Download PHGY 216 Final Study Set Exam: Water Balance and Osmolarity and more Exams Advanced Education in PDF only on Docsity!
PHGY 216 Final Study Set Exam
Water Balance - Answer - Every cell in the body requires a well-regulated environment in order to survive and function
- Body water plays a large role in creating this environment, which is comprised of intracellular and extracellular fluid
- When we talk about the major body compartments for water (fluid) there are three key pools: ICF, plasma, and interstitial fluid, which remain distinct due to the presence of "barriers" between them
Intracellular Fluid (ICF) - Answer This is the fluid within the cells and comprises about two thirds of total body fluid
Extracellular Fluid (ECF) - Answer - This is the fluid surrounding the cells which includes the plasma, the interstitial fluid, lymph, and transcellular fluid (such as cerebrospinal fluid) and comprises about one third of total body fluid
- The plasma is about one fifth of the ECF, the interstitial fluid is about four fifths of the ECF, and the lymph and the transcellular fluid are considered negligible
Transcellular Fluid - Answer The portion of total body water contained within epithelial lined spaces
Barriers Between Body-Fluid Compartments - Answer - Several barriers separate the body-fluid compartments
- This limits the movement of water and solutes between the various compartments to differing degrees
Barriers Between the Plasma and Interstitial Fluid - Answer - The plasma and the interstitial fluid are separated by the blood vessel walls
- At the level of the capillaries, water and everything else in the plasma (except proteins) can freely exchange with the interstitial fluid
- Because of this, the composition of the plasma and the interstitial fluid are essentially identical, except for the plasma protein
- Consequently, any change in one of these compartments is quickly reflected in the other compartments
Barriers Between the Intracellular Fluid and the ECF - Answer - This barrier is the plasma membrane that surrounds each cell in the body
- The ICF contains proteins that do not exchange with the ECF
- There is an unequal distribution of ions across this barrier because the barrier does
not allow the passive movement of either ICF or ECF constituents across the plasma membrane, preventing them from equilibrating through diffusion
ECF Volume and Osmolarity - Answer - All exchanges of water and other constituents between the ICF and the external world are dependent upon the ECF
- Even though cells tightly regulate their own ICF, it can be said that overall control of fluid balance is dependent upon regulating the ECF
- In order to maintain fluid balance in the body the ECF volume and osmolarity are regulated
ECF Volume - Answer - This is closely regulated to maintain blood pressure
- The maintenance of salt balance is important in the long-term regulation of ECF volume
ECF Osmolarity - Answer - This is closely regulated to prevent the swelling or shrinkage of cells
Control of ECF Volume - Answer - ECF volume directly influences blood pressure by changing plasma volume
- Increasing ECF volume will increase plasma volume ,and thus increase arterial blood pressure
- Consequently there are mechanisms in place to adjust blood pressure until the ECF volume is returned to normal values
- There are short term control factor including the baroreceptor reflex and fluid shifts
- There are long term control factors including fluid input and output
The Baroreceptor Reflex - Answer - Baroreceptors are mechanoreceptor that are located in the carotid artery and the aortic arch (areas within major arteries of the body), and they detect changes in arterial blood pressure
- Through the effects of the autonomic nervous system on the heart and blood vessels, teh baroreceptor reflex regulates blood pressure
- When pressure falls too low, cardiac output and total peripheral resistance will increase to raise blood pressure
- When blood pressure rises above normal, both decrease to reduce blood pressure
Total Peripheral Resistance - Answer - The resistance to blood flow due to the constriction of blood vessels
- Higher total peripheral resistance leads to increased blood pressure
Cardiac Output - Answer The amount of blood pumped by the heart per minute
- This loss of water from cells can cause them to shrink
- When there is an increase in ECF water, the osmolarity would decrease and the ECF would become hypotonic
- This would result in water moving into the cells until the osmotic pressures were equalized and this movement would cause the cells to expand
- If the ECF was very hypotonic, the amount of water moving into the cells would cause them to burst
Osmolarity - Answer - Defined as a measure of the concentration of a particular solute in solution
- A high osmolarity means that there is more solute, and therefore less water, in solution
Hypertonic - Answer A hypertonic solution is one in which the concentration of solutes within that solution is greater than that of another solution that is separated by a membrane
Hypotonic - Answer A solution that has a lower osmotic pressure than the surrounding cells
Hypotonicity - Answer - Hypotonicity of the ECF is usually associated with overhydration, or excess free water, and has three major causes: renal failure, rapid water ingestion, and over secretion of vasopressin
- The osmolarity of the ECF must be regulated to prevent these undesirable shifts of water into or out of the cell
Renal Failure - Hypotonicity - Answer These individuals are not able to produce a concentrated urine
Rapid Water Ingestion - Hypotonicity - Answer This can occur in healthy individuals if they drink volumes of water in excess of what the kidneys can deal with in a timely manner
Over Secretion of Vasopressin - Hypotonicity - Answer Vasopressin promotes water retention
Hypertonicity - Answer - Hypertonicity of the ECF, the excessive concentration of ECF solutes, is usually associated with dehydration and has three major causes:
- Insufficient water intake, or not drinking enough
- Diabetes insipidus, which involves a deficiency in vasopressin
- Excessive water loss due to heavy sweating during extreme exercise, prolonged bouts of vomiting, or diarrhoea
Isotonic Fluids - Answer - An isotonic solution has an equal osmolarity to that of normal body fluids
- In a patient that is given a therapeutic intravenous administration of an isotonic solution, the saline solution is being injected into the blood plasma within the veins, which makes up approximately one fifth of the ECF
- When isotonic fluid is injected into the ECF compartment, the ECF volume increases, but the concentration of ECF solutes remain unchanged; the ECF remains isotonic
- Since the ECF osmolarity has not changed, the ECF and ICF are still in osmotic equilibrium, and there is no net fluid shift between the two compartments
- Cells would neither shrink nor swell, illustrating the need for intravenous fluid therapy to be isotonic in order to prevent fluctuations of intracellular volume
- In isotonic fluid loss the loss is confined to the ECF, with no corresponding loss of fluid from the ICF and no osmotic gradient is created that would result in net fluid shifts
Regulation of Water Balance - Answer - Within the hypothalamus, near the vasopressin-secreting cells and thirst centre, are hypothalamic osmoreceptors which constantly monitor the osmolarity of the fluid surrounding them in order to quickly counteract any fluctuations in water balance
- The hypothalamic osmoreceptors detect osmolarity and as osmolarity increases, both vasopressin secretion and thirst are stimulated
- The vasopressin acts on the kidneys to increase water reabsorption, while thirst stimulates the intake of water to the body through drinking
- This continues until the hypertonicity is relieved
- If the fluid around the osmoreceptors is hypotonic, then vasopressin secretion and thirst are not stimulated, promoting water loss
- Large losses of ECF volume can also impact these pathways
- Within the left atrium of the heart are what are called left atrial volume receptors, which monitor the pressure of the blood in the left atrium
- They are activated when there is a greater than 7% loss of ECF volume and blood pressure
- Once activated, they also stimulate the hypothalamic pathways to stimulate vasopressin release and thirst
The Kidneys - Answer - The kidneys are controlled by both neural and endocrine inputs
- The primary function of the kidneys are to maintain the ECF volume, electrolyte composition, and osmolarity
- The vascular component supplies blood to the nephron
- The tubular component carries the filtrate throughout the nephron
The Vascular Component - Answer - The major part of the vascular component of the nephron is the glomerulus, which is a ball-like capillary through which water and solutes are filtered from the plasma
- When blood enters the kidney via the renal artery, the renal artery subdivides into many small afferent arterioles, each of which supplies a nephron
- Leaving the nephron are efferent arterioles, which transport the unfiltered blood to the glomerulus to be filtered
- The capillaries of the nephron are different in that arterial blood enters and then leaves, with no oxygen extracted
- Instead, the efferent arteries subdivide into capillaries, the peritubular capillaries, that deliver oxygen to the renal tissues
The Tubular Component - Answer - Filtered blood enters a hollow tube, formed by a single layer of epithelial cells, that transports urine to the renal pelvis
- Even though it is technically one long continuous tube, it is divided along its length based on differences in structure and function
- It begins with the Bowman's capsule, which encircles the glomerulus to collect the fluid filtered from the glomerular capillaries
- The fluid then passes into the proximal tube within the renal cortex, which is highly coiled along its length
- Next is the loop of Henle, which forms a hairpin loop that dips down into the renal medulla
- The descending limb of the loop of Henle travels from the cortex to the medulla while the ascending limb travels from the medulla back to the cortex
- The ascending limb passes through the fork of the afferent and efferent arteries
- The tubule now coils again and is called the distal tubule, also entirely within the cortex
- The distal tubule empties into a collecting duct, which travels deep into the medulla and ultimately drains into the renal pelvis
Juxtaglomerular Apparatus - Answer The fork of the afferent and efferent arteries
