osmoregulation2

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Published on October 10, 2007

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II. Osmoregulation in aquatic environments:  II. Osmoregulation in aquatic environments Marine birds and reptiles Salt glands Unable to produce a hyperosmotic urine Drink seawater to obtain water Integument has low permeability to Na+ Salt gland located above the orbit of the eye in some sea birds and in iguanas and empty into the nasal cavity; in sea snakes (mouth); in estuarine crocodiles (tongue) Slide2:  estuarine crocodile marine iguana sea bird II. Osmoregulation in aquatic environments:  II. Osmoregulation in aquatic environments Avian salt gland Operates in the same fashion as a chloride cell Flow of blood through capillaries is in the opposite direction to the flow of secretory fluid (countercurrent system) Countercurrent system minimizes concentration gradient of Na+ from the blood to the tubular lumen Salt gland is activated in response to increased salt uptake or an increase in ECF volume Slide4:  Hyperosmotic secretions Active secretion of salts occurs across secretory tubules Avian Salt Gland Slide5:  Avian Salt Gland initially, water moves into gut from ECF SW Na+ moves from gut into ECF as [Na+] in ECF rises, water from gut moves into ECF Net result of drinking 25 ml of SW is a gain of 12.5 ml of osmotically free (pure) water II. Osmoregulation in aquatic environments:  II. Osmoregulation in aquatic environments Marine mammals Do not have salt glands and do not drink seawater Obtain water from food and metabolism Highly efficient kidneys produce a hypertonic urine Nursing females produce milk with high fat but low water content Some juvenile animals can use water derived from the oxidation of body fat Modifications in nasal passages to reduce water loss Ability to lower metabolic rate Slide7:  Water-salt relations in a marine mammal -obtain water from food and metabolism -conserves water by producing a hypertonic urine The kidney- a fluid processing organ:  The major function of the animal kidney is to regulate the composition of blood plasma by removing water, salts, and other solutes from the plasma in a controlled fashion Effects of the kidney on blood composition can be studying by comparing the urine composition (U) to plasma composition (P) or the U/P ratios The kidney- a fluid processing organ The kidney- a fluid processing organ:  The kidney- a fluid processing organ The effects of kidney function on osmotic regulation depend on the osmotic U/P ratio If U/P = 1, urine is isosmotic to plasma, no effect on water or solute excretion, plasma osmotic pressure unaltered If U/P < 1, urine is hyposmotic to plasma, urine contains more water relative to solutes than plasma, plasma osmotic pressure is raised If U/P > 1, urine is hyperosmotic to plasma, urine contains less water relative to solutes than plasma, plasma osmotic pressure is lowered The kidney- a fluid processing organ:  The kidney- a fluid processing organ The effects of of kidney function on volume regulation depends on the amount of urine produced Kidneys can play a role in volume regulation without a direct role in osmotic regulation Freshwater crabs of tropical regions -experience both volume and osmotic challenges -kidneys deal with volume challenge by excreting an equivalent amount of water that is gained by osmosis but are unable to produce a hypoosmotic urine -other tissues are involved in maintaining osmotic balance The kidney- a fluid processing organ:  The kidney- a fluid processing organ The effects of kidney function on ionic regulation depend on ionic U/P ratios Kidneys can play a role in ionic regulation without playing a direct role in osmotic regulation Marine teleost fish hyposmotic to SW (lose water osmotically and gain ions by diffusion) Produce a urine that is isosmotic to plasma (U/P=1), therefore urine production plays no direct role in osmotic regulation However, urine ionic composition differs greatly from plasma , U/P ratios for Mg2+, SO42-, and Ca2+ >>>1 (lowers internal ionic composition) III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Osmoregulatory challenges on land Water and salts must be obtained from food Animals are constantly losing water through respiration, evaporative cooling, urine and feces Physiological and behavioral mechanisms used to retain water and ions Some earthworms and terrestrial molluscs can cope with 40-80% water loss; insects (30-35%), mammals and birds (5-10%) III. Osmoregulation in the terrestrial environment:  Desert mammals Kangaroo rat is exposed to excessive heat and lack of water Avoids daytime heat by remaining in burrow and forages at night Excretes a highly concentrated urine Rectal reabsorption of water from food Receives water from diet and metabolism Respiratory moisture condensed in nasal passages III. Osmoregulation in the terrestrial environment Slide14:  Water conservation by the kangaroo rat III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Functions of the mammalian kidney Maintain water balance Regulate concentration of ions in the ECF Maintains long term arterial pressure Maintains acid-base balance Maintain proper ECF/ICF osmolarity Excrete end products of metabolism Excrete foreign compounds Secrete erythropoietin and renin Converts vitamin D into its active form III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Urinary system Kidneys  Urine formation  Renal pelvis  Ureter  Urinary bladder  Urethra III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Structure of the mammalian kidney Cortex (outer layer) In contact with the renal capsule Possesses many capillaries Medulla (deeper region) Composed of renal pyramids separated by renal columns Renal pyramids project into minor calyces Minor calyces unite to form major calyx Major calyces form renal pelvis Slide18:  Mammalian kidney (Eckert, Fig. 14-17) III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment The nephron Functional unit of the kidney Two major components of the nephron: Vascular component (glomerulus) A tuft or ball of capillaries Filters fluid from blood as it passes through Tubular component Filtered fluid from from the glomerulus (ultrafiltrate) passes to the tubular component and is converted to urine III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Renal circulation (2 capillary beds) Glomerular capillaries High pressure (50-60 mm Hg) Allows for rapid filtration Peritubular capillaries Low pressure (10 mm Hg) Allows for reabsorption Some vessels form the vasa recta III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Blood flow through the kidney Afferent arterioles  Glomerular capillaries (ultrafiltration)  Efferent arterioles  Peritubular capillaries (wrapped around nephrons)  Renal tubules Renal venules  Renal vein III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Parts of a nephron Bowman’s capsule Invagination around the glomerulus which collects filtered fluid from the glomerulus Juxtaglomerular apparatus Specialized tubular and vascular cells lying next to the glomerulus Produces renin III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Proximal tubule Within the cortex Reabsorption of selected solutes Loop of Henle U-shaped loop that dips into the medulla Two sections: descending limb (cortexmedulla) and an ascending limb (medullacortex) Establishes an osmotic gradient in medulla Allows kidney to produce urine of varying concentration III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Distal tubule Lies within the cortex Empties into the collecting duct Highly regulated reabsorption of Na+ and water Secretion of H+ and K+ Collecting duct Drains fluid from the nephrons Enters medulla and empties into the renal pelvis Similar functions to the distal tubule Slide25:  The nephron (Sherwood, Fig. 14-3) III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Types of nephrons Cortical nephrons Glomeruli in the outer cortex Descending limb of the loop of Henle enters partially into the medulla No vasa recta Juxtamedullary nephrons Glomeruli lie in the inner cortex Descending limb enters entire length of medulla Abundant in desert species Vasa recta present Slide27:  Cortical and juxtamedullary nephrons (Eckert, Fig 14-18) III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Processes contributing to urine formation Glomerular filtration Reabsorption from renal tubules into the peritubular capillaries Secretion of substances from peritubular capillaries into the renal tubules Rate of urinary = Filtration – Reabsorprtion+Secretion excretion rate rate rate Slide29:  (Silverthorn, Fig. 18-3) Processes contributing to urine formation III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Glomerular filtration rate: the amount of fluid that filters into the Bowman’s capsule per unit time In humans, about 180 l/day Kidneys excrete about 1 l/day, therefore most of the filtrate is returned to the vascular system (>99% reabsorbed) GFR is about 20% of renal blood flow III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Glomerular capillary membrane Three major layers: Endothelium Basement membrane Podocytes (epithelial cells) III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Podocytes Surround outer surface of the capillary membrane; cell body with several ‘arms’ or pedicels (foot processes) Narrow slits between pedicels allow for the passage of molecules based on MW and charge Glomerular capillaries are fenestrated, allowing for a high filtration rate Most substances except large proteins are filtered Slide33:  (Silverthorn, Fig. 18-4) Structure of the glomerulus Slide34:  Structure of the podocytes (Silverthorn, Fig. 18-4) III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Forces involved in glomerular filtration PG: glomerular hydrostatic pressure; promotes filtration (60 mm Hg) PB: hydrostatic pressure in Bowman’s capsule; opposes filtration (18 mm Hg) G: colloidal osmotic pressure of the glomerular capillary; opposes filtration (32 mm Hg) B: colloid osmotic pressure of the Bowman’s capsule; promotes filtration (0 mm Hg) III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment GFR depends largely on two factors Net filtration pressure Filtration coefficient Surface area of glomerular capillaries Permeability of glomerular capillary-Bowman’s capsule interface III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Regulation of GFR Prevents extreme changes in renal excretion from occurring in response to small arterial pressure changes Regulation is generally achieved by adjusting resistance to flow in the afferent arteriole Afferent arteriole has large diameter and short length (low resistance) Efferent arteriole and vasa recta have smaller diameter and are longer (offer higher resistance) Slide38:  Creation of high filtration pressure at the renal glomerulus (Eckert, Fig. 14-20) Slide39:  Control of GFR by modulating arteriolar resistance (Silverthorn Fig. 18-8) Slide40:  Effect of vasoconstriction of the afferent arteriole on GFR (Silverthorn, Fig. 18-8) Slide41:  Effect of vasoconstriction of the efferent arteriole on GFR (Silverthorn, Fig. 18-8) III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Mechanisms controlling GFR Intrinsic (autoregulation) Myogenic response of the arteriolar smooth muscle Hormonal control Involves the juxtaglomerular apparatus (JGA) JGA- a specialized renal structure where regions of the nephron and afferent arteriole are in contact with each other Macula densa and juxtaglomerular cells (granular cells) Slide43:  Juxtaglomerular apparatus (Eckert, Fig. 14-24) III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Nervous control Afferent arteriole innervated by sympathetic nervous system Sympathetic activation causes constriction of glomerular cells and causes podocytes to contract Nervous mechanism overrides autoregulatory mechanisms if there is a sharp decrease in BP Slide45:  (Sherwood, Fig. 14-15) Nervous control of podocyte contraction III. Osmoregulation in the terrestrial environment:  III. Osmoregulation in the terrestrial environment Tubuloglomerular feedback Changes in fluid flow sensed by macula densa Paracrine factors can either cause vasoconstriction or vasodilation Endothelin (vasoconstrictor); bradykinin and nitric oxide (vasodilators) Slide47:  Tubuloglomerular Feedback (Fig. 18-10, Silverthorn)

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