Module 4: Renal PHGY Flashcards
how is homeostasis in the renal sys controlled
- Controlled by the kidneys
- Responsible for regulation of electrolyte composition, volume, osmolarity, and ph.
- Kidneys work to eliminate all the waste products from bodily metabolism with the exception of carbon dioxide removed via respiration
Describe the major structures of a juxtamedullary nephron and discuss the importance of each section with respect to reabsorption and secretion
- Renal Corpuscle: Filters blood and collects filtrate
- Proximal Convoluted Tubule (PCT): Reabsorbs essential substances like glucose and ions, and secretes waste products
- Loop of Henle: Establishes concentration gradient in the medulla, enabling water reabsorption
- Distal Convoluted Tubule (DCT): fine-tunes reabsorption and secretion processes initiated earlier
- Collecting duct: Responds to hormones to regulate water reabsorption and urine concentration, and maintains acid-base balance
Describe blood flow through the kidneys and its physiological importance in the generation of urine
Blood Flow:
1. Renal Artery Entry: Oxygenated blood enters each kidney through the renal artery.
2. Afferent Arterioles: Branch off the renal artery to form the glomerular capillaries within the renal corpuscle.
3. Glomerular Filtration: Blood pressure forces fluid and solutes from the glomerular capillaries into Bowman’s capsule, forming filtrate.
4. Efferent Arterioles: Carry blood away from the glomerulus, leading to two pathways:
- Peritubular Capillaries: Surround the renal tubules for reabsorption and secretion.
- Vasa Recta: Form long, hairpin-like capillaries around the loop of Henle, maintaining osmotic gradient in the medulla.
5. Renal Vein Exit: Filtered blood exits the kidney via the renal vein, carrying reabsorbed substances and wastes.
6. Urine Formation: Filtrate undergoes reabsorption (essential substances reclaimed) and secretion (additional substances added) throughout the nephron, resulting in concentrated urine.
Importance:
- Filtration: Removes waste products and excess substances from the blood, maintaining internal environment balance.
- Reabsorption: Retrieves essential substances like glucose, ions, and water, conserving body resources.
- Secretion: Excretes waste products and regulates electrolyte and acid-base balance.
- Concentration Gradient: Establishes osmotic gradient in the medulla, crucial for water reabsorption and concentration of urine.
- Blood Pressure Regulation: Renin-angiotensin-aldosterone system and other mechanisms adjust blood volume and pressure.
Using your knowledge of osmotic gradients, describe how the kidney can make urine either more dilute or more concentrated than other bodily fluids
The kidney regulates urine concentration through osmotic gradients. By adjusting the reabsorption of water and solutes along the renal tubules, it can produce urine that is either more dilute or more concentrated than other bodily fluids. This process involves creating and maintaining osmotic gradients in the renal medulla, primarily through countercurrent mechanisms in the loop of Henle. Hormonal regulation, such as antidiuretic hormone (ADH), also plays a crucial role in modulating water reabsorption and urine concentration. Overall, the kidney’s ability to manipulate osmotic gradients allows for precise control of urine concentration, essential for maintaining fluid and electrolyte balance in the body.
Using dehydration as an example, describe the physiological responses and processes that occur in order to reduce water loss through urine production
- Dehydration Detection:
Osmoreceptors and baroreceptors detect decreased blood volume and increased osmolality. - Release of ADH:
Hypothalamus signals pituitary gland to release ADH into bloodstream. - Increased Water Reabsorption:
ADH increases collecting duct permeability, promoting water reabsorption.
Aquaporin channels facilitate water movement from ducts to interstitium. - Concentration of Urine:
Reabsorbed water reduces urine volume, making it more concentrated.
Helps conserve water in the body. - Thirst Mechanism Activation:
Dehydration triggers thirst, prompting fluid intake. - RAAS Activation:
Renin-Angiotensin-Aldosterone System may activate to enhance sodium and water retention.
Importance:
Maintains fluid balance, blood pressure, and electrolyte levels during dehydration, aiding overall homeostasis.
Identify the major fluid compartments and describe how they are interrelated
Fluid within cells, constituting about 2/3 of total body water.
Contains electrolytes, proteins, and other solutes necessary for cellular function.
Extracellular Fluid (ECF):
Fluid outside cells, comprising interstitial fluid, plasma, and transcellular fluid.
Interstitial fluid: Surrounds cells, exchanges nutrients and waste with blood.
Plasma: Fluid portion of blood, carries nutrients, hormones, and waste products.
Transcellular fluid: Small, specialized compartments like cerebrospinal fluid, synovial fluid, and digestive juices.
Fluid Movement: Exchange occurs between compartments via osmosis, diffusion, and active transport.
Homeostasis: Fluid balance is maintained through intricate regulatory mechanisms.
Blood Pressure Regulation: ECF volume influences blood pressure, impacting fluid movement between compartments.
Nutrient Distribution: Plasma transports nutrients to cells, while waste products are removed via interstitial fluid.
Cellular Function: ICF provides the environment for cellular metabolism and signaling.
Osmotic Regulation: Electrolyte concentrations in ECF and ICF are regulated to prevent osmotic imbalances.
Hormonal Control: Hormones like ADH and aldosterone regulate fluid balance by affecting water reabsorption in the kidneys.
Compare and contrast short-term vs long-term control of the extracellular fluid
Short-term Control:
Rapid Response: Acts within minutes to hours.
