Transcript Unit 3-6 Urinary System Notes File

I. Osmoregulation = Solute (salts) / Solvent (H2O) Balancing
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Physiological systems of animals operate in a fluid environment (cells are in liquid)
Relative concentrations of water and solutes must be maintained within fairly narrow limits
Osmoregulation regulates solute concentrations and balances the gain and loss of water
Controls movement of solutes and/or solvents between internal fluids and the external environment
Examples:
• Freshwater animals
– Too much water  dilutes solutes
– Have adaptations that reduce water uptake and conserve solutes (excrete less Na +)
• Desert and marine (i.e. saltwater) animals face desiccating environments that can quickly deplete body
water
Homeostasis of body fluids
• Maintain the salt (solute) concentration
• Maintain water concentration
• Maintain the osmotic concentration
Also:
• Maintain H+ concentration (blood pH)
• Excrete waste products (e.g. nitrogenous wastes)
II. Excretion of Wastes
An animal’s nitrogenous wastes reflect its phylogeny and habitat
• The type and quantity of an animal’s waste products may greatly affect its water balance
• Among the most important wastes are nitrogenous breakdown products of proteins and
nucleic acids  ammonia (NH3)
• Some animals convert toxic ammonia (NH3) to less toxic compounds prior to excretion
Requires large
volumes of urine
Small volumes of urine
but energy expended to
produce
Ammonia
• Animals that excrete nitrogenous wastes as ammonia need lots of water
• They release ammonia across the whole body surface or through gills
Urea
• The liver of mammals and most adult amphibians converts ammonia to less toxic urea
• The circulatory system carries urea to the kidneys, where it is excreted
• Conversion of ammonia to urea is energetically expensive; excretion of urea requires less
water than ammonia
Uric Acid
• Insects, land snails, and many reptiles, including birds, mainly excrete uric acid
• Uric acid is largely insoluble in water and can be secreted as a paste with little water loss
 not urine
• Uric acid is more energetically expensive to produce than urea
Transport Epithelia
• specialized epithelial cells that regulate solute movement
• essential components of osmotic regulation and metabolic waste disposal
• arranged in complex tubular networks
Example: ialt glands of marine birds remove excess sodium chloride from the blood
Protonephridia: The Flame-cell system of planaria
• network of dead-end tubules connected to
external openings
• smallest branches capped by a cellular unit
called a flame bulb
• These tubules excrete a dilute fluid and
function in osmoregulation
Metanephridia
Each segment of earthworm has pair
of open-ended metanephridia:
collect coelomic fluid and produce
dilute urine for excretion
Malpighian Tubules
• Arthropods
• Insects produce a relatively dry waste
matter, an important adaptation to
terrestrial life
Excretory Processes
Most excretory systems produce urine by refining a filtrate derived from body fluids
Key functions of most excretory systems:
– Filtration: pressure-filtering of body fluids
– Reabsorption: reclaiming valuable solutes
– Secretion: adding toxins and other solutes from
the body fluids to the filtrate
– Excretion: removing the filtrate from the system
Kidney Functions
1. Filters 200 liters of blood daily
toxins, metabolic wastes, and excess ions leave the body in urine
2. Regulate volume and chemical makeup of the blood
3. Maintain the proper balance between water and salts, and acids and bases
Other Renal Functions
4. Gluconeogenesis during prolonged fasting
5. Production of renin to help regulate blood
pressure and erythropoietin to stimulate
RBC production
6. Activation of vitamin D
The Nephron
Nephrons are the structural and functional units that form urine
Two main parts
1. Glomerulus: a tuft of capillaries
2. Renal tubules:
• Glomerular (Bowman’s) capsule –cup-shaped end that completely surrounds the glomerulus
- Renal corpuscle – the glomerulus and its Bowman’s capsule
• Proximal Convoluted Tubules
• Loop of Henle
• Distal Convoluted Tubues
• Collecting Duct
(technically not part of nephron)
Capillary Beds of the Nephron
Every nephron has two capillary beds
1. Glomerulus - Each glomerulus is:
• Fed by an afferent arteriole
• Drained by an efferent arteriole
2. Peritubular capillaries or vasa recta
• Arise from efferent arterioles
• low-pressure & porous
• Cling to adjacent renal tubules
• Empty into the renal venous system
Peritubular
capillaries
Vasa recta
Filtration: Blood pressure forces fluids and solutes out of the glomerulus into the renal tubules