Cortical Nephrons - Answer - The glomeruli of this type of nephron lie in the outer layer of the cortex
- About 80% of all nephrons are cortical nephrons that primarily serve secretory and regulatory functions
- Their loop of Henle only slightly dips into the renal medulla
- The peritubular capillaries from this type of nephron wrap around the short loops of Henle
Juxtamedullary Nephrons - Answer - This type of nephron is found on the inner layer of the cortex and are responsible for the concentration and dilution of urine
- The peritubular capillaries from these nephrons form hairpin loops of vasculature that are in close proximity to the loops of Henle
Vasa Recta - Answer Hairpin loops of vasculature that are in close proximity to the loops of Henle in juxtamedullary nephrons
The Basic Renal Processes - Answer When it comes to the formation of urine, there are three basic process of the kidneys
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion
Glomerular Filtration (GF) - Answer - About 20% of the blood that flows through the glomerular capillaries is filtered into the Bowman's capsule
- This plasma filtrate is normally protein free, but does contains the same solutes as the plasma
- Collectively about 125 ml of glomerular filtrate is formed each minute
Tubular Reabsorption (TR) - Answer - As the filtrate flows through the tubules, important
- The rate at which blood is filtered through all of the glomeruli, the measure of the overall renal function, is the glomerular filtration rate (GFR)
Glomerular Filtration - Answer - In order for blood to be filtered, the fluid must pass through three layers that make up the glomerular membrane
- These layers re the glomerular capillary wall, the basement membrane, and the inner layer of Bowman's Capsule
The Glomerular Capillary Wall - Answer - Like most capillaries, it consists of a single layer of endothelial cells
- However, it contains many large pores that make it 100 times more permeable to fluids and solutes than regular capillaries
- The pores are of such size that large plasma proteins cannot pass through, but the smaller ones, such as albumin, can
The Basement Membrane - Answer - This layer contains no cells and is composed of collagen to provide structural strength, and glycoproteins to discourage the filtration of small plasma proteins
- Because the glycoproteins are negatively charged, they help to repel any proteins that do not get through the capillary walls
- Only about 1% of filtered albumin will pass into Bowman's capsule
The Inner Layer of Bowman's Capsule - Answer - This layer is composed of podocytes that form narrow filtration slits between them that allow fluid to pass into Bowman' capsule
Podocytes - Answer Cells that wrap around the capillaries of the glomerulus
Forces that Regulate Glomerular Filtration - Answer - The forces involved in glomerular filtration are very conceptually similar to the forces involved in bulk flow across any capillary wall
- These forces are glomerular capillary blood pressure, plasma-colloid oncotic pressure, and Bowman's capsule hydrostatic pressure
Glomerular Capillary Blood Pressure - Answer - This is the pressure exerted by the blood in the glomerular capillaries
- While regular capillaries have a blood pressure of about 18 mmHg, glomerular capillary pressure is on average 55 mmHg
- This is mainly due to the afferent arteriole diameter being larger than the diameter of the efferent arterioles, which increases resistance to blood leaving the glomerular capillaries
- This also prevents glomerular capillary pressure from decreasing along their length, further favouring filtration
Plasma-Colloid Oncotic Pressure - Answer - The presence of large proteins in the plasma that cannot be filtered produces an oncotic force that resists the movement of water into the Bowman's capsule
- The plasma-colloid oncotic pressure is about 30 mmHg
Bowman's Capsule Hydrostatic Pressure - Answer - This is the pressure of the fluid in Bowman's capsule and it also resists the movement of water out of the glomerular capillaries
- Bowman's capsule hydrostatic pressure is around 15 mmHg
Net Filtration Pressure - Answer - Net filtration pressure equals the glomerular capillary blood pressure minus the sum of the plasma-colloid oncotic and Bowman's capsule hydrostatic pressures
- Net filtration pressure = 55 mmHg - (30 mmHg + 15 mmHg) = 10 mmHg
Glomerular Filtration Rate - Answer - GFR is not only dependent upon filtration pressure, but also the glomerular surface area available and how permeable the membrane is
- Collectively, these properties are called the filtration coefficient
- Glomerular Filtration Rate (GFR) = Filtration pressure x Filtration coefficient
- In the average male the GFR is 125 ml/min and in females it is 115 ml/min
- Specialized tubular cells in this area are collectively called the macula densa, which can sense changes in the salt level of the tubular fluid
- If there is an increased arterial pressure that increases the GFR, more fluid than normal will flow through the distal tubule
- This also means there is an increased salt delivery
- In response, the macula densa releases ATP, which is degraded to adenosine
- This adenosine acts on the afferent arterioles to cause constriction and reduce GFR