Mechanisms:
Neural Regulation: Baroreceptors in blood vessels detect changes in blood pressure, signaling the nervous system to adjust.
Hormonal Regulation: Rapid release of hormones like ADH (vasopressin) and aldosterone in response to changes in blood pressure or osmolality.
Effects:
ADH increases water reabsorption in the kidneys, reducing urine output and conserving water.
Aldosterone enhances sodium and water reabsorption, promoting blood volume and pressure.
Long-term Control:
Gradual Adjustment: Occurs over days to weeks.
Mechanisms:
Renal Regulation: Kidneys play a central role in long-term fluid balance through processes like reabsorption, secretion, and filtration.
Thirst Mechanism: Controlled by osmoreceptors in the hypothalamus, prompting fluid intake to restore hydration.
Effects:
Renal mechanisms adjust over time to maintain fluid and electrolyte balance.
Thirst mechanism prompts increased fluid intake to replenish lost fluids and restore homeostasis.
Describe the differences between isotonic, hypertonic, and hypotonic, and how cells in each of these solutions would be affected
Isotonic:
Same solute concentration as cell cytoplasm.
No net water movement.
Cells maintain normal shape and volume.
Hypertonic:
Higher solute concentration than cell cytoplasm.
Water moves out of cells.
Cells shrink or undergo crenation/plasmolysis.
Hypotonic:
Lower solute concentration than cell cytoplasm.
Water moves into cells.
Cells swell or undergo lysis/turgor pressure.
Effects on Cells:
Isotonic: No change.
Hypertonic: Cells shrink.
Hypotonic: Cells swell.
Describe the pathways involved in the regulation of water balance in terms of intake and output
Intake Pathways:
Thirst Mechanism:
Triggered by increased plasma osmolality or decreased blood volume.
Stimulates the sensation of thirst, prompting individuals to drink fluids to restore hydration.
Regulated by osmoreceptors in the hypothalamus.
Output Pathways:
Renal Regulation:
Filtration: Blood is filtered in the kidneys, producing urine.
Reabsorption: Essential substances like water, ions, and nutrients are reabsorbed from the filtrate back into the bloodstream.
Secretion: Additional substances, such as waste products and excess ions, are secreted from the bloodstream into the filtrate for excretion.
Hormonal Regulation:
Antidiuretic Hormone (ADH):
Released by the pituitary gland in response to dehydration or increased plasma osmolality.
Increases water reabsorption in the kidneys, reducing urine output and conserving water.
Aldosterone:
Released by the adrenal glands in response to low blood pressure or low blood volume.
Enhances sodium reabsorption in the kidneys, indirectly affecting water reabsorption and blood volume.
Thirst Mechanism:
Regulates fluid intake in response to changes in fluid output and hydration status.
ICF
Intracellular Fluid
Fluid within cells
Comprises 2/3rds of total body fluid
ECF
Extracellular Fluid
Fluid surrounding the cells (plasma, interstitial fluid, lymph, and transcellular fluid
Comprises 1/3rd of total body fluid
what do barriers btwn body-fluid compartments do
They limit the movement of water and solutes between the various compartments
plasma-interstitial fluid
Separated by the blood vessel wall
Plasma and interstitial fluid are identical (except plasma proteins)
2 barriers btwn bodu-fluid compartments
- plasma-interstitial fluid
- ICF and ECF
- plasma membrane barrier
- intracellular contains proteins that do not exchange with extracellular
- greater [c] of K+ in ICF
- greater [c] of Na+ in ECF
overall umbrella def for ECF volume and osmolarity
Overall control of fluid balance is dependent upon regulating the ECF
ECF volume
is closely regulated to maintain blood pressure (salt-balance is important for long-term regulation)
ECF Osmolarity
is closely regulated to prevent swelling and shrinkage of cells
To maintain fluid balance, the extracellular volume and osmolarity is regulated closely
Short-term ECF volume control
- Only temporary and can compensate for fairly minor changes in ECF volumes
- Baroreceptor reflex
- Fluid shifts
baroreceptor reflex
Baroreceptor Reflex is mechanoreceptors that are located in the carotid artery and the aortic arch, and they detect changes in arterial blood pressure, the autonomic NS then affects the heart and blood vessels
The 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
Fluid Shifts
a decrease in plasma volume can temporarily be compensated for by a shift of the fluids out of interstitial compartment to the plasma, the opposite is also true, as an increase in plasma volume can cause fluid to shift to the interstitial compartment
Long term ECF volume control
Kidneys and thirst mechanism (control of urine output)
Describe the Control of Salt
1) describe sodium and how it is controlled
Sodium, and the anions that are associated with it (mainly chloride) accounts for more than 90% of the ECF’s solutes
When salt is transported across a membrane, water follows due to osmosis
Control salt=ECF volume is controlled
To maintain salt balance (salt input=salt output)
describe salt input
is dependent on dietary salt, replace salt loss through feces and sweat (0.5g/day)
describe salt output
excess salt must be eliminated (feces, sweat and kidneys)
describe hypotonicity + causes
Associated with overhydration, or excess of free H20
Causes
Renal Failure (can’t produce concentrated urine)
Rapid water ingestion (kidneys can’t deal with it quick enough)
Over secretion of vasopressin (promotes water retention)
describe hypertonicity and causes
Associated with dehydration
Causes
Insufficient water intake, not drinking enough
Diabetes insipidus, vasopressin deficiency
Excessive water loss from exercise, vomiting, or diarrhoea
describe isotonic
- Has an equal osmolarity to that of normal body fluids
- Isotonic fluids are injected into blood plasma within the veins (1/5th of ECF)
- When isotonic fluids are injected into the ECF compartments, the ECF volume increases, but the concentration of the ECF remains unchanged (remains isotonic)
- There will be no fluid shift between compartments because ECF and ICF are still in osmotic equilibrium (cells would not shrink nor swell)
- Isotonic fluid loss, is confined to the ECF, with no corresponding fluid loss from the ICF.
