Osmoregulation and Ionic Balance

I. Osmoregulation = the mean by which animals control water and salt content of the body.

A. Generalities

1. Most vertebrates are 65-80% water

a. ~75% of water is intracellular

b. ~20% is extracellular

c. 5-10% is vascular

2. Solutes give water osmotic pressure. The higher the concentration of solutes, the higher the osmolarity, or osmotic pressure (P_os).

3. Basic osmotic environments

a. Aquatic

i. Sea water = generally 1 Osm/L

ii. Freshwater

- ranges from distilled water to brackish

- ranges from 10 mOsm/L to 300 mOsm/L

b. Terrestrial

i. Mesic (moist with plenty of water available)

ii. Xeric (dry, little water available)

4. Most animals regardless of the environment in which they live try to maintain body fluids at 300 – 350 mOsm/L.

a. In sea water, animals face the problem of constantly losing water and gaining salts.

b. In fresh water, animals face the problem of constantly losing salts and gaining water.

c. Terrestrial organisms have the problem of locating and retaining water.

5. The overall consequence of osmoregulation is osmotic steady state in which water in = water out and salts in = salts out.

a. In terrestrial animals, water comes in via drinking and exits via urine, feces, perspiration and breathing.

b. In fresh water aquatic animals, water comes in through gills and exits through urine and feces.

B. Principal Organs of Osmoregulation

1. Kidney

2. Bladder

3. Skin

4. Salt gland

5. Gut

6. Gills

II. Kidney

A. Functions in osmoregulation (also functions indirectly in gas exchange b/c of role in regulating RBC production)

1. Regulating amount of water in body, retaining or getting rid of it, depending on animal’s needs.

2. Regulates amount of salts and the ratios of various ions to each other.

3. Removes waste products. This is an osmotic problem because water is required for their elimination.

B. Mechanisms involved in performing functions

1. Ultrafiltration

a. Blood is filtered to form a filtrate. Substances move from the blood to the filtrate.

b. Substances are forced out of the blood by high hydrostatic pressure.

c. Kidney forms a tubule filtrate.

2. Reabsorption

a. Substances move from the filtrate back into the blood.

b. Selective reuptake of substances from filtrate.

c. Water moves back into the blood because of high osmotic pressure.

3. Secretion

a. Substances move from the blood to the filtrate.

b. The kidney actively and selectively transports solutes into the filtrate.

C. Structures required to perform function

1. Glomerulus

a. Components

i. Ball of capillaries supplied by an afferent arteriole and drained by an efferent arteriole.

ii. Bowman’s capsule

- epithelial cells called podocytes surround each capillary and form filtration slits through which the filtrate passes

- together with other epithelial cells not in contact with the capillaries, podocytes form a space in which the initial filtrate is collected

b. Function

i. Formation of the ultrafiltrate

ii. Hydrostatic pressure of blood in the capillary is high, and fluid is forced out of the capillary into the Bowman’s capsule.

iii. Rate of formation of filtrate = glomerular filatration rate (GFR), and different organisms have a characteristic GFR.

- GFR depends on difference between osmotic pressure and hydrostatic pressure in capillary.

- Regulation of GFR can be achieved by altering P_osm or P_hydrostatic.. Hydrostatic pressure is regulated by constriction or dilation of afferent or efferent arterioles. GFR also depends on rate of flow of blood in the rest of the body and the permeability of the glomerular capillaries. Permeability of capillaries can be changed by prostaglandins and other hormones.

- Human GFR = ~120 ml/min

c. What’s filtered

i. Water

ii. Everything in the blood with a MW < 5000

d. Consequence of filtration àOsmotic pressure in capillaries rises

2. Proximal tubule (a.k.a., proximal convoluted tubule)

a. Function

i. tubular reabsorption

ii. PCT cells have very active Na⁺ pumps, which have a net effect of pumping Na⁺ out of the filtrate into the blood. Water and chloride follow because the tubule is permeable to both.

b. Consequences

i. 75% decrease in volume of the filtrate

ii. Filtrate becomes isoosmotic WRT blood

iii. Secondary active transport is used to return other valuables to the blood, e.g. glucose and a.a.’s.

3. Loop of Henle

a. Descending loop

i. Characteristics

- impermeable to salts

- highly permeable to water

ii. Consequences

- Water leaves tubule passively because outside of tubule, there’s a high concentration of salt.

- Filtrate becomes more concentrated

b. Ascending loop

i. Characteristics

- impermeable to water

- highly permeable to salts; pumps NaCl actively out of the lumen, into the surrounding interstitium

ii. Consequences

- Salt leaves; water stays

- Filtrate becomes more dilute

c. Net effects of entire loop

– establishes an osmotic gradient in the medulla of the kidney

– concentrates nitrogenous waste urea in the tubule

– conserves water

4. Distal Tubule

a. Functions

i. absorption and secretion

ii. Controls blood ion concentration and ratios (especially Na⁺:K⁺. Has pumps for Na⁺, K⁺, Cl^-, HCO₃^-, and H⁺.

iii. Controls blood pH by secreting or withholding bicarbonate ions and protons.

iv. Removes toxic substances and waste. Liver conjugates wastes and toxins to glucuronic acid. Transporters in the kidney export detoxified wastes into the urine.

5. Collecting duct

a. Function: concentrate urine

b. Mechanism: collecting duct passes through medullary gradient set up by loop of Henle. Water leaves the duct; urine is concentrated.