A&PII Ch.24 Urinary System pt. 2Urine Formation Flashcards
How is Filtrate formed?
- Blood flows through glomerulus
- Blood, water, and solutes filtered from blood plasma
- Moves across wall of glomerular capillaries and into capsular space
- Forms filtrate
Glomerular Filtration
creates a plasma-like filtrate of the blood
Filtration and Layers of the Filtration barrier
- the first step of urine formation is filtration
1) Capillary Endothelium - Fenestrated; very permeable
- allows passage of anything smaller than a cell
2) Basement Membrane - fused; not as permeable
- blocks all but small proteins
3) Podocytes of Glomerular Capsule - pedicels create filtration slits
- prevents passage of most molecules
Urine is formed through 3 interrelated processes..
filtration, reabsorption, and secretion
Glomerular Filtration
- First step of urine formation
- in glomerular capillaries
- separates some water and dissolved solutes from blood plasma
- water and solutes enter capsular space of renal corpuscle
-due to pressure differences across filtration membrane - Separated fluid is called filtrate
Filtration Membrane
- endothelium (blocks formed elements)
- basement membrane (blocks large proteins)
- filtration slits of visceral layer (block small proteins)
- filtrate includes water, glucose, amino acids, ions, urea, some hormones, vitamins B and C, ketones, and very small amounts of protein
Filtration is driven by pressure differences
- GBHP= glomerular blood hydrostatic pressure
-blood pressure within glomerulus
-drives filtration - CHP= capsular hydrostatic pressure
-Hydrostatic pressure inside glomerular capsule
-opposes filtration - BCOP= blood colloid osmotic pressure
-osmotic pull of proteins not being filtered
-opposes filtration - NFP= net filtration pressure
NFP= GBHP- (CHP + BCOP)
Determining net filtration pressure
*If pressures promoting filtration are greater than pressure opposing
- difference is net filtration pressure (NFP)
HPg-(OPg+HPc)=NFP
60mm Hg - (32 mm Hg + 18 mm Hg) = NFP
60 mm - 50 mm Hg= 10 mm Hg
Glomerular Filtration Rate (GFR) and Regulation of Filtration
- GFR is the total volume of filtrate formed by all of the glomeruli of both kidneys each minute
- the magnitude of NFP is directly proportional to GFR
Renal clearance and Glomerular Filtration
- Renal clearance is a measurement of how quickly the kidneys remove a substance from plasma and excrete it in urine.
- Renal clearance is used to determine how quickly a drug/chemical I’d eliminated by the kidneys
- A substance with a high renal clearance is quickly removed from the blood
- The renal clearance of a substance that is neither reabsorbed nor secreted by the tubules is equal to the GFR
Approximating GFR using Renal Clearance
C= rate of renal clearance, typically in mL/min
U= concentration of substance in the urine
V= rate of urine formation
P= concentration of substance in the blood plasma
Assumptions for substance to approximate GFR:
* It must freely pass through the filtration membrane
* It must neither be reabsorbed from nor secreted into the filtrate by the renal tubules
The renal clearance rate of insulin is equal to GFR. Based on the values below, calculate GFR.
Insulin concentration in urine= 50 mg/mL
Insulin concentration in blood plasma= 1 mg/mL
Rate of urine formation= 2 mL/min
100 mL/min meaning the renal clearance for insulin is 100mL/min and clearance rate for drug is 100 mL
Tubular Fluid
- this is the new name for filtrate when it enters the PCT
- flows through
PCT, nephron loop, DCT, enters collecting tubules, empties into collecting ducts, enters papillary duct within renal papilla; and is then called urine
Tubular Reabsorption
- the second step of urine formation
- movement of components within tubular fluid
- move by diffusion, osmosis, or active transport
- move from lumen of tubules and collecting ducts across walls
- return to blood within peritubular capillaries and vasa recta
- all vital solutes and most water reabsorbed
- excess solutes, waste products, some water remaining in tubular fluid
Tubular reabsorption in the collecting ducts and loops
- PCT:
-Na+ is reabsorbed by primary transport
-Glucose, AA, Proteins, and vitamins are reabsorbed by secondary active transport
-HCO3-, Ca2+, Mg2+, PO43-, K+ also actively reabsorbed
-water and other ions passively reabsorbed by osmosis - Ascending and descending loops:
-majority of remaining water, Na+, Cl-, and K+ reabsorbed
-opposing permeability: descending loop is permeable to water, ascending loop is permeable to solutes
Tubular Secretion
- 3rd step of urine formation
- movement of solutes, usually by active transport
- move out of blood within peritubular and vasa recta capillaries
- move into tubular fluid
- materials moved selectively into tubules to be excreted
Filtrate, Tubular Fluid, and Urine Flow
Urine enters papillary duct located within renal papilla
*minor calyx-> major calyx-> renal pelvis
* renal pelvis connects to ureter
* ureter connects to urinary bladder
- stores and excretes from body through urethra
Overview of Structures that influence reabsorption and secretion
- Simple epithelium of tubular wall= transport barrier
- Paracellular transport is the movement of substances between epithelial cells
- Transcellular transport is the movement of substances acroiss epithelial cells
-must cross luminal membrane in contact with fluid
-must cross basolateral membrane on basement membrane
-order depends on whether being reabsorbed or secreted
Overview of transport processes
- Transport proteins embedded within luminal and basolateral membranes, controls the movement of various substances
- Peritubular Capillaries (Vasa Recta)
-low hydrostatic pressure and high oncotic pressure
-facilitate reabsorption of substances through bulk flow - Most reabsorption in PCT
-aided by microvilli increasing surface area
Transport Maximum (Tm) and Renal Threshold
- Tm is the maximum rate of substance that can be reabsorbed (or secreted) across tubule epithelium per a certain time
-depends on number of transport proteins in membrane
–if no more than 375 mg/min, glucose in tubule are all reabsorbed
–if greater than 375 mg/min, excess glucose excreted in urine - Renal threshold is the max concentration of a substance that can be transported in the blood without appearing in the urine
- Renal threshold for glucose= 180 mg/dl
If an ion moves between two renal tubule cells to enter the PCT, it is using…
paracellular transport
If transport maximum for a molecule is exceeded, that molecule will..
be excreted into the urine
Substances Eliminated as waste products (Nitrogenous Waste)
- Nitrogenous waste is metabolic waste containing nitrogen
-the main nitrogen waste products are urea
–molecule produced from protein breakdown, both reabsorbed and secreted, 50% excreted in the urine, helps establish concentration gradient in the interstitial fluid
-uric acid
– produced from nucleic acid breakdown in the liver, reabsorbed and secreted
-Creatinine
–produced from creatine metabolism in muscle and is only secreted
Substances Eliminated as Waste Products
- Elimination of drugs and bioactive substances
-most secretion occurring in PCT - Certain drugs
-for example, penicillin, sulfonamides, asprin - Other metabolic wastes
-for example, urobilin, hormone metabolites - Some hormones
-for example, human chorionic gonadotropin, epinephrine
Urea recycling
- urea is a toxic chemical at high levels, but moderate amounts can help drive osmotic gradient
- help concentrating process in interstitial fluid
- urea removed from tubular fluid in collecting duct by uniporters
- diffuses back into tubular fluid in thin segment of ascending limb
- remains within tubular fluid until it reaches collecting duct
- urea “cycled” between collecting tubule and nephron loop
Sodium Ion imbalance
- Hyponatremia
-Low plasma Na+
-renