Osmoregulation - The Kidney (1) PDF

Osmoregulation The Kidney A.K. Daemicke  š Explain how kidneys function to maintain fluid balance in terrestrial vertebrates š Describe the chemical composition of urine š Recognize the three processes that are invol...

Osmoregulation The Kidney A.K. Daemicke  š Explain how kidneys function to maintain fluid balance in terrestrial vertebrates š Describe the chemical composition of urine š Recognize the three processes that are involved in creating urine forms and identify ways in which urine composition Learning can be altered š Explain how the kidney’s countercurrent Objectives mechanism works š Discuss the renin angiotensin aldosterone system and understand how it influences blood pressure š Understand how ADH levels influence the volume and concentration of urine excreted by animals  Animals live in varying environments that each pose unique challenges to osmotic balance Saltwater Freshwater Land  What is the difference between an osmoconformer and an osmoregulator? Osmoconformers š Most marine invertebrate organisms that maintain an internal environment which is isotonic to their external environment (saltwater) š Saltwater = 1,000 mOsm/L š NO net movement of water Osmoregulators šMarine, freshwater and terrestrial vertebrates that maintain an internal environment which is hypertonic/hypotonic to their external environment Tonicity Review Isotonic Hypertonic Hypotonic  Passive Transport – Osmosis 100 mOsm/L 200 mOsm/L 150 mOsm/L 150 mOsm/L Dilute Glucose Concentrated Movement of water Plasma membranes are Solution Glucose Solution permeable to water, but frequently not to solutes When two solutions exchange water by osmosis, water always moves from the one with lower osmotic pressure into the one with higher osmotic pressure Semipermeable Membrane Glucose molecule How do osmoregulators make sure their fluids stay within a narrow range? For blood and other body fluids to be maintained at a normal composition, animals require organs that serve to counteract any deviation away from normal  The kidneys are the primary organ that serves to maintain fluid homeostasis in terrestrial vertebrates. This work was supported and approved by the Human Anatomical Sciences Graduate Program at Northern Illinois University. All Human Bodies donated to the Department of Biological Sciences Body Donation Program are treated humanely, and with the highest ethical standards Urinary System Organs š Kidneys are our major excretory organs. š Ureters transport urine from kidneys to urinary bladder. š Urinary bladder is temporary storage reservoir for urine. š Urethra transports urine out of body. Qualities of the Kidney š Consist of tubular elements š Produce and void aqueous solutions derived from blood š Do so to regulate the composition and volume of blood  o A complex solution of various organic and inorganic solutes, as well as water that are drawn from the blood o Therefore, the composition of urine impacts the composition of an animal’s blood. o 95% water and 5% solute o Water in urine consists of two parts Amount that is require to accompany excreted solutes Amount that is additional that may or may not be excreted à this is regulated o Wastes Urea: from amino acid breakdown Creatinine: metabolite of creatine phosphate o Other normal solutes Na+, K+, PO43–, and SO42–, Ca2+, Mg2+ and HCO3– o Abnormally high concentrations of any constituent, or abnormal components, e.g., blood proteins, WBCs, bile pigments, may indicate pathology Chemical Composition of Urine  š Osmolarity of body fluids Regulation of Expressed in milliosmols (mOsm) Urine Kidneys maintain osmolarity of plasma at ~275-325 mOsm by Concentration regulating urine concentration and volume. and Volume Kidneys function with a countercurrent mechanism. Renal cortex Kidney have a rich blood supply Renal medulla in order to filter blood  Nephrons š Structural and functional units that form urine š > 1 million per kidney š Two main parts Renal corpuscle Renal tubule Urine Formation Primary urine becomes definitive urine by way of three events… (1)Glomerular filtration (2)Tubular reabsorption (3)Tubular secretion While all three occur for some substances, that is not true for all substances. Renal cortex Let’s make Renal medulla some urine! Fun fact! Our