum rate at which a substance can be reabsorbed

Secretion: H+, nitrogenous wastes (ammonia), drugs

Distal tubule reabsorption and secretion

fine tuning and water regulation

Reabsorption: Na+, Cl-, Ca2+,water, bicarbonate

Distal tubule responds to hormones

Secretion: K+, H+

collecting duct reabsorption and secretion

Reabsorption: water, Urea

Secretion or reabsorption: Bicarbonate, K+, H+, Cl-, Na+

production of dilute urine

Late distal tubule and collecting duct not reabsorbing H2O

Occurs when extracellular H2O ish hight (excess fluid)

Ions still absorbed as needed along way

concentrating urine location

nephron loop

used energy (ATP) to create osmotic gradient

Driving water reabsorption

thick limb (ascending limb)

Na/K/2Cl symporter

NaCl pumped out of filtrate

Interstitial fluid osmolarity increased

Thin limb (descending limb)

Freely permeable to H2O

High interstitial osmolarity (NaCl and urea) draws H2O out

concentration of filtrate

more: approaches bottom of nephron loop less: in the ascending limb

High NaCl concentration in filtrate increases symporter activity

symporter activity

causes filtrate to become more dilute as it ascends

Urea contributes to interstitial osmolarity

Urea passively moves out of collecting duct

(30-50% remains in filtrate and is excreted)

Urea entering interstitial is recycled (maintain osmolarity) (vasa recta highly involved)

osmotic gradient

necessary to produce concentrated urine

Water moves according to gradient

Countercurrent exchange

H2O and NaCl changed between nephron loop, interstitial fluid, and blood in vasa recta

hormones affecting kidney

antidiuretic hormone (ADH), aldosterone, angiotensin II, atrial natriuretic peptide (ANP

kideys are involved in

maintaining fluid volume and electrolyte balance, regulating circulating levels of Na and K, and regulating blood pH

hypothalamus

receptors sense osmotic changes in blood and dehydration

antidiuretic hormone (ADH)

is a neurohormone

Secreted from posterior pituitary

acts on aquaporin-2 (water channels) in collecting duct - more permeable to H2O (h2o reabsorbed)

release decreases when blood becomes diluted and osmotic pressure falls

angiotensin II synthesized in response

low blood pressure

angiotensin II causes

efferent arteriole constriction, increases GFR and FF, Na reabsorption in proximal and distal nephron

angiotensin II net effect

preservation of renal function in low volume state with simultaneous Na reabsorption to maintain circulating volume

ANP secreted in response

to increased atrial pressure

ANP causes

increased GFR and Na filtration with no Na reabsorption in distal nephron

ANP net effect

NA loss and volume loss

ANP effects

afferent arteriole and distal convoluted tubule

inhibits release of renin, aldosterone, ADH

Causes kidneys to excrete Na (inhibits aldosterone) and H2O (inhibits ADH)

Dilates afferent arteriole

angiotensin II effects

efferent arteriole, proximal convoluted tubule, ascending limb of loop of henle, and distal convoluted tubule

aldosterone secreted in response

blow blood volume and high K levels in plasma

aldosterone causes

high Na reabsorption, high K secretion, and high H secretion

aldosterone effects

distal convoluted tubule and collecting duct

ADH secreted in response

high plasma osmolarity and low blood volume

ADH causes

high numbers of aquaporins due to binding on receptors on principal cells and high H2O reabsorption

ADH effects

collecting duct

Juxtaglomerular cells

Location: mainly in afferent arteriole, some in efferent

stimulus: vessel stretch (BP)

Sympathetic innervation: low BP, less stretch

Stimulates renin (enzyme) release from juxtaglomerular cells

macula densa cells

Location: distal tubule, basilar membrane contracts afferent arteriole

Stimulus: filtrate concentration and flow in distal tubule

Paracrine signals to juxtaglomerular cells: simulates renin release

RAAS

Renin is released by kidneys in response to decreased blood volume; causes angiotensinogen to split & produce angiotensin I; lungs convert angiotensin I to angiotensin II using ACE enzyme; angiotensin II stimulates adrenal gland to release aldosterone & causes an increase in peripheral vasoconstriction

kideys only conserve water

Cannot add fluid or additional solutes

kidneys clear excess fluid or solutes

Physiological response to severe dehydration

Nestor osmolarity first (avoid cell shrining and swelling)

ADH

Volume second

regulating acid-base balance

hemoglobin buffers much of H

bicarb major buffer of non-respiratory H

have discussed respiratory compensatory mechanisms (depth and rate of ventilation)

kidneys excrete or reabsorb H and bicarb

excrete H when excess acid

excrete bicarb when excess base

collecting duct response to acidosis

excretion of H

production and absorption of bicarb

K reabsorption

collecting duct response to alkalosis

absorption of H

excretion of bicarb

excretion of K

Multisystemic regulation of acid-base balance

Involves buffers, lungs, and kidneys

Kidneys take longer in response to (secretion and absorption

metabolic regulation)

Multiple buffering mechanisms in circulation and cells

Lungs (ventilation) changes occur rapidly and reflexively

quick turn around type of regulation