Regulation of Water Balance (describe this)
- Hypothalamus, near the vasopressin-secreting cells and thirst centre, there is hypothalamic osmoreceptors (which monitor osmolality of fluid around them)
- If osmolality increases, so does vasopressin secretion and thirst
- Vasopressin acts on the kidneys to increase water reabsorption
Thirst stimulates the intake of water - If it becomes hypotonic, then vasopressin secretion and thirst are not stimulated.
- Large losses of ECF volume also impact these pathways (Left atrial volume receptors) monitor pressure of blood in left atrium, they are activated with more than 7% loss of ECF volume and blood pressure, they also stimulate vasopressin and thirst
describe the roles of the kidneys
Filtration: Removes waste products, toxins, and excess ions from the bloodstream.
Fluid and Electrolyte Balance: Regulates body fluid volume and composition by reabsorbing water and electrolytes.
Excretion of Waste: Eliminates waste products like urea, creatinine, and uric acid through urine.
Acid-Base Balance: Maintains pH levels by excreting or retaining hydrogen and bicarbonate ions.
Hormone Production:
Renin: Regulates blood pressure.
Erythropoietin (EPO): Stimulates red blood cell production.
Calcitriol (Active Vitamin D): Regulates calcium and phosphate levels.
Blood Pressure Regulation: Controls blood volume and vascular resistance to maintain optimal blood pressure.
describe the major structural components of a nephron
Renal Corpuscle:
Glomerulus: Network of capillaries where blood filtration occurs.
Bowman’s Capsule: Surrounds the glomerulus, collecting filtrate from blood.
Renal Tubule:
Proximal Convoluted Tubule (PCT): Initial segment for reabsorption and secretion.
Loop of Henle: Descending and ascending limbs involved in water and ion reabsorption.
Distal Convoluted Tubule (DCT): Fine-tunes reabsorption and secretion processes.
Collecting Duct: Collects tubular fluid from multiple nephrons, regulates water reabsorption.
describe the basic renal processes involved in urine excretion
Filtration:
Occurs in the renal corpuscle.
Blood pressure forces fluid and solutes from the glomerulus into Bowman’s capsule, forming filtrate.
Reabsorption:
Takes place in the renal tubules.
Essential substances like water, ions, and nutrients are reabsorbed from the tubular fluid back into the bloodstream.
Proximal Convoluted Tubule (PCT) is a major site for reabsorption.
Secretion:
Occurs in the renal tubules.
Additional substances, such as waste products and excess ions, are actively transported from the bloodstream into the tubular fluid for excretion.
Distal Convoluted Tubule (DCT) and collecting ducts are involved in secretion.
Concentration:
Involves the loop of Henle and collecting ducts.
Establishes a concentration gradient in the medulla, allowing for water reabsorption and concentration of urine.
Hypothalamus regulates water reabsorption via Antidiuretic Hormone (ADH).
Excretion:
Final step where urine is excreted from the body.
Urine, containing waste products and excess substances, exits the kidneys via the ureters, bladder, and urethra.
The kidneys are controlled by…
Both neural and endocrine inputs
functions of the kidneys
Maintain water balance in the body
Maintain body fluid osmolarity
Maintain proper plasma volume
Help maintain acid-base balance
Regulates ECF solutes (NA, K, Cl, Ca, PO4)
Excretes wastes of metabolism
Excretes foreign compounds ingested
Produces erythropoietin
Produces renin
Activated Vitamin D
describe the structure of the kidney
Bean shaped organ
10cm in length
An adrenal gland on top of the kidney
Outside of the kidney is the renal cortex
Inner part is renal medulla
Renal pelvis is at the core of the kidney (urine empties)
Urine channeled to the ureter
Functional unit of the kidney is the nephron (blood is filtered to produce urine and reabsorb fluids and molecules
describe the vascular component of the nephron
- Glomerulus (ball-like capillary), is where water and solutes are filtered from plasma
- Blood enters the kidney via the renal artery, and this subdivides into afferent arterioles (each supplies a nephron)
- Leaving the nephron are efferent arterioles, which transport unfiltered blood from the glomerulus (arterial blood enters and leaves with no oxygen extracted)
- Capillaries of the nephron, efferent arterioles subdivide into capillaries, the peritubular capillaries deliver oxygen to renal tissues
describe the tubular component of the nephron
(Begins with) Bowmans capsule, encircles the glomerulus to collect filtered fluid
Fluid then passes into the proximal tubule within the renal cortex
Loop of Henle forms a hairpin loop that dips down into the renal medulla
The descending limb of loop of Henle travels from the cortex to the medulla back to the cortex
Ascending limb of loop of Henle passes through a fork of the afferent and efferent arteries in a region called juxtaglomerular apparatus.
