Urinary System physiology Flashcards
What are the main functions of the kidneys?
Excretion of wastes, H2O balance, Plasma volume control, Blood pressure control, Renin production, Acid-base balance, Blood cell production (erythropoietin), Vitamin D activation
What components make up the urinary system?
Kidneys, Ureters, Urinary bladder, Urethra, Blood supply (20% of total flow)
What is the functional unit of the kidney?
Nephron
How many nephrons are typically found in one kidney?
Approximately 1 million
What are the parts of the kidney?
-Renal calyces
-Renal cortex (outer)
-Renal medulla (inner)
-Renal pelvis
What are the two types of nephrons?
- Cortical (85%): shorter
- Juxtamedullary (15%): longer looped and important in establishing the osmostic gradient
Vascular components of nephron
1) renal artery
2) afferent arteriole
3) glomerulus
4) efferent arteriole
5) peritubular capillaries
6) renal vein
What is the glomerulus?
-Tuft of capillaries that filters a protein-free plasma into the tubular component
-Very permeabled because they are genestrated capillaries
What do the peritubular capillaries do?
supply the renal tissue with exchanges from the blood
Tubular components of nephrons
1) bowman’s capsule (surrounding glomerulus)
2) proximal tubule
3) loop of henle - ascending and descending
4) distal tubule
5) collecting duct
What is glomerular filtration?
Nondiscriminant filtration of protein-free plasma from the glomerulus (blood) into Bowman’s capsule (tubule)
What is tubular reabsorption?
Movement from tubule back into blood
What is tubular secretion?
Molecules that are too big that we move from blood into the tubule
What processes result in urine formation?
Glomerular filtration, Tubular reabsorption, Tubular secretion
Sites of action for filtration, reabsorption, secretion
Filtration: bowman’s capsule
Reabsorption: loop of henle, proximal tubule, distal tubule, collecting ducts
Secretion: proximal tubule, distal tubule, collecting ducts
Is the proximal or distal tubule free for all or hormonally controlled?
Proximal: free for all
Distal: hormone controlled
What does glomerular filtration filter?
All but RBCs and proteins (too big)
-Peptides are ok though
What gets reabsorbed?
Na, Cl, Ca, PO4, water, glucose
What gets secreted?
potassium, hydrogren, large organics
What is the role of podocytes in the glomerulus?
They encircle the glomerulus tuft and can change shape of filtration slit to control filtration
Which layers does glomerular filtration have to pass?
-Glomerular capillary wall through pores
-Basement membrane: acellular gelatinous layer made of collagen and glycoproteins
-Inner layer of Bowman’s capsule
How much glomerular filtration occurs per day?
160 to 180 L/day
Urine is composed of _______% of filtrate
1%
What happens to podocytes in renal failure?
Large slits which allow proteins and RBCs in
What are the three main physical forces involved in glomerular filtration?
- Glomerular capillary blood pressure
- Plasma-colloid osmotic pressure
- Bowman’s capsule hydrostatic pressure
Glomerular capillary blood pressure
-Greatest force
-Pushes water into tubule
-Favours filtration
55 mmHg
Plasma-colloid osmotic pressure
-Pull water into capillary
-Opposes filtration
-30 mmHg
Bowman’s capsule hydrostatic pressure
-Pulls water into capillary
-Opposes filtration
-15 mmHg
What is the net filtration pressure in the glomerulus?
10 mm Hg
What is the Glomerular Filtration Rate (GFR)?
The rate at which fluid is filtered through the glomeruli
What influences the Glomerular Filtration Rate (GFR)?
- Net filtration pressure
- Glomerular surface area
- Permeability of the glomerular membrane (podocytes and slit sizes)
Tubuloglomerular feedback
What is the effect of the afferent arteriole constricting on GFR?
Lower GFR
What is the effect of the efferent arteriole constricting on GFR?
Higher GFR
What is the effect of the efferent arteriole dilating on GFR?
Lower GFR
What is the effect of the afferent arteriole dilating on GFR?
Higher GFR
What is the role of the macula densa in regulating GFR?
It senses flow through the tubule and adjusts the afferent arteriole resistance
Extrinsinc control on GFR
-Sympathetic control
-Long-term regulation of arterial BP
-Input to afferent arterioles
-Lower blood pressure means lower GFR and retention of fluids
How do changes to plasma-colloid osmotic pressure affect GFR?
-Loss of proteins from blood to repair sites (in the case of severe injury) lowers osmotic pressure, and GRF increases
-Diarrhea results in loss of fluids, increasing osmotic pressure, and GRF decreases
How do changes to bowman’s capsule hydrostatic pressure affect GFR?