Tubular Reabsorption
process whereby most tubule contents are returned to the blood
Transported substances move through three membranes
Luminal and basolateral membranes of tubule cells
Endothelium of peritubular capillaries
Only Ca2+, Mg2+, K+, and some Na+ are reabsorbed via paracellular (inbetween cells) pathways
Sodium Reabsorption: Primary Active Transport
Na+ enters the tubule cells at the luminal membrane, because of conc. gradient derived from
 Na+-K+ ATPase pump - sodium actively transported out of the tubules
From there it moves to peritubular capillaries due to:
• Low hydrostatic pressure (not much water in capillary)
• High osmotic pressure of the blood (relatively low amounts of salts in blood)
+
Na reabsorption provides the energy and the means for reabsorbing most other solutes
Reabsorption by PCT Cells - Site of most reabsorption
• 65% of Na+ and water, All nutrients, Ions, Small proteins
Loop of Henle
• Descending limb: H2O
• Ascending limb: Na+, K+, Cl
DCT and collecting duct - Reabsorption is hormonally regulated
• Ca2+ (regulated by PTH)
• Water (regulated by ADH)
• Na+ (regulated by aldosterone and ANP)
Tubular Secretion
Reabsorption in reverse –substances move from peritubular capillaries or tubule cells into filtrate
Tubular secretion is important for:
Disposing of substances not already in the filtrate
Eliminating undesirable substances (urea and uric acid)
Ridding the body of excess potassium ions
Controlling blood pH
The loop of henle and collecting duct form a complex countercurrent system
Countercurrent trades osmolarity
ie- [NaCl] (NaCl and H2O)
In the descending loop H2O leaves
 Increasing Osm (conc)
In the ascending loop NaCl leaves
 Decreasing Osm (conc)
With a small amount of active
transport (energy), a lot of H2O &
NaCl can be reabsorbed passively
(without energy consumption)
In the collecting duct H2O leaves
 Increasing Osm (conc)
Nonreabsorbed Substances
A transport maximum (Tm):
Reflects the number of carriers in the renal tubules available
Exists for nearly every substance that is actively reabsorbed
When the carriers are saturated, excess of that substance is excreted
Substances are not reabsorbed if they:
Lack carriers
Are not lipid soluble
Are too large to pass through membrane pores
Urea, creatinine, and uric acid are the most important nonreabsorbed substances
DCT and collecting duct - Reabsorption is hormonally regulated
• Ca2+ (regulated by PTH)
• Water (regulated by ADH)
• Na+ (regulated by aldosterone and ANP)
-ADH
Juxtaglomerular
Apparatus (JGA)
↑ Blood NaCl
or ↓BP
↑ renin
↑ ADH
+ADH
Antidiuretic hormone (ADH) inhibits diuresis (urine ouput)
This equalizes the osmolality of the filtrate and the interstitial fluid
In the presence of ADH, 99% of the water in filtrate is reabsorbed
Diabetes Insipidus – inability to produce ADH
Diuretics
Chemicals that enhance the urinary output include:
Any substance not reabsorbed
Substances that exceed the ability of the renal tubules to reabsorb it
Substances that inhibit Na+ reabsorption
Osmotic diuretics include:
High glucose levels – carries water out with the glucose – Diabetes Mellitus
Alcohol – inhibits the release of ADH
Caffeine and most diuretic drugs – inhibit sodium ion reabsorption
Lasix and Diuril – inhibit Na+-associated symporters
Urine
Urine is 95% water and 5% solutes
Nitrogenous wastes: urea, uric acid, and creatinine
Other normal solutes include:
Sodium, potassium, phosphate, and sulfate ions
Calcium, magnesium, and bicarbonate ions
Color and transparency
• Clear, pale to deep yellow (due to urochrome) - Concentrated urine has a deeper yellow color
• Drugs, vitamin supplements, and diet can change the color of urine
• Cloudy urine may indicate infection of the urinary tract
Odor
• Fresh urine is slightly aromatic
• Standing urine develops an ammonia odor
• Some drugs and vegetables (asparagus) alter the usual odor
pH
• Slightly acidic (pH 6) with a range of 4.5 to 8.0 - Diet can alter pH
Specific gravity (density, water = 1)
• Ranges from 1.001 to 1.035 (means more solutes in it)
• Is dependent on solute concentration
Renal Clearance
The volume of plasma that is cleared of a particular substance in a given time
Renal clearance tests are used to:
Determine the Glomerular Filtration Rate (GFR)
Detect glomerular damage
Follow the progress of diagnosed renal disease
For any substance freely filtered and neither reabsorbed nor secreted by the kidneys (e.g., inulin),
RC = GFR = 125 ml/min (~2%)
If RC < 125 ml/min (2%), the substance is reabsorbed
If RC = 0, the substance is completely reabsorbed
If RC > 125 ml/min (2%), the substance is secreted (most drug metabolites)