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Introduction to Physiological Principles Physiology
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Category: Anatomy
Type: Lecture Notes
Tags: water, urine, transport, cells, blood, example, tubule, solutes, osmotic, control, pressure, reabsorption, kidney, freshwater, excretion, increases, collecting, tissues, across, primary, reabsorbed, produced, excreted, specific, hormone, henle, salts, aff
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Description
Chapter 8 - Ion and Water Balance
Transcript
Ion and Water Balance
Overview
Environment has many different meanings
External world for the whole animal
Extracellular fluid for a cell
Cytoplasm for intracellular enzymes
Epithelial tissues provide a barrier with environment
Maintains a favorable profile of solutes and solutions
Overview, Cont.
Animals use combinations of tissues to control ion and water balance in their environments
These tissues regulate three homeostatic processes
Osmotic regulation
Driving force for movement of water
Movement of solutes across membranes
Ionic regulation
Those involved in osmoregulatory strategies
Nitrogen excretion
Toxic end product of protein catabolism
Ionic and Osmotic Challenges
Marine
Animals tend to gain salts and lose water
Freshwater
Animals tend to lose salts and gain water
Terrestrial
Animals tend to lose water
Many animals move between environments and must be able to alter their homeostatic mechanisms
Ionic Regulation
Strategies to meet ionic challenges
Ionoconformer
Exert little control over ion profile within the extracellular space
Exclusively found in marine animals
For example, many invertebrates
Ionoregulator
Control ion profile of extracellular space
For example, most vertebrates
Strategies to meet osmotic challenges
Osmoconformer
Internal and external osmolarity similar
For example, marine invertebrates
Omoregulator
Osmolarity constant regardless of external environment
For example, most vertebrates
Ionic and Osmotic Regulation
Ability to cope with external salinities
Stenohaline
Can tolerate only narrow range
Euryhaline
Can tolerate wide range
Ionic and Osmotic Regulation
Table 10.1
Sources of Water and Solutes
Water
Dietary water
Water preformed in plant and animal tissue
Metabolic water
Water generated as result of oxidative phosphorylation
Drinking
Solutes
K+: intracellular fluids
Na+: extracellular fluids
Classification of Solutes
Distinguished by their effects on macromolecules
Perturbing
Disrupt macromolecular function
Na+, K+, Cl–, SO4+, charged amino acids
Compatible
Little affect on macromolecular function
Polyols (glycerol, glucose) and uncharged amino acids
Counteracting
Disrupt function on their own
Counteract disruptive effects of other solutes when employed in combination
Cell Volume
Cells transport solutes in and out of ECF to control cell volume
Water follows solutes by osmosis
Animal regulates composition of the ECF
to maintain appropriate cell volume
Interstitial fluid especially
RVI RVD
How Controlled?
Epithelial Tissue
Epithelial tissues form boundary between animal and environment
External surfaces
For example, skin
Internalized surfaces
For example, lumen of digestive and excretory systems
Epithelial tissues have physiological functions in respiration, digestion, and ion and water regulation
Integument
Animals change flux of water across body surface by mediating permeability of the integument
Aquaporin proteins increase water permeability 100-fold
Typically animals need to reduce water flux
Cover external surfaces with layer of hydrophobic molecules
Mucus
Cornified stratum corneum with keratin
For example, terrestrial amniotes
Cuticle with chitin
For example, arthropods
Epithelial Tissue Properties for
Ion Movement
Four features of transport epithelia
Asymmetrical distribution of membrane transporters
Cells interconnected to form impermeable sheet of tissue
High cell diversity within tissue
Abundant mitochondria
Solute Movement
Epithelial cells use two main routes of transport
Transcellular transport
Movement through the cell across membranes
Paracellular transport
Movement between cells
“Leaky” vs. “tight” epithelia
Types of transporters
Na+/K+ATPase
Ion channels (Cl–, K+, Na+)
Electroneutral cotransporters
Electroneutral exchangers
Epithelial Cells in Fish Gills
Fish gill lamellae composed of
Mitochondria-rich chloride cells (PNA+)
Pavement cells
Some mitochondria-rich (PNA-)
Some mitochondria-poor
Transport likely carried out by mitochondria-rich cells
Ion Transport by Fish Gills
Direction of ion transport depends on water salinity
Figure 10.11
H+ and HCO3- out for Na+ and Cl-
From 0.01 mM to 100-200 mM!!