Sympathetic Control of GFR - Answer - In addition to intrinsic controls, GFR is also under extrinsic control, independent of fluctuations in arterial blood pressure
- This is controlled by the sympathetic nervous system, which innervates the afferent arterioles
- In a haemorrhage, for example, baroreceptors would sense the pressure drop and would initiate responses to normalize blood pressure
- At the level of the kidney, this increased sympathetic activity would constrict the afferent arterioles, which would decrease glomerular capillary pressure, decreasing GFR, and reducing urine production
- This is a mechanism by which depleted plasma volumes can be corrected
Baroreceptors - Answer Mechanoreceptors that detect changes in blood pressure
The Kidneys and Cardiac Output - Answer - In a healthy person, about 20% of the plasma enters the kidneys and becomes the glomerular filtrate
- This means that if GFR = 125 ml/min, the total blood flow to the kidneys must be 5 x 125 or 625 ml/min
- Additionally, considering that only 55% of whole blood is filterable plasma, we can adjust renal blood flow to 1140 ml/minute
- Since total cardiac output equals around 5000 ml/min at rest, we can now calculate that the kidneys receive around 22% of total cardiac output
- This proportion far exceeds what would be expected based on tissue size since kidney weight is only about 1% of total body weight
Two-Step Process of Tubular Reabsorption - Answer 1. Reabsorption begins with either active or passive movement of substances from the tubule into the interstitial space
- Reabsorption then continues with passive movement of substances from the interstitial space back into the bloodstream
The Fate of Various Substances Filtered by the Kidneys - Answer - Unlike glomerular filtration, tubular reabsorption is highly selective and variable
- In general, the tubules have a high reabsorptive capacity for substances needed by the body, and a low reabsorptive capacity for substances not needed by the body
- Since water and solutes are critical for maintaining homeostasis, their tubular reabsorption is high
Transepithelial Transport - Answer - The tubule is composed of a single layer of epithelial cells
- The area of the epithelial cells that are in contact with the tubule lumen is the luminal membrane, and the area of the epithelial cells that are in contact with the interstitial fluid is the basolateral membrane
- Transepithelial transport is defined asa the movement of solutes across an epithelial cell layer through the cell
- The membranes from neighbouring epithelial cells are not in contract, other than where there are tight junctions connecting them
- Because of this, any substance that enters an epithelial cell cannot transport it to a neighbouring cell, the substance must move through the cell into the interstitial space
5 Steps of Transepithelial Transport - Answer 1. The substance must cross the luminal membrane
- The substance must pass through the cytosol
- The substance must cross the basolateral membrane
- It must diffuse through the interstitial fluid
- It must cross the capillary wall to enter the plasma
Locations of Sodium Reabsorption - Answer - Since 99.5% of all filtered Na is
- These nutrients are transferred by secondary active transport, as they use the concentration gradient of Na established by the Na-K ATPase pump to be transported against their concentration gradient, along with the passive transport of NA
- In contrast, in the collecting duct, Na passively enters the epithelial cells through a Na channel
Hormonal Regulation of Na - Answer - In the proximal tubule and the loop of Henle, a constant percentage of the filtered Na is reabsorbed regardless of the total amount of Na within the body fluids
- In the distal tubule, however, the reabsorption of a small percentage of the filtered Na is subject to hormonal control
- The most important, and most well-known, hormonal system involved in the regulation of Na is the renin-angiotensin-aldosterone system (RAAS)
- Within the juxtaglomerular apparatus, there are granular cells that secrete renin into the blood
Three primary triggers of renin secretion - Answer - When the granular cells detect a drop in blood pressure, they secrete renin
- The granular cells are innervated by the sympathetic nervous system and will release renin when sympathetic activity increases
- The macula densa cells in the tubular portion of the juxtaglomerular apparatus are sensitive to the Na and when there is a decrease in luminal Na, the macula densa cells trigger the granular cells to secrete renin
Renin and Na - Answer - Once renin has been secreted into the blood, a series of events occur to regulate Na within the blood
- Once secreted, renin acts like an enzyme to convert angiotensinogen into angiotensin I
- When circulating angiotensin I passes through the lungs, it is converted to angiotensin II by the enzyme angiotensin converting enzyme (ACE)
- Angiotensin II, in turn, stimulates the adrenal cortex to release aldosterone
- Aldosterone then causes an increase in Na reabsorption in the distal and collecting tubules
Angiotensinogen - Answer A protein made in the liver that is present at high concentrations in the plasma
Actions of Aldosterone - Answer - Under the influence of aldosterone, tubular epithelial cells increase the insertion of Na channels in the luminal membrane and Na-K ATPase carriers in the basolateral membrane