Tubule coils again and is called the distal tubule (within the cortex)
The distal tubule empties into a collecting duct, which travels deep into the medulla and drains in the renal pelvis
what are the 2 types of nephrons
- Cortical Nephrons- lie in the outer layer of the cortex (secretory and regulatory functions)
- Juxtamedullary Nephrons - inner layer of cortex (concentration and dilution of urine function) and they form hairpin loops called vasa recta
What is glomerular filtration (GF)
20% of blood flowing through the glomerular capsule is foltered in Bowmans capsule
what is tubular reabsorption (TR)
As filtrate flows through the tubules, important substances are returned to the peritubular capillaries by the process of tubular reabsorption
Tubular Secretion (TS)
Second route for substances in the blood to enter renal tubules
Selective transfer of substances from the peritubular capillaries into the tubules (remaining 80%)
Describe the forces that regulate glomerular filtration
Hydrostatic Pressure in Glomerular Capillaries (PGC):
Blood pressure within the glomerular capillaries.
Forces fluid and solutes out of the blood into Bowman’s capsule.
Hydrostatic Pressure in Bowman’s Capsule (PBS):
Pressure exerted by the filtrate in Bowman’s capsule.
Opposes filtration by pushing fluid back into the capillaries.
Osmotic Pressure in Glomerular Capillaries (πGC):
Created by proteins (e.g., albumin) in the blood.
Draws water back into the capillaries, opposing filtration.
Filtration Pressure (Net Filtration Pressure, NFP):
NFP = PGC - (PBS + πGC).
Represents the overall pressure gradient driving filtration.
Positive NFP favors filtration, while negative NFP opposes it.
Describe how glomerular filtration is regulated by the body, both intrinsically and extrinsically
Intrinsic Regulation:
Myogenic Mechanism:
Description: Autoregulatory response to changes in blood pressure.
Process: Increased blood pressure stretches afferent arterioles, triggering smooth muscle contraction and vasoconstriction to maintain a steady glomerular filtration rate (GFR).
Purpose: Protects glomeruli from damage due to excessive pressure.
Tubuloglomerular Feedback (TGF):
Description: Feedback mechanism involving the juxtaglomerular apparatus.
Process: Macula densa cells in the DCT sense changes in tubular fluid composition, particularly NaCl levels. Increased NaCl concentration triggers vasoconstriction of afferent arterioles, reducing GFR.
Purpose: Maintains a stable flow of filtrate to prevent electrolyte imbalances.
Extrinsic Regulation:
Sympathetic Nervous System (SNS):
Description: Neural regulation of renal blood flow.
Process: Activation of the SNS leads to vasoconstriction of both afferent and efferent arterioles, reducing GFR and renal blood flow.
Purpose: Redirects blood flow to vital organs during times of stress or low blood pressure.
Hormonal Regulation:
Description: Hormones control renal blood flow and GFR.
Example: Angiotensin II, released in response to low blood pressure or low sodium levels, constricts arterioles, increasing blood pressure and GFR.
Purpose: Maintains blood pressure and fluid balance.
Explain why the kidneys receive a greater proportion of cardiac output relative to its weight
Vital Function:
The kidneys play crucial roles in maintaining fluid balance, electrolyte levels, and blood pressure regulation.
Adequate renal blood flow is essential for efficient filtration and waste removal.
High Metabolic Demand:
Despite their small size relative to body weight, the kidneys have high metabolic activity.
They require a significant amount of oxygen and nutrients to support filtration, reabsorption, and secretion processes.
Filtration Efficiency:
Renal blood flow directly affects glomerular filtration rate (GFR), the rate at which blood is filtered by the kidneys.
Higher renal blood flow allows for more efficient filtration and waste removal.
Regulatory Role:
The kidneys play a critical role in regulating blood pressure through mechanisms like the renin-angiotensin-aldosterone system (RAAS).
Adequate blood flow ensures proper activation of these regulatory pathways.
describe the glomerulus
Is a network of capillaries located at the beginning of a nephron
Blood is filtered across the walls of this capillary networks through the glomerular membranes (which yields its filtrations into Bowmans capsule)
The glomerulus receives its blood supply from an afferent arteriole and the glomerular capillaries exit into efferent arterioles
The rate at which blood is filtered through all the glomeruli and the measure of its overall function is the glomerular filtration rate (GFR)
describe glomerular filtration
The glomerular capillary wall- large pores, albumin can pass through
The basement membrane- collagen for structural strength, and glycoproteins to discourage the filtration of small plasma proteins (1% albumin gets through)
The inner layer of Bowman’s capsule (podocytes) cells that wrap around the capillaries of the glomerulus, and they form a narrow filtration slit between them to allow fluid to pass into Bowman’s capsule
3 forces that regulate glomerular filtration
- Glomerular Capillary Blood Pressure
- Plasma-Colloid Oncotic Pressure
- Bowmans Capsule Hydrostatic Pressure
glomerular capillary blood pressure
55 mm Hg
Plasma-Colloid Oncotic Pressure
The presence of large proteins in the plasma that cannot be filtered produces an oncotic force that resists the movement of water into Bowmans capsule
30 mmHg*
bowmans capsule hydrostatic pressure
Pressure of fluid in Bowman’s capsule
Resists the movement of water oyt of glomerular capillaries
15 mm Hg *
glomerular filtration rate
Dependent on filtration pressure, glomerular SA and permability
Those factors are also called the filtration coefficient (Kf)
autoregulation
Regulates the diameter of afferent arterioles
Constricting the afferent arterioles = decreased glomerular capillary blood pressure
Dilating the afferent arterioles = increased glomerular capillary blood pressure
myogenic activity
Increased pressure stretches the afferent arteriole walls, they automatically constrict to reduce blood flow, and thus preventing a GFR increase
tubuloglomerular feedback (TFG)
- Macula densa-sense changes in salt level of the tubular fluid
- Increased arterial pressure increases GFR, more fluid will flow through the distal tubule = an increased salt delivery
- In response to increased salt, the macula densa will release ATP, degraded to adenosine, which then causes constriction and reduces GFR
vasoconstrition
Decrease in glomerular capillary blood pressure
A decrease in net filtration pressure
A decrease in GFR
vasodilation
Increase in glomerular capillary blood pressure
Increase in net filtration pressure
An increase in GFR
Sympathetic control of GFR
- Extrinsic control, independent of arterial blood pressure
- Baroreceptors sense the drop in arterial blood pressure
- Kidneys increase sympathetic activity, constricting afferent arterioles, and decreasing GFR, and reducing urine production
cardiac output
Signifies the importance of the kidneys
Not for oxygen, but to deliver blood for cleaning
Also allows kidneys to maintain tight control of volume and electrolyte concentrations of the body’s water pools and eliminate wastes efficiently
Describe the process of transepithelial transport using sodium as an example
Sodium Entry:
Sodium ions (Na⁺) enter epithelial cells from the lumen through specific transport proteins like sodium channels or co-transporters.