total blood volume filters into the renal tubules about every 22 minutes Glomerular Filtration š Passive process š No metabolic energy required š Hydrostatic pressure higher in glomerulus (55 mm Hg) forces fluids and solutes through filtration membrane Opposed by osmotic pressure of blood and hydrostatic pressure of glomerular capsule Net filtration pressure (NFP) š No reabsorption into capillaries of glomerulus š Large molecules (e.g. plasma proteins and blood cells) cannot get filtered š Glucose, ions and amino acids can pass Glomerular Disease š Caused by an infection or a drug that is harmful to your kidneys In other cases, it may be caused by a disease that affects the entire body, like diabetes or lupus. Many different diseases can cause swelling (inflammation) or scarring (sclerosis) of the glomerulus š Symptoms Albuminuria Hematuria Where does the filtrate enter next?  š Most of tubular contents reabsorbed to blood Proximal š Includes active and passive tubular reabsorption Convoluted š Two routes: Tubule (PCT) 1. Transcellular 2. Paracellular  Tubular Reabsorption š Transcellular route Apical membrane of tubule cells à Cytosol of tubule cells à Basolateral membranes of tubule cells à Endothelium of peritubular capillaries š Paracellular route Between tubule cells limited by tight junctions, but leaky in proximal nephron Reabsorption in PCT š Na+ uptake is driven by electrochemical gradient established by Na+/K+ pump. š Aquaporins cause PCT to be highly permeable to water. š Glucose and amino acid uptake driven by secondary active transport. Reabsorption 3 1 in PCT 4 2 These aquaporins are always open à obligatory water reabsorption  Glucose reabsorption in the proximal tubule … When transporters are saturated, the Transport excess in maximum (Tm ) for excreted in every substance urine. reabsorbed by a transport protein and is proportional to the number of proteins available for transport. Diabetes Where does the filtrate enter next? Loop of Henle š Provides capacity to make hyperosmotic urine š An important adaptation to have in order to live on land š In the ascending limb, ions are passively and actively transported out of filtrate. š The descending limb is only permeable to water and not NaCl. Hyperosmotic interstitial fluid created by NaCl active transport in the ascending limb drives water reabsorption in the descending limb.  The Countercurrent Multiplier š Expends energy to create a concentration gradient š The only way to move water is to move Na+ first. Hyperosmotic region created in by ascending limb, drives water movement out of descending limb. š Limbs are close enough to influence each other’s exchange with the interstitial fluid they share. The Countercurrent Multiplier  PCT DCT The Countercurrent Exchanger The steady influx of 300 mOsm filtrate at the beginning of the descending limb, coupled with Descending Ascending the dilution of filtrate moving Limb Limb up the ascending limb, serve to keep the osmotic pressure of at the outer cortical end of the loop near 300 mOsm.  The Countercurrent Exchanger š Vasa recta runs alongside the loop. š Function: preserve medullary gradient Prevent rapid removal of salt from interstitial space Remove reabsorbed water š Water entering ascending vasa recta either from descending vasa recta or reabsorbed from nephron loop and collecting duct. Volume of blood at end of vasa recta greater than at beginning  Role of countercurrent mechanisms Countercurrent o All the loops and their combined action establish and maintain Mechanism osmotic gradient. (~300 mOsm to ~1200 mOsm) from renal cortex through medulla o Helps with drives concentration of urine in collecting duct which aids land animals conserve water. What sorts of animals do we expect to have long loops of Henle? Species with a high relative medullary thickness tend to produce especially concentrated urine as it is a gauge long loop concentration. 