M/A=J=(D/X)(C₁-C₂)

M=Molecules/second

A=Area

J=Molecules/cm²sec (net flux)

D=diffusion constant

X=distance of diffusion

voltage

concentration gradient

Ions cross membranes via selective gated ion channels

Gating is affected by voltage, stretch, and chemical interactions (ligand binding, phosphorylation)

Properties of a solution that depend on the concentration of dissolved entities.

increase in osmolality

decrease water vapor pressure

decrease freezing point

diffusion through phospholipid

or

channel-mediated water transport through water-specific aquaporitns (rbc, some kidney tubules)

faster than diffusion

faster flux due to bulk flow strreaming, but still passive

Permeant concentrations: in matches out

cell volume changes (water movement) so osmolarity in matches out

total impermeant concentrations in and out balance

A- is impermeant

K+ and Na+ are permeant, but pump makes Na+ non-penetrating

Pump needed for [Na+] to balance [A-] for stable cell volume

The concentration gradient of a permeable ion can create an electrical gradient

Involves more than one permeating ion plus nonpermeating ions.

Cell volume is maintained by a double Donnan system (nonpermeating intracellular anions and effectively nonpermeating extracellular Na+)

transported by carrier proteins

Ions, sugars, amino acids bind reversibly to specific transporters

transport shows saturation kinetics

conformational changes in the transporter protein are key

use energy to transport solutes away from electrochemical equilibrium

-Located in the basolateral membrane of epithelia

Na+ ions bind, phosphorylation changes affinity, Na+ are occluded, then exit. K+ sites increase affinity, K+ bind, dephosphorylation changes affinity, K+ are occluded, then enter

can exist in different forms in different tissues and species

can undergo noncovalent and covalent modulation, insertion/retrieval, and changes in gene expression, often under hormonal control

paracellular- movement through tight junctions

transcellular - through apical and basolateral membranes via various mechanisms

Proteins in the membranes of cells that line up and form a channel between them

Permit 1 KD of momement between cell interiors

Epithelia rest on a secreted basement membrane

made of glycoproteins and collagen

exchange with capillaries is through basement membrane

protein conformation is affected by fluid composition

membrane electrical gradients are affected by fluid composition

humans regulate Na⁺, K⁺, Ca²⁺

60% of weight in men, 50% of weight in women

ECF = 1/3 TBW, ICF = 2/3 TBW

Plasma = 20% ECF

Partial pressure of gaseous water in equilibrium with a liquid solution in a closed system at a given temperature

Affected by dissolved solutes (colligative property)

Water vapor pressure from 1 to 2

(body to air)

J = (K/X)(WVP₁-WVP₂)

K = permeability of integument

X = thickness of integument

U/P ratios indicate the effect of the kidneys on plasma composition

Kidneys can independently regulate ion concentration, volume, and osmolarity

Ranges from .1 to 4

U/P = 1 does not change Plasma concentration

U/P < 1 concentrates Plasma concentration

U/P > 1 dilutes Plasma concentration

Include respiration, urine and feces

-insensible water loss (dry air on skin)

-inspired air may be dry and cold while expired air is warm and saturated

-Permits changes in intracellular solute amount while maintaining cell volume and intracellular ion concentrations

-Compatible solutes (which have minimal side effects on macromolecules) or occur in counteracting pairs (like urea and methylamines)

Permits the intracellular concentrations of inorganic ions to be similar in all animals

Stimulated by increased osmolarty (or by low blood volume or pressure in emergency)

Promotes retention of solute free water

Stimulated by low blood volume, pressure via renin-antiotensin system

controls ECFV including blood plasma volume

INcreased Na+ amt leads to ECFV expansion

also affects salivary, sweat glands, intestines, salt appetite

Atrial Natriuretic Peptide

"P Na"

Stimulated by atrial stretch (blood volume increase)

Inhibits aldosterone secretion, promotes Na+ and water excretion

Inhibits distal Na+ reabsorption, vasopressin, renin, aldosterone secretion

Increases GFR (glomerular filtration rate)

The proximal tubule reabsorbs "goodies" such as HCO₃^-, glucose, and amino acids

Isosmotically reabsorbs enough salt and water to reduce the quantity of tubular fluid and solute to amounts that the later nephron segments can manage (about 70% reabsorption)

Makes tubular fluid hyposmotic and medullary interstitial fluid hyperosmotic

Reabsorbs additional salt and water

fine regulation of Na⁺, K⁺, and H⁺ balance

reabsorbs additional salt and water

Mammals produce primary urine by ultrafiltration

Push:

BP in glomerular capillary: 50mmHg

Colloid osmotic pressure: -25 mmHg

Capsular fluid hydrostatic pressure: -15 mmHg

=total pressure pushing fluid out: 10 mmHg

Glomerular filtration pathway?