- Combined, this results in a greater passive flow of Na out of the tubular fluid
- This enhanced Na retention also causes increased water retention (water follows Na) and will therefore increase arterial blood pressure
Atrial Natriuretic Peptide (ANP) - Answer - ANP is another hormone involved in the regulation of Na and water
- Its actions, however, are opposite to those of aldosterone in that ANP release reduces Na load and blood pressure
- When blood volume increases, or there is an increase in venous return, stretch receptors in the left atrium, aortic arch, adn carotid sinus stimulate the release of ANP
Three Main Actions of ANP - Answer - It directly inhibits Na reabsorption in the distal tubules so there is more Na excreted in the urine
- It inhibits both renin and aldosterone secretion
- It dilates the afferent arterioles and increases GFR --> as more salt and water are filtered, more salt and water are excreted
Actively Reabsorbed Substances - Answer - Any substance that is actively reabsorbed will bind to a specific carrier protein in the plasma membrane
- Because there are a limited number of carrier proteins in a membrane ,there is a limit to how much of a substance can be reabsorbed
- This is designated as the tubular, or transport, maximum
- For any given substance, if its concentration in the tubular fluid exceeds its transport maximum, then the excess will be excreted in the urine
- The plasma concentration at which the transport maximum is exceeded is called the renal threshold
- In the presence of reduced PTH, phosphate reabsorption by the kidneys increases, returning plasma phosphate concentration towards normal
- Second, a fall in plasma calcium also increases activation of vitamin D within the kidney, which then promotes absorption in the intestine
Glucose and the Kidneys - Answer - Glucose plasma concentrations are not regulated by the kidneys
- The plasma concentration of glucose is normally 100 mg per 100 ml of plasma
- Glucose is also small enough that it is freely filterable and the concentration in the Bowman's capsule filtrate is the same as it is in the plasma
- Given that the normal GFR is 125 ml/min, we can calculate that 125 ml/min glucose is filtered
- This is what is called the filtered load of a substance
- Any increase in GFR results in a proportional increase in its filtered load
- The normal transport maximum for glucose is 375 mg/min which means that for all filtered loads of glucose below 375 mg/min, 100% of the glucose will be reabsorbed
- Only when the filtered load of glucose exceeds 375 mg/min will glucose appear in the urine
- Ordinarily, urine contains no glucose because the kidneys are able to reabsorb it all
- Bowman's capsule collects the filtrate that the glomerulus forms which includes urea, electrolytes, and glucose
- The filtrate then passes into the proximal tubule to be reabsorbed
- The proximal tubule, however, can only reabsorb a limited amount of glucose
- When blood glucose levels exceed about 160-10 mg/100 ml, the proximal tubule is overwhelmed and begins to excrete glucose in the urine
Chloride, Water, and Urea Reabsorption - Answer - Not only is the secondary active reabsorption of glucose and amino acids linked to the Na-K ATPase pump, but the passive reabsorption of chloride, water, and urea also depends on this active Na reabsorption mechanism
Reabsorption of Water - Answer - Water is passively reabsorbed all along the tubule as it
follows sodium
- 65% of water is reabsorbed in the proximal tubule (117 L/day)
- 15% of water is reabsorbed in the loop of Henle
- 20% of water is reabsorbed in the distal and collecting tubules
- The fraction of water reabsorbed in the proximal tubule and the loop of Henle is constant, despite the sodium and water load in the body
- The proportion of water reabsorbed int he distal tubule and collecting tubule can vary depending on hormonal influences and the hydration state of the body
Reabsorption of Chloride - Answer - Despite the high concentration of chloride in the plasma, the kidneys do not directly regulate it
- The majority of chloride does not undergo transepithelial transport, rather it leaves the tubular fluid by moving between the epithelial cells
- It goes down its electrochemical gradient, essentially following the amount of Na reabsorption
- Therefore, the amount of chloride reabsorbed is determined by the amount of sodium reabsorbed
Reabsorption of Urea - Answer - Though urea is a waste product from the breakdown of protein, a large amount of urea is reabsorbed
- The concentration of urea at the beginning of the proximal tubule is the same as the plasma concentration of urea so that there is no net diffusion
- However, as fluid moves through the proximal tubule, its volume is reduced by 2/3 as water is reabsorbed so the tubular concentration of urea increases three-fold
- Because of this, it is passively reabsorbed
- With each pass through the nephron, only about 40-50% of plasma urea is filtered and excreted from the body
- Blood urea, measured as blood urea nitrogen (BUN), has historically been used as a measure of renal failure
- During renal failure, less urea is excreted so it accumulates in the plasma and can be clinically measured