29.10 Relative medullary thickness also decreases allometrically with body size Longer loop of Henle = greater concentration gradient Expect desert mammals to have longer loops of Henle and to produce more concentrated urine https://ib.bioninja.com.au/higher-level/topic-11-animal-physiology/113-the-kidney-and-osmoregu/water-balance.html Distal Convoluted Tubule (DCT) š Movement of many substances depends on the body’s needs š Regulated by hormones Aldosterone: Increases active transport Na+ out and in K+ to filtrate when blood volume/ concentration of Na+ is low or K+ is high š pH regulation Can absorb bicarbonate and secrete H+, raising pH of bloodstream. Can release bicarbonate and absorb H+, lowering pH of bloodstream In order to fully appreciate what occurs in the collecting duct, let’s consider how the kidney helps maintain blood pressure. Juxtaglomerular Complex (JGC) o Three cell populations o Macula densa Tall, closely packed cells of ascending limb Chemoreceptors; sense NaCl content of filtrate o Granular cells (juxtaglomerular, or JG cells) Enlarged, smooth muscle cells of arteriole Secretory granules contain enzyme renin Mechanoreceptor sense blood pressure in afferent arteriole o Extraglomerular mesangial cells Between arteriole and tubule cells Passes signals between macula densa and granular cells  Main mechanism for Extrinsic increasing blood pressure Controls: Three pathways to renin Renin- release by granular cells Angiotensin- Direct stimulation of granular cells by sympathetic nervous system Aldosterone Stimulation by activated macula densa cells when filtrate NaCl Mechanism concentration low Reduced stretch of granular cells Renin- Angiotensin Aldosterone System  DCT and collecting duct: reabsorption hormonally regulated o Antidiuretic hormone (ADH): released when an increase in blood osmolarity or a decrease in blood volume is detected Released by posterior pituitary gland Reabsorption Causes principal cells of collecting ducts to insert aquaporins in apical membranes à water Capabilities of reabsorption As ADH levels increase, water reabsorption Renal Tubules increases. Function: decrease blood osmolarity; increase and o blood volume Aldosterone: released when decreased blood Collecting pressure is detected Released by adrenal glands Ducts Targets principal cells of collecting ducts and distal DCT and promotes synthesis of luminal Na+ and K+ channels, and basolateral Na+-K+ ATPases for Na+ reabsorption à water follows Functions: increase blood volume/ concentration of Na+ is or decrease K+ Let’s draw it out! ADH NaCl concentration in medullary interstitial ADH high ADH low fluids increases with increasing depth of the medulla because blood supply removes water from medullary interstitial fluids 29.14 Diabetes insipidus š A rare condition caused by a lack of ADH or a failure of the kidneys to respond to ADH š Treatment includes a medication called desmopressin acetate š What are some symptom one would have with diabetes insipidus? Frequent urination Dehydration Excessive thirst  Takeaway Animals can control the excretion of water independently of the excretion of solutes. Have you ever noticed that in the morning your urine is very dark? Color of urine is Since an animal hasn’t had water all night, it must evidence of reabsorb osmotically free osmoregulation! water and only excretes water that is required to be excreted with solutes The RAAS is not the only way ADH secretion is stimulated!  Stimulus Receptors Afferent pathway Control center Negative Ta Feedback awa kes Efferent pathway stim y the ulus ! Effector Response Stimuli Animals monitor various measures of water 1. Blood volume 2. Extracellular ion concentration  What happens to these variables as we lose water to the environment? Decrease Increase extracellular Lose water blood volume ion concentration Receptors sensing extracellular ion o Osmoreceptors are sensory receptors in the thirst concentration center in the hypothalamus that monitor the concentration of solutes (osmolality) of the blood. (Osmoreceptors) o If blood osmolality increases above its ideal value, the hypothalamus transmits signals that result in a conscious awareness of thirst. Control Center  Stimulus: Dehydration Receptors: Osmoreceptors in hypothalamus Control center: Hypothalamus initiates… thirst Negative Feedback Release of ADH from posterior pituitary Response: drink ADH travels through blood to… Effector: CT cells à insert aquaporins es Tak the y awa ulus! stim Response: Reabsorb water into blood Formation of Dilute and Concentrated Urine  Great summary video! https://www.youtube.com/watch?time_continue=3&v=XbI8eY-BeXY&feature=emb_title The Real Thing!