Rate?

Pathway: endothelium, basement membrane, epithelial podocytes - proteins are filtered out on the basis of size and charge

Combined rate = 120 ml/min

Increased urine flow

Lack of ADH causes water diuresis: copious dilute urine

Increased solute excretion causes a solute (or osmotic) diuresis

Cortex contains glomeruli and convoluted tubules

Medulla contains loops of Henle, collecting ducts, and vasa recta, which are all required to produce concentrated urine.

reabsorb solute, but make water stay in the tubules (active reabsorption)

-The walls of the TkAL and DCT are water-impermeable and so are the late DT and CD in the absence of ADH

Osmotic gradient in the medulla, where osmolarity is higher in the interstitial fluid than the tubules

Single effect: the TkAL reabsorbs solute without water

-Na/K pump causes lower Na concentration inside cell

Na movement helps to bring 2Cl and 1K into the cell using the gradient (secondary active transport)

Results in large end-to-end osmotic gradient

Uses gradient created with countercurrent multiplication

occurs if ADH is present to insert aquaporin-2 proteins into CD apical membranes

Urea is concentrated in the CD, then moves from high concentration through UT proteins into the ISF and AL (Urea recycling)

Carry blood into the medulla and back out

Act as countercurrent exchangers: blood loses water and gains solute as it goes toward the turn, and gains water and loses solute as it returns.

Solute from ISF that enters blood in descending limb is returned to ISF from ascending limb, so solute is not carried away blood.

This preserves the osmotic gradient.

Sodium, glucose cotransporter for glucose

Similar pathway for amino acids, but with different proteins.

Facilitated diffusion

Specificity

Saturation

Inhibition

Competition

Steroid hormone produced in adrenal cortes of adrenal gland

Increases expression of genes for basolateral Na/K pumps, apical Na channels, and apical K channels

Increases Na reabsorption and K secretion

Ammonia

Urea

Uric acid and urates

Requires little energy to synthesize

Its toxicity means it must be kept at concentrations below .3mM, so it requires a lot of water

most ammonotelic organisms are aquatic

We use ammonia excretion to maintain acid/base balance, but it is not a principal means of nitrogen excretion

Requires 4-5 ATP to make one urea (2 N atoms)

Less toxic than ammonia - 5mM

Less water is required to excrete urea than ammonia - most urotelic organisms are terrestrial vertebrates, including mammals

low toxicity and low solubility

can be stored as deposits during a water shortage

can be excreted with less water than urea (birds)

Uric acid requires more energy to make than urea

Marine mammals maintain body fluids that are more dilute than their surroundings

Kidneys can concentrate to a U/P of about 6

Don't lose water through respiration or low-premeability integument

Don't drink sea water

Evaporative water loss

Respiratory water loss

Excretory water loss

Reduced by lipids in the low-permeability integument (e.g. glycolipids between skin cells)

Increased by larger surface area/kg or high metabolic rate/kg

-Carrying out respiration in invaginated lungs

-Cooling expired air and retaining the condensed water (countercurrent exchange)

Kidneys that concentrate well lose less water

excreting nitrogen in poorly soluble compounds would help, but mammals don't do it

Urea recycling is used to improve the concentrating ability of kidneys

greater evaporative water loss due to greater surface area to volume ratio

greater respiratory water loss due to greater metabolic rate per kg

greater concentrating ability

turnover more water per gram than large animals

Renal clearance expresses...

What is the formula?

Renal clearance expresses excretion in terms of the equivalent volume of plasma cleared by the kidney per minute

C_x=VU_x/P_x=excretion of X/P_x

V=urine flow rate

U_x=urine concentration of X

P_x=plasma concentration of X

GFR equals the clearance of a substance that is freely filtered but not secreted or reabsorbed.

-Inulin

-to a close approximation, Creatinine

Filtered Load equation

Excretion exquation

Filtered load = (GFR)(P_x)

Excretion=VU_x

or

Excretion=filtered load + secreted amt - reabsorbed amt