-Obstructions such as kidney stones elevate capsular hydrostatic pressure
-Decreases filtration
What substance is used to measure GFR?
Inulin
What is the main process of sodium reabsorption in the kidneys?
Active transport via Na+/K+ ATPase pump
Why is paracellular transport for reabsorption preferable?
-Because it is only passive transport through diffusion
Where in the nephron is the majority of sodium reabsorbed?
Proximal tubule (67%)
How is sodium reabsorbed?
-sodium potassium pump in tubular cell membrane lowers sodium concentration inside cell, and releases sodium into interstitial fluid
-sodium diffuses down its concentration gradient into the cell from lumen
-sodium in interstitial fluid diffuses into peritubular capillary
which other substances follow the reabsorption of sodium?
-water follows osmotic gradient created (into capillaries)
-Cl follows electrical gradient of positive charges (into capillaries)
-glucose by carriers
glucose reabsorption (in proximal tubule)
-sodium pulls glucose with SGLT protein into the cell
-glucose diffuses out using the GLUT protein
tubular maximum
-point where all the glucose carriers are full
-excess glucose stays in the tubules and is lost in the urine
renal threshold
-blood glucose level where the carriers are full and glucose is seen in the urine
(as seen in diabetes mellitus)
What is the function of the loop of Henle?
Creates an osmotic gradient critical for urine concentration
What role does the distal and collecting tubules play in sodium reabsorption?
Variable reabsorption subject to hormonal control (8%)
True or False: The majority of plasma entering the kidney is excreted as urine.
False
Fill in the blank: The _______ is the site of glomerular filtration.
Bowman’s capsule
What is the primary method of urea reabsorption? How much is reabsorbed
Passive process to equilibrium
50%
What percentage of Na+ is typically reabsorbed in the kidneys?
99.9%
What percentage of Cl and water is reabsorbed in kidneys?
99%
what percentage of glucose is reabsorbed under normal circumstances
100%
what percentage of potassium is secreted and reabsorbed in kidneys?
86%
What hormone controls Na+/K+ ATPase pumps?
Aldosterone
When is aldosterone release?
when blood volume is low
What is the effect of high aldosterone levels on Na+ reabsorption?
Increases Na+ reabsorption and water reabsorption, leading to decreased urine volume
What is the role of Atrial Natriuretic Peptide (ANP) in kidney function?
Antagonist to aldosterone that inhibits Na+ reabsorption by inactivating the Na+/K+ pump
What triggers the release of ANP?
higher blood pressure/volume, large quantities of sodium, stretch of the atria (increase blood volume)
Which things are secreted?
potassium, hydrogen (acid-base balance), and large organics (alcohol)
What is the primary site of water reabsorption controlled by ADH? What function does this serve?
Collecting ducts (medulla region)
-Concentrates the urine by pulling the water out
What is the counter-current mechanism in the Loop of Henle?
Creates a zone of high solute concentration in renal medulla by selective reabsorption of salt and urea
Characteristics of descending loop of Henle
-Permeable to water
-Impermeable to salts
-Filtrate becomes more concentrated
Characteristics of ascending loop of Henle
-Permeable to salts (actively reabsorbs NaCl)
-Impermeable to water
-Filtrate becomes less concentrated
What is the role of the vasa recta in renal physiology?
-Maintains osmotic gradient in blood supply
-Follows the loop of Henle
What does ADH stand for?
Anti-diuretic hormone
What is the effect of low ADH levels on urine?
-Due to water loading
-Makes collecting ducts impermeable to water, resulting in dilute urine
What is the effect of high ADH levels on urine?
-Due to high blood osmolarity and dehydration
-makes collecting ducts permeable to water
-concentrates urine
Effects of dehydration on ADH, aldosterone, ANP. List some behavioural mechanisms.
increases in ADH, aldosterone, water reabsorption
decreases in ANP, urine
Behavioural: thirst, low sodium stimulates salt appetite, avoidance behaviours
Effects of water loading on ADH, aldosterone, ANP.
increases in ANP, urine volume
decreases in ADH, aldosterone, water reabsorption
What’s happening in the proximal tubule?
-67% of Na, Cl, and water reabsorption.
-100% of glucose and amino acids reabsorbed
-K is secreted and reabsorbed (small amount)
-Variable H secretion
-Organic ion secretion
-Phosphate and electrolytes reabsorbed
-Urea reabsorption
What’s happening in the distal tubule?
-Variable Na and water reabsorption (aldosterone and ANP control)
-Variable K secretion/reabsorption (aldosterone control)
-Variable H secretion (depends on acid-base balance)
What’s happening in the collecting ducts?