Intracellular Transport:
Na⁺ is transported across the epithelial cell via diffusion or active transport mechanisms.
Basolateral Exit:
Na⁺ exits the cell into the interstitial fluid through sodium-potassium pumps located on the basolateral membrane.
Secondary Active Transport:
Na⁺ exit creates an electrochemical gradient, driving the co-transport of other solutes into the cell against their concentration gradient.
Water Reabsorption:
Reabsorption of Na⁺ and co-transported solutes increases interstitial osmolarity, promoting water reabsorption from the lumen into the interstitial space
Describe how the reabsorption of sodium is regulated
Hormonal Regulation:
Aldosterone:
Released by the adrenal glands in response to low sodium levels or high potassium levels in the blood.
Stimulates the reabsorption of sodium ions in the distal convoluted tubule (DCT) and collecting ducts by increasing the activity of sodium channels and sodium-potassium pumps on the apical and basolateral membranes of epithelial cells.
Atrial Natriuretic Peptide (ANP):
Released by the heart in response to high blood pressure or volume.
Inhibits the reabsorption of sodium ions in the distal nephron segments, promoting sodium excretion and water loss.
Autoregulation:
Tubuloglomerular Feedback (TGF):
Macula densa cells in the distal convoluted tubule sense changes in sodium chloride concentration in the tubular fluid.
Increased sodium chloride concentration triggers vasoconstriction of afferent arterioles, reducing glomerular filtration rate (GFR) and sodium reabsorption to maintain a stable flow of filtrate.
Myogenic Mechanism:
Smooth muscle cells in the afferent arterioles respond to changes in blood pressure by constricting or dilating to regulate renal blood flow and sodium filtration.
Neural Regulation:
Sympathetic Nervous System (SNS):
Activation of the SNS leads to vasoconstriction of renal blood vessels, reducing renal blood flow and sodium filtration, particularly in response to low blood pressure or acute stress.
Define Tm and explain why it is important in the reabsorption of necessary substances
Definition:
Tm (Transport Maximum):
The maximum rate at which a substance can be reabsorbed by the renal tubules.
Represents the saturation point of transporters responsible for reabsorption.
Importance:
Efficient Reabsorption:
Ensures that essential substances like glucose, amino acids, and vitamins are fully reabsorbed from the filtrate into the bloodstream.
Prevents Losses:
Prevents the loss of valuable nutrients and other solutes in the urine.
Maintains Homeostasis:
Helps maintain optimal levels of nutrients and other solutes in the body fluids, supporting overall physiological function
Describe the reabsorption of water along the entire tubule
Proximal Convoluted Tubule (PCT):
Description: Site of major water reabsorption, driven by osmotic forces created by solute reabsorption.
Process: Water follows the reabsorption of solutes like sodium, glucose, and amino acids from the tubular fluid into the bloodstream.
Descending Limb of the Loop of Henle:
Description: Highly permeable to water but not to solutes.
Process: Water moves out of the tubular fluid through osmosis into the interstitial space, driven by the increasing osmolarity of the renal medulla.
Thin Ascending Limb of the Loop of Henle:
Description: Impermeable to water but allows passive transport of solutes.
Process: No water reabsorption occurs in this segment, contributing to the concentration gradient in the medulla.
Thick Ascending Limb of the Loop of Henle:
Description: Impermeable to water but actively transports solutes like sodium, chloride, and potassium out of the tubular fluid.
Process: Creates a dilute tubular fluid, contributing to the osmotic gradient in the medulla.
Distal Convoluted Tubule (DCT) and Collecting Ducts**:
Description: Water reabsorption occurs under the influence of antidiuretic hormone (ADH).
Process: ADH increases the permeability of the tubular epithelium to water, allowing water to move out of the tubular fluid and into the interstitial space, following the osmotic gradient established by solute reabsorption.
describe tubular reabsorption
Processes by which water and other necessary solutes are returned to the plasma, while allowing waste products to remain in the filtrate
1. Reabsorption begins with either active or passive movement of substances from the tubule into the interstitial space
2. Reabsorption then continues with passive movement of substances from the interstitial space back into the bloodstream
fate of various substances filtered by the kidneys
Water-mostly reabsorbed
Sodium-mostly reabsorbed
Urea-half reabsorbed
Phenol-none reabsorbed
transepithelial transport
- The tubule is composed of a single layer of epithelial cells
- 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 tubule lumen is the luminal membrane
- Epithelial cells that are in contact with the interstitial fluid is the basolateral membrane
- Transepithelial transport is defined as the movement of solutes across an epithelial cell layer throughout the cell
- Tight junctions between them (substance must move through the cell into the interstitial space
steps for transepithelial transport
Steps :
1. Substance moves across luminal membrane
2. Substance passes through cytosol
3. Substance moves across basolateral membrane
4. Substance diffuses through interstitial fluid
5. Substance crosses capillary wall to enter plasma
Na Reabsorption Locations (3)
The proximal tubule
- 76% reabsorbed Na+ (needed for reabsorption of glucose, amino acids, water, Cl, and urea(
The ascending limb of the loop of Henle
- 25% reabsorbed (Na and Cl are essential for concentration or dilution of urine)
The distal and collecting tubules
- 8% (hormonal control and plays a key role in regulating ECF volume and secretion of both K and H
active transport of Na
- Active and passive for Na
- Movement of Na across basolateral membranes is active transport with Na K ATPase pump
- 80% of energy needs from kidney is for this
passive transport of Na
- Luminal membrane transport
- Na crosses by a cotransporter in the proximal tubule that also moves glucose and amino acids
- These nutrients are transferred by secondary active transport (they use the gradient concentration of Na established by the K-Na pump to be transported against their concentration gradient along with Na
- In the collecting duct, Na passively enters the epithelial cell
Hormonal Regulation of Na
- Granular cells detect a drop in blood pressure, secretes renin
- Granular cells innervated by the sympathetic nervous system will release renin when sympathetic activity increases
- Macula densa cells are sensitive to Na, and a decrease in luminal Na triggers secretion of renin
Renin and Na
- Renin acts like an enzyme and converts angiotensin into angiotensin I
- Circulating angiotensin I passes through the lungs, it is converted to angiotensin II by the enzyme angiotensin converting enzyme (ACE)
- Angiotensin II stimulates the adrenal cortex, and the release of aldosterone
- Aldosterone increases Na reabsorption in the distal and collecting tubules
describe aldosterone
- 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
- This makes a greater flow of Na out of tubular fluid
- This enhanced Na retention increases water retention
- Water follows Na
Atrial Natriuretic Peptide (ANP)
- Opposite of aldosterone
- Reduces Na load
- Reduces blood pressure
Actions
1. Inhibits Na reabsorption in the distal tubules (more Na excreted in urine)
2. Inhibits renin and aldosterone secretion
3. Dilates the afferent arterioles and increases GFR (more salt and water filtered= more salt and water excreted)
describe actively reabsorbed substances
A limited number of carrier proteins in a membrane
Limit to how much substance can be absorbed
Tubular or transport maximum ™
For any substance, if its concentration in the tubular fluid exceeds Tm, then the excess will be secreted in the urine
Plasma concentration at which Tm is exceeded is called the renal threshold
how does phosphate work w the kidneys
Kidneys regulate phosphate plasma concentration
Renal threshold for phosphate is the same as normal plasma concentration of phosphate
Phosphate and calcium are hormonal regulation
describe glucose and the kidneys
Not regulated by kidneys
Have a Tm for reabsorption
Urine usually does not have glucose because kidneys reabsorb it
Proximal tubule can only reabsorb a limited amount of glucose, if it gets over this then glucose appears in urine
describe chloride, water, and urea in context of the kidneys
- Water is passively reabsorbed as it follows sodium
- Water flows through water channels called aquaporins (proximal tubule are controlled by osmosis) (distal tubule are controlled by vasopressin so not always open)
- Chloride is not directly regulated by kidneys, moves between epithelial cells, and goes down electrochemical gradient, following the amount of Na reabsorption (amount of chloride is determined by amount of sodium)
- Urea is a waste product of protein, renal failure=less urea is excreted so it accumulates in plasma
Describe the secretion of hydrogen and potassium ions
Hydrogen Ion Secretion:
Description: Process of actively transporting hydrogen ions (H⁺) from the blood into the renal tubular fluid.
Location: Primarily occurs in the proximal tubule and collecting ducts.
Process: H⁺ ions are secreted across the tubular epithelium into the tubular fluid via hydrogen pumps, primarily in exchange for sodium ions (Na⁺) or bicarbonate ions (HCO₃⁻).
Purpose: Helps regulate blood pH by eliminating excess acid from the body.
Potassium Ion Secretion:
Description: Process of actively transporting potassium ions (K⁺) from the blood into the renal tubular fluid.
Location: Occurs primarily in the distal tubule and collecting ducts.
Process: K⁺ ions are secreted across the tubular epithelium into the tubular fluid via potassium pumps, typically in exchange for sodium ions (Na⁺).
Purpose: Regulates potassium balance in the body, ensuring optimal cellular function and electrical activity.