Saltwater-Freshwater Transitions
Some fish migrate between saltwater and freshwater (diadromous fish)
Catadromous
Live in freshwater, migrate to saltwater to spawn
For example, eels
Anadromous
Live in saltwater; migrate to freshwater to spawn
For example, salmon
Ion transport functions of epithelia must change during migration
Reverse direction of active NaCl transport
Controlled by hormones
Digestive Epithelia
Water and salts from drinking and food transported across digestive epithelium
Mediate ion and water transfers
Transcellular and paracellular transport involved
Driven by osmotic gradients and facilitated by aquaporins
Internal and external fluids isosmotic – ions secreted into interstitial space to create a transepithelial osmotic gradient driving movement of water across tight junctions
Absorbed water and salts enter blood
Salt Glands
Reptiles and birds possess salt glands
Glands located near the eye, drain into ducts that empty near nostril
Excrete hyperosmotic solutions of Na+ and Cl–
Large amount of salt excreted in small volume of water
Hyperosmotic solutions produced by ion pumps and a countercurrent multiplier
Rectal Glands
Accessory excretory organ in elasmobranchs
Empties into digestive tract
Actively transport Na+ and Cl– from blood into lumen of the gland
Ion transport similar to chloride cell and salt glands
Rate of salt excretion regulated by hormones
Nitrogen Excretion
Ammonia produced during amino acid breakdown is toxic and must be excreted
Ammonia nitrogen excreted in three forms
Ammonia (ammonioteles)
Uric acid (uricoteles)
Urea (ureoteles)
Figure 10.15
Ammonia Excretion
Advantages
Ammonia released by deamination of amino acids
Requires little energy to produce
Disadvantages
Highly toxic
Requires large volumes of water to store and excrete
Figure 10.16
Uric Acid Excretion
Earliest evolutionary solution to permit nitrogen excretion without water
Advantages
Few toxic effects
Can accumulate
But, In humans, Gout
Can be excreted in small volume of water
Disadvantages
Expensive to produce
Figure 10.17
Figure 10.18
Urea Excretion
Advantages
Only slightly toxic
Relatively inexpensive to produce
Disadvantages
Urea is a perturbing solute
Cartilaginous fish use urea to increase tissue osmolarity
Helps prevent water loss in marine environment
Urea’s perturbing effects counteracted by methylamines (TMAO, betaine, sarcosine)
The Kidney
Most animals maintain ion and water balance using some form of internal organ
Multiple cell types combine to produce a tubelike structure
Vertebrate kidneys have six roles in homeostasis
Ion balance
Osmotic balance
Blood pressure
pH balance
Excretion of metabolic wastes and toxins
Hormone production
Kidney Structure
Mammalian kidney has two layers
Outer cortex
Inner medulla
Urine leaves kidney via ureter
Ureters empty into urinary bladder
The Nephron
Functional unit of the kidney
Composed of
Renal tubule
Lined with transport epithelium
Various segments with specific transport functions
Vasculature
Glomerulus
Ball of capillaries from a single arteriole
Surrounded by Bowman’s capsule
Capillary beds surrounding renal tubule
The Nephron and Its Vasculature
Figure 10.20 and Figure 10.21
Efferent always more constricted
Increases pressure in the glomerulus
Urine Production
Four processes
Filtration - Filtrate of blood formed at glomerulus
Reabsorption - Specific molecules in the filtrate removed
Secretion - Specific molecules added to the filtrate
Excretion - Urine is excreted from the body
Regulated by numerous hormones and neurotransmitters that also affect cardiovascular properties
Filtration
Liquid components of the blood are filtered into Bowman’s capsule
Water and small solutes cross glomerular wall
Blood cells and large macromolecules are not filtered
Glomerular capillaries are very leaky
Podocytes with foot processes form filtration structure
Mesangial cells control blood pressure and filtration within glomerulus
Filtrate flows from Bowman’s capsule into proximal tubule
Retrievedfrom http://sitemaker.umich.edu/ransom.lab/files/glomerulus.jpg, 13, 03,09
Reabsorption
Primary urine
Initial filtrate filtered in Bowman’s capsule that is isosmotic to blood
Most water and salt in primary urine reabsorbed using transport proteins and energy
Rate of reabsorption limited by number of transporters
Renal threshold
Concentration of a specific solute that will overwhelm reabsorptive capacity
Each zone of the nephron has transporters for specific solutes
e.g. Reabsorption of Glucose
Glucose is reabsorbed by secondary active transport
Reabsorbed molecules taken up by the blood
Figure 10.23
Renal Threshold
Figure 10.24
Secretion
Similar to reabsorption, but in reverse
Molecules removed from blood and transported into the filtrate
Molecules secreted include
K+, NH4+, H+, pharmaceuticals, and water-soluble vitamins
Requires transport proteins and energy
Tubule Regions
Different regions of the tubule have different transport functions and permeabilities
Proximal tubule
Most of solute and water reabsorption
Loop of Henle