-Variable water reabsorption (ADH control)
-Variable H secretion
-Variable urea reabsorption (related to loop of Henle)
Fill in the blank: Excretion = filtration – ______ + secretion.
reabsorption
What is the formula for renal clearance (RC)?
RC = UV/P
U = concentration (mg/ml) of the substance in urine
V = flow rate of urine formation (ml/min aka GFR)
P = concentration of the same substance in plasma
What is renal clearance?
-Rate at which a solute disappears from the body
-Opposite of reabsorption
-Inulin clearance is equal to GFR
What is the clearance rate for urea if 50% is reabsorbed?
50 mL/min
What is the typical renal clearance for penicillin if more penicillin is excreted than was filtered?
150 mL/min
What is the micturition reflex?
Autonomic control of sphincters and detrusor muscle for urination
What can override the micturition reflex?
CNS input
Bladder and sphincters during filling vs micturition
Filling: bladder (detrusor) muscle is relaxed, sphincters are contracted
Micturition: stretch receptors increase their firing, detrusor muscle contracts, sphincters relax
What are common causes of renal failure?
Acute: infections, toxic agents, inappropriate immune responses, obstruction of urine flow, insufficient renal blood supply
Chronic: hypertension, diabetes, chronic exposure to toxins/drugs
What happens during renal failure?
-Build up of wastes to toxic levels
-Loss of calcium
-Na and K imbalance (affects nerve and muscle)
-Loss of proteins
-Loss of RBCs
-Low blood pressure (decreased renin)
What are the effects of renal failure on edema?
-Loss of proteins makes water stay in tissues and creates edema
What are the effects of renal failure on calcium levels?
Loss of calcium leads to osteoporosis
Kidney stones
-crystallization of minerals in either the kidney, ureters, or bladder
-composed of calcium
-dehydration causes
What is the normal pH range for the blood? tissue?
7.35–7.45
7.38 - 7.42
What can abnormal pH levels affect?
Tertiary structure of proteins and the nervous system
What is the primary role of buffers in acid-base balance?
Combines with or releases H+ to regulate hydrogen ion and pH balance in the body
What are some buffers?
-HCO3
-Proteins, hemoglobin, phosphates in cells
-Phosphates and ammonia in urine
What happens to neurons during acidosis?
Neurons become less excitable and CNS depression occurs
What happens to neurons with alkalosis?
they become hyperexcitable
What is metabolic acidosis caused by?
- Metabolic organic acid production
- Lactic acid (exercise)
- Ketoacids (diabetes)
- Diarrhea
- Organic acids intake (diet)
What is respiratory acidosis primarily due to?
Production of CO2
What conditions can cause metabolic alkalosis?
- Vomiting
- Dietary sources of bases (e.g., Tums)
- Pyloric stenosis (where bicarbonate builds up)
What triggers respiratory alkalosis?
Hyperventilation (high altitude)
Which mechanism of acid-base balance is the fastest?
Buffers (response within seconds)
What is the role of renal regulation in acid-base balance? Is it fast or slow?
-Directly excreting or reabsorbing H+
-Slowest (within hours)
How does ventilation respond to pH disturbances?
-By changing the rate and depth of breathing
-Responds within minutes
-Reflex pathway can correct for metabolic acidosis (increased breathing)
What is the role of type A intercalated cells?
-Secrete H+ and reabsorb bicarbonate
-Functions in acidosis
What is the function of type B intercalated cells?
-Secrete bicarbonate and reabsorb H+
-Functions in alkalosis
What is the correction for acidosis in the body?
1) buffers bind to H+
2) breathing increases to decrease CO2 and H via carbonic acid
3) kidneys excrete H and keep bicarbonate
What occurs in the body to correct alkalosis?
1) Buffers release H+
2) breathing slows down, retains CO2 and H+
3) kidneys retain H+ and secretes bicarbonates
What are the three mechanisms of acid-base balance?
- Buffers
- Ventilation
- Renal regulation
What is the impact of abnormal pH on proteins?
It can alter the tertiary structure of proteins
True or False: Acidosis makes the nervous system hyperexcitable.
False
Fill in the blank: The plasma pH is maintained by _______.
[buffers, ventilation, renal regulation]
What is the primary function of hemoglobin in acid-base balance?
Acts as a buffer
How does the body raise pH during acidosis?
By increasing breathing to decrease CO2 and H+
What is the role of chemoreceptors in respiratory compensation?
Monitor plasma PCO2 and H+ (pH)
What is the relationship between H2O, CO2, and H2CO3 in acid-base balance?
H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-