Describe why it is necessary to secrete organic anions and cations
xic Waste Removal:
Description: Organic anions and cations include metabolites, drugs, and toxins that need to be eliminated from the body.
Importance: Secretion of these substances into the renal tubular fluid facilitates their removal from the bloodstream, preventing accumulation and potential toxicity.
Renal Excretion of Drugs:
Description: Many drugs and their metabolites are organic compounds that require renal excretion to be eliminated from the body.
Importance: Secretion of these substances ensures effective clearance of drugs and metabolites, optimizing therapeutic outcomes and preventing adverse effects.
Regulation of Body Homeostasis:
Description: Organic anions and cations include endogenous substances like hormones, neurotransmitters, and metabolites involved in various physiological processes.
Importance: Proper secretion of these substances helps regulate body homeostasis by controlling their levels in the bloodstream, ensuring optimal physiological function.
Maintenance of Acid-Base Balance:
Description: Organic anions like citrate and cations like ammonia are involved in buffering systems that help maintain acid-base balance in the body.
Importance: Secretion of these compounds contributes to the regulation of blood pH, preventing acidosis or alkalosis.
describe tubular secretion
- Involves the movement of substances from the peritubular capillaries to the tubule lumen, for removal of substances
- Substances that undergo tubular secretion: Hydrogen ions, potassium ions, organic anion and cations
describe hydrogen ion secretion
Secreted in the proximal, distal and collecting ducts
Too much H+ in plasma = secretion
Low H+ in plasma = decreases tubular secretion of H+
Renal H+ regulates the acid-base balance
describe potassium ion secretion
Tubular reabsorption and secretion
Kidneys regulate plasma K levels
K+ ion secretion in the distal tubule is an active process dependent upon Na - K - ATPase pump
describe secretion of organic anions and cations
Increasing secretion-adding more organic ions to tubular fluid can increase the organic ion excreted
Excrete poorly soluble organic ions
Removal of foreign compounds
control of K+ secretion
A rise in plasma K+ = release of aldosterone
Aldosterone increases Na reabsorption, secreting more K
H+ secretion effects it
Describe what is meant by plasma clearance
Definition:
Plasma clearance refers to the volume of plasma that is completely cleared of a substance by the kidneys per unit of time.
Process:
It measures the efficiency of the kidneys in removing a specific substance from the bloodstream.
Clearance is determined by the rate at which the substance is filtered by the glomeruli, reabsorbed from the tubular fluid, and secreted into the tubular fluid.
Calculation:
Clearance (C) is calculated using the formula: C = (U x V) / P, where:
C = clearance,
U = urinary concentration of the substance,
V = urine flow rate (volume of urine produced per unit of time),
P = plasma concentration of the substance.
Importance:
Plasma clearance provides valuable information about kidney function and the rate at which a substance is eliminated from the body.
It is used clinically to assess renal function, monitor the progression of kidney diseases, and adjust drug dosages based on renal clearance rates.
Describe what is meant by the vertical osmotic gradient and why it is important
The vertical osmotic gradient in the renal medulla is crucial for concentrating urine and conserving water. It is generated by countercurrent exchange mechanisms in the loop of Henle and maintained by the vasa recta. This gradient allows the kidneys to regulate urine concentration and maintain fluid balance, making it essential for overall kidney function and homeostasis.
Describe the role of the loop of Henle and why its structure is important for its function
The loop of Henle is essential for generating the osmotic gradient necessary for water reabsorption and urine concentration. Its unique structure, with a descending limb permeable to water and an ascending limb actively transporting solutes, facilitates the establishment and maintenance of the osmotic gradient. Dysfunction of the loop of Henle can lead to impaired kidney function and electrolyte imbalances, highlighting its critical role in renal physiology.
Describe the countercurrent exchange
Countercurrent exchange is a physiological process essential for efficient urine concentration in the kidneys. By creating and maintaining osmotic gradients through countercurrent flow, it maximizes water reabsorption and facilitates electrolyte balance, contributing to overall renal function and fluid homeostasis.
Describe the ,micturition reflex
The micturition reflex is an involuntary neurological process that coordinates bladder contraction and sphincter relaxation to facilitate urination. Initiated by bladder distension, the reflex arc involves sensory input, spinal cord processing, and efferent motor output to ensure controlled voiding. Dysfunction of this reflex can lead to urinary issues and is associated with various neurological conditions.