Descending limb
Ascending limb
Distal tubule
Reabsorption completed for most solutes
Collecting duct
Drains multiple nephrons
Carries urine to renal pelvis
Transport in Tubule Regions
Figure 10.25
Obligatory (must occur because of osmolarity of surrounding interstitial fluid)
Facultative (under hormonal control)
Transport in Tubule Regions
Differences in transport and permeability due to differences in epithelium along the tubule
Figure 10.26
Transport in the Proximal Tubule
Most reabsorption of solutes and water from glomerular filtrate
Active transport of Na+, glucose and AA into cortex
Many solutes reabsorbed by Na+ cotransport
Water follows by osmosis
Taken up by peritubular capillaries and returned to venous blood
Proximal tubule also carries out secretion
Figure 10.27
Transport in the Loop of Henle
Descending limb is permeable to water
Water is reabsorbed
Volume of primary urine decreases
Primary urine concentrated
Ascending limb is impermeable to water
Ions are actively transported (thick) and reabsorbed
Primary urine becomes dilute
Reabsorbed ions accumulate in interstitial fluid
Osmotic gradient created in medulla
Osmotic Gradient
Figure 10.28
Countercurrent Multiplier
Loop of Henle does not directly concentrate urine, rather it acts as countercurrent multiplier
Creates an osmotic gradient in medulla that facilitates reabsorption of water
Low osmolarity near cortex
High osmolarity deep in medulla
Figure 10.28
The ability of the kidney to produce urine hyperosmotic to blood plasma is due to the loop of Henle.
Countercurrent Multiplier
Animation
Thick ascending limb
Active Na+ transport, not permeable to water
? solute concentration in surrounding fluid
Descending limb
Permeable to water, not to Na+ and Cl-
Water flows into tubular fluid due to osmosis
Thin ascending limb
Permeable to ions but not water
Ions move down concentration gradient
Maintenance of Medullary Osmotic Gradient
Vasa recta countercurrent multiplier
Transport in the Distal Tubule
Distal tubule can reabsorb salts and water
Distal tubule can secrete potassium
Transport function of distal tubule affected by hormones
Parathyroid hormone increases Ca2+ reabsorption
Aldosterone increases K+ secretion
Figure 10.30
Excretion
After urine is produced, it leaves kidney and enters urinary bladder via ureters
Urine temporarily stored in bladder
Urine leaves bladder via urethra
Sphincters of smooth muscle control flow of urine out of bladder
Opening and closing of sphincters controlled by a spinal cord reflex arc (micturition reflex) and can be influenced by voluntary controls
For any substance, the amount excreted = amount in primary filtrate + amount secreted – amount reabsorbed
Regulation of Urinary Function
Hormones affect kidney function
Steroid hormones
For example, aldosterone
Slow response
Peptide hormones
For example, vasopressin
Rapid response
Dietary factors that affect urine output
Diuretics
Stimulate excretion of water
Antidiuretics
Reduce excretion of water
Glomerular Filtration Rate (GFR)
Is determined by pressure across glomerular wall
Three main forces
Glomerular capillary hydrostatic pressure
Bowman’s capsule hydrostatic pressure
Oncotic pressure – osmotic pressure due to protein concentration in blood
Intrinsic Regulators of GFR
Two intrinsic pathways: both affect vasoconstriction/vasodilation of afferent arteriole
Myogenic regulation
Arteriolar smooth muscle
Tubuloglomerular feedback
Juxtaglomerular apparatus
Macula densa cells in distal tubule
Juxtaglomerular (JG) cells in afferent arteriole
Juxtaglomerular Apparatus
Figure 10.32
Intrinsic Controls of GFR
Figure 10.33
Extrinsic Regulators of GFR
Hormones
Vasopressin (antidiuretic hormone, ADH)
Renin-Angiotensin-Aldosterone (RAA) pathway
Atrial natriuretic peptide (ANP)
Vasopressin
Also called antidiuretic hormone (ADH)
Peptide hormone produced in hypothalamus and released by posterior pituitary gland
? water reabsorption from the collecting duct by ? number of aquaporins
Release stimulated by ? plasma osmolarity detected by osmoreceptors in the hypothalamus
Release is inhibited by ? bp detected by stretch receptors in atria and baroreceptors in carotid and aortic bodies
Vasopressin Increases Cell Permeability
Figure 10.34a
Urine Concentration
Osmotic concentration of final urine depends on permeability (aquaporins) of the collecting duct, which can be regulated by vasopressin
Impermeable
Water not reabsorbed from collecting duct
Dilute urine (formed in ascending limb) excreted
Permeable
Water reabsorbed from collecting duct
Concentrated urine (formed in collecting duct) excreted
Aldosterone
Mineralcorticoids control ion excretion
Steroid hormone produced by adrenal cortex in tetrapods
Aldosterone is the mineralcorticoid in tetrapods
Targets cells in distal tubule and collecting duct
Stimulates Na+ reabsorption and enhances K+ excretion
Release stimulated by Ag II and increases in circulating K+
Aldosterone Stimulates Na+ Reabsorption
Figure 10.34b
What transport proteins would you expect and where would they be inserted?
