Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Water & Electrolyte Homeostasis:  Osmoregulation & the Kidney
 

I.  Homeostasis & the need for osmoregulation
    Organisms must maintain the volume and content of their internal fluids within a tolerable range.  To complicate matters, they must be able to do this no matter what type of environment.  For example, marine, freshwater and terrestrial animals all maintain homeostatic conditions in their interstitial fluids even though the external environments present very different "problems".  Here we  focus on how terrestrial vertebrates deal with osmoregulation.

A.  Fluid Content in Vertebrates
    The water content of an individual is a balance of the water input and the water output.  Or, in other words:  water content = input - output.

1. Input - water derived from food, drink, osmotic uptake from the environment, and water from metabolism (i.e., respiration)

2. Output - water lost from evaporation (i.e., sweating, breathing) and excretion (i.e., urine, feces).

3. Normally, input = output.  If input > output, then the organism/system will swell;  If input < output, then the organism/system will shrivel, dry out.

B.  Water deficits (input < output)
     This is a major concern for terrestrial vertebrates (even marine fish) - therefore these organisms have a series of adaptations for minimizing water loss which include:

1. water impervious covering (i.e., skin);

2. kidneys for efficient water use/recycling; and 

3. various behavioral modifications (i.e., thirst, nocturnal habit in dry environments)

4. marine fish drink lots

C.  Anhydrobiosis:  Surviving desiccation
    My favorite adaptation is anhydrobiosis, which literally translates into �life without water�.  This phenomenon, which was first discovered by the famous Dutch microscopist Leeuwenhoek, is relatively common in organisms that live in ephemeral, aquatic environments.  These animals are capable of drying down and then being rehydrated and returning to life.  Examples include tardigrades, nematodes and rotifers.   Active dry yeast is another good example.  

    To read more about anhydrobiosis in fungi, click here.

D.  Water surplus (input > output)
    Problem for freshwater fish - produce lots of dilute urine

II.  A brief review of osmosis

A.  Osmosis
    Refers to the diffusion of water across a membrane.  The rate and direction of osmosis is determined by (among other things) the difference in solute concentration on either side of the membrane.  Recall that solutes lower the energy state of water and hence water will move from an area of lower solutes to one of higher solutes.

B.  Osmolarity
    Is a measure of the concentration of all osmotically-active solute particles in a solution.  If 1 mol of sucrose is dissolved in 1 kg of water it will yield a solution with an osmolarity of 1.0.  However, if 1 mol of NaCl is dissolved in 1 kg of water the resultant solution has an osmolarity of 2.0 because salt dissociates to form 2 osmotically-active particles (Na⁺ and Cl^-).

    The actual units of osmolarity used in studies of animal water regulation are given in terms of milliosmoles Liter^-1.  As a generalization, 1 mosm L^-1 = 10^-3 M.  Some examples: blood = 300 mosm L^-1; seawater = 1000 mosm L^-1.

C.  Water moves osmotically in response to a gradient in osmolarity.
    Other factors being equal, water moves from:  LOW osmolarity → HIGH osmolarity.  Osmosis will continue until the difference between the two areas is zero (i.e., dynamic equilibrium).

D.  Two solutions with the same osmotic concentration are called iso-osmotic.  If one solution has fewer solute particles than another it is termed hypo-osmotic (remember hypodermic - beneath the skin).  A solution that has more solute particles than another is termed hyperosmotic.  Note - these terms are relative. 

    All other factors being equal - water moves osmotically from a:  HYPOosmotic solution → HYPERosmotic solution.

III.  Kidney/excretory System -  Meet "Headphones Dude�
    The basic parts of the excretory system include:

A.  Kidney

1.  Structure.  The kidney is comprised of 

• renal capsule (covering of kidney);

• Cortex -  outer layer of kidney;

• medulla -  inner layer of kidney; and

• renal pelvis - central cavity.

2.  General

• Kidneys are about the size of your fist.

• They are only about 1% of body size but receive about 20% of the blood from each heartbeat (a total of about 2000 L day^-1)

B.  Ureters - transport urine from kidney to bladder

C.  Bladder - stores urine

D.  Urethra - transport urine from the bladder out of the body

E.  Renal artery/veins

IV.  Nephron structure

A.  General

• the main functional unit of the kidney.  

• Made up of blood vessels and a slender tubule. 

• There are about 10⁶ nephrons in each human kidney.

B.  Meet "Snake Dude" - A Saupian nephron diagram

1.  Structure.  The nephron consists of:

• Bowman's capsule

• Glomerulus - cluster of capillaries

• Proximal (convoluted) tubule

• loop of Henle (not found in all nephrons)

• Distal (convoluted) tubule

• Collecting duct

2.  The nephron is oriented perpendicularly to surface of kidney.

3.  The nephron is associated with capillaries (peritubular capillaries -  surround the  convoluted tubules;  vasa recta - surround the Loop of Henle)

IV. Nephron Action
    Nephrons are involved in three main activities:  (1) filtration; (2) secretion; and (3) resorption.

A.  Filtration
    Blood pressure forces fluid across epithelium of glomerulus.  This process is NON-SELECTIVE - any small molecules will move across membrane including water, glucose, salts, vitamins, drugs, and nitrogenous wastes like urea.  Larger molecules like proteins do not normally enter glomerulus.  The product is called filtrate (like the stuff in a funnel in an icky chemistry lab).  Out of the 2000 L of blood delivered daily to kidney, about 180 L of filtrate result (of this only about 1.5 L excreted in urine).

B.  Secretion
    This refers to the SELECTIVE dumping of excess protons, potassium ions, etc. into the filtrate.  In other words, moving materials out of the interstitial fluids into the nephron.  This occurs at both the distal and proximal tubules.  Involves passive and active transport.

C.  Resorption
    Reclaiming water and other goodies (solutes) from the filtrate.  In other words, moving materials from the nephron into the interstitial fluids.  Selective.

D.  Together, reabsorption and secretion serve to regulate the composition of the filtrate.

V.  Specifics of Renal Function

A.  Proximal tubule
    Located in the cortex.  Important for controlled secretion (ammonia, protons) and resorption (NaCl, water, glucose, bicarbonate).  The majority of water and salt is reabsorbed here.   Important for pH control (produces and excretes protons, absorbs bicarbonate). 

B.  Descending Loop of Henle
    In the medulla.  Permeable to water, not salt.  Water follows gradients in osmolarity.  Salt concentration increases in the filtrate.

C.  Ascending Loop of Henle
    In the medulla.  There is a thick and a thin region.  Both are impermeable to water and both permit the passage of salt.  The thin (lower) region allows for passive transport, the thicker (upper) region actively transport salt out.  The movement of salt out of the loop helps maintain high osmolarity in interstitial fluid to permit osmotic recovery of water.

D.  Distal Tubules
    In the cortex.  Like the proximal tubules, it is also important for controlled secretion and absorption.  Secretes potassium and hydrogen ions; reabsorbs sodium ions, water, bicarbonate.  Thus, important in regulation of blood pH (remember the carbonic acid/bicarbonate buffer system in blood?)

 E.  Collecting duct
    permeable to water, not salt.  Permeable to urea, which accumulates in inner medualla region, also responsible for high osmolarity.

VI.  Kidney Regulation

A.  Response to low blood pressure

kidney → renin →  converts angiotensinogenin to angiotensin  which:

1. increases thirst →  increase water input  →  increase blood pressure

2. blood vessels constrict →  increases blood pressure

3. adrenal gland →  aldosterone →  increases permeability convoluted tubules →  increases sodium resorption →  increases water resorption →  increases blood volume →  increases blood pressure

B.  Response to high blood pressure

stimulates atrial stretch receptors →  atrial naturietic hormone →  kidney →  decreases sodium resorption  →  decreases water resorption →  decreases blood volume →  decreases blood pressure

C.  Response to high osmolarity (> 300 mOsm)

hypothalamus  posterior pituitary  release ADH (antidiuretic hormone)  stimulate aquaporin synthesis & insertion into membrane  increase collecting duct permeability  increase water resorption  increase blood volume which:

1. decreases blood osmolarity

2. decreases urine output

3. increases [urine]

 D.  General

• a diuretic is something that increases the flow of urine.  An anti-diuretic thus reduces urine volume.  In this case, ADH stimulating water reabsorption thereby reducing urine output.

• alcohol blocks ADH release - thus dehydrated after drinking = hangover.