describe plasma clearance and the type
Substances have been cleaned from the plasma (any substance as the volume of plasma cleared of that substance by the kidneys per minute
Plasma clearance expresses the effectiveness of the kidneys to remove a substance from internal fluids
Types
1. Substances that are filtered, not absorbed (insulin)
2. Substances that are filtered and reabsorbed (glucose)
3. Substances that are filtered and secreted (hydrogen)
The kidneys and urine concentrations
Concentrate urine occurs because there is a vertical osmotic gradient in the interstitial fluid of the medulla
describe the structure of the cortical nephron
the loop of Henle only dips slightly into the medulla
describe the juxtamedullary nephron
the loop of Henle dips all the way down to the renal pelvis. the vasa recta of these nephrons also goes all the way to the renal pelvis. flow in the loop of Henle and the vasa recta goes in opposite directions in what is called countercurrent flow
describe the medullary vertical osmotic gradient
- Fluid leaves Bowman’s capsule and enters proximal tubule (strong osmotic reabsorption drive)
- Isotonic tubular fluid 965% of filtrate volume absorbed at proximal tubule)
- Loop of Henle, additional 15% of filtered water is reabsorbed
describe countercurrent multiplication
- Descending and ascending limbs are in close proximity, and so important interactions occur between them to establish the vertical osmotic gradient
- Filtrate is constantly flowing
- Fluid from the proximal tubule enters the descending loop of Henle (300 mOsm/L) and the interstitial space is also 300 mOsm/L = both isotonic and no net movement
- Na moves into the interstitial space until it is 200 mOsm/L more concentrated than the ascending limb (tubular fluid is 200 mOsm/L and the interstitial fluid is 400 mOsm/L
- As new fluid moves into the descending loop of Henle, fluid shifts so we have 300 mOsm/L (pushing the 400 mOsm/L fluid deeper into the medulla
- While the ascending limb is still transporting Na out, water continues to passively leave the descending limb until 200 mOSM/L difference
- A fresh 300 mOsm/L enters the descending loop, stopping the vertical gradient until it re equilibriates
- Again, fresh filtrate enters,, and the osmolarity of the interstitial fluid increases, and the osmolarity of the ascending loop decreases to maintain 200 mOsm/L difference
- Eventually, equilibrium is achieved and even the (300 mOsm/L) filtrate, the vertical osmotic gradient results in it being 1200 mOsm/L as it enters the ascending limb, and the tubular fluid is at 100 mOsm/L as it enters the distal tubule
purpose of countercurrent multiplication
The isotonic fluid that enters the loop becomes progressively more concentrated as it flows down the descending limb, only to become more dilute as it flows up the descending limb
benefits of countercurrent multiplication
Allows the collecting ducts to both form more concentrated and more dilute urine than normal bodily fluids
Allows the overall volume of urine to be significantly reduced (conserves both salt and water)
Vasopressin-Controlled Water Reabsorption
Vasopressin (antidiuretic hormone) released from the posterior pituitary gland is released in response to a water deficit (Hypertonic ECF)
It’s release is inhibited when the ECF is hypotonic
Once released into circulation, it travels to the kidneys where it acts on distal tubular cells to increase the number of aquaporin molecules in the luminal membrane (increases amount of water reabsorbed into the epithelial cells)
Once inside the epithelial cells, water passively moves into the interstitial fluid and plasma (only works is proximal and distal tubules)
regulation of water reabsorption
Tubular fluid entering the distal tubule is around 100 mOsm/L
Interstitial fluid of the renal cortex is 300 mOsm/L, and gets higher approaching 1200 mOsm/L
These gradients mean that water wants to leave the tubular fluid due to osmosis, but can only do so in the presence of vasopressin
Deficit of water-dehydration = vasopressin hormone, increasing aquaporin channels in the distal and collecting ducts
Excess of water- body fluid osmolarity below 300 mOsm/L, so hypotonic that vasopressin is supressed, this prevents the insertion of aquaporins in the luminal membrane
what do kidney and wide range of urine concentrations ensure
Ensures tubular fluid is always hypotonic to interstitial fluid
describe countercurrent exchange within vasa recta
- vasa recta (blood supply to the renal medulla)
1. as efferent arteriolar blood leaves the renal cortex, its osmolarity is 300 mOsm/L, isotonic to the interstitial fluid
2. as the descending loop moves towards the renal pelvis, the plasma remains isotonic to the surrounding interstitial fluid by reabsorbing Na+ and water leaving.
3. At the bottom of the loop, the plasma osmolarity is 1200 mOsm/L
4. As the blood flows up the ascending limb, the opposite occurs with water being reabsorbed and Na+ leaving, to keep the plasma isotonic with the different levels of the medulla
5. as the vasa recta re-enters the cortex its osmolarity is back to 300 mOsm/L, again isotonic to the interstitial fluid
water reabsorption
Tubular segments permeable to water, solute reabsorption always leads to water reabsorption due to osmosis
Osmotic diuresis (increased secretion of both water and excess un-reabsorbed solute
Excess glucose in tubules attract water and increase urine production, as seen with diabetics
Water diuresis (an increases excretion of water when there is little to no change in the excretion of solutes) (alcohol consumption because vasopressin is suppressed)
urine storage
- The bladder is composed of smooth muscle and can expand to increase storage (parasympathetic nervous system = bladder contraction)
- Urethra is the exit, guarded by an internal urethral sphincter and external
- Internal=voluntary control (relaxed bladder=closed)
- External=closed constant, tonic firing motor neurons (Voluntary control by deliberately tightening)
describe micturition (urination reflex)
Micturition is the process of bladder emptying and is governed by 2 mechanisms
Micturition reflex- adult bladder holds 250-400 mL before the internal pressure on the bladder wall initiates the micturition reflex (this stretch activates afferent fibres to the spinal cord where interneurons activate the parasympathetic system) (bladder contraction and relaxation of external sphincter)
Voluntary Control- micturition can be overrode by voluntary control, learning the perception of bladder filling prior to the activation of the reflex, voluntary excitatory signals from the cerebral cortex can override this reflex.
describe the major structures of a juxtaglomerular nephron and discuss the importance of each section with respect to reabsorption and secretion
describe blood flow through the kidneys and its physiological importance in the generation of urine
using your knowledge of osmotic gradients, describe how the kidney can make urine either more dilute or more concentrated than bodily fluids
using dehydration as an example, describe the physiological responses and processes that occur in order to reduce water loss through urine production