Renin-Angiotensin-Aldosterone
(RAA) Pathway
Juxtaglomerular cells secrete enzyme renin
Secretion of renin controlled in three ways:
Baroreceptors in JG cells release renin in response to low bp
Sympathetic neurons in cardiovascular control center of medulla trigger renin secretion in response to low bp
Macula densa cells in distal tubule respond to ? in flow by releasing a paracrine signal that induces JG cells to release renin
Renin secreted when blood pressure or GFR lower than normal
Renin-Angiotensin-Aldosterone Pathway
Angiotensinogen
? renin
Angiotensin I
? ACE
Angiotensin II
Helps regulate MAP
Angiotensin II a vasoconstrictor
Raises blood pressure by ? resistance
Aldosterone increases Na+ (and water) retention
Raises blood pressure by ? blood volume
Atrial Natriuretic Peptide (ANP)
Produced in specialized cells within the atria
Secreted in response to stretch associated with increase in blood volume
ANP increases urine output and consequently lowers blood volume and pressure
Acts as an antagonist with RAA pathway
Increases excretion of Na+ in urine
Inhibits secretion of vasopressin
Thirst
Detected and controlled by hypothalamus
Osmoreceptors monitor ionic concentration of body fluids
Receptors monitor levels of angiotensin II
Integrating Systems – Regulation of BP
Invertebrates
Simple animals like worm taxa have protonephridia
Similar to vertebrate tubule
Fluids taken from interstitial space
Most developed in freshwater organisms
Molluscs and annelids have more complex metanephridia
Fluid taken from blood or coelom
Sponges use simple contractile vacuoles to expel cellular waste, including water
Insects
Malpighian tubules are the insect equivalent to vertebrate kidney
Empties into hindgut
Primary urine formed by secretion, not filtration
Reabsorption in hindgut modifies primary urine
Diuretic hormones increase urine formation
CRF-related diuretic hormones
Insect (myo)kinins
Cardioacceleratory peptides
Less is known about antidiuretic hormones
Chondrichthian Kidneys
Sharks slightly hyperosmotic to seawater due to high urea concentrations
Countercurrent arrangement recovers up to 90% of the urea from primary urine
Final urine slightly hyposmotic relative to shark tissues and isosmotic to seawater
Fish Kidneys
Role of the kidney differs in freshwater and seawater
Freshwater
Ions reabsorbed from primary urine
Excretion of very dilute urine
Seawater
Produce only small amounts of urine
Most ion, water, and nitrogen excretion responsibilities met by gills and skin
Some marine fish lack glomeruli (aglomerular kidney)
All fish nephrons lack a loop of Henle
Amphibian Kidney
Structure and function of changes with metamorphosis
Structure
Pronephros in larval forms
Tubule opens into coelom
More mammal-like nephron in adult
Function
In aquatic life, little need for water retention
Excretion of dilute urine
Conserve water on land
Reduce the GFR
Reabsorb water from bladder
Comparison of Vertebrate Nephrons
Figure 10.37
Terrestrial Animals
Major innovation was loop of Henle, allowing production of concentrated urine
Mammals producing more concentrated urine have longer loop of Henle and relatively thicker medulla
Birds and reptiles without a loop of Henle conserve water by excreting uric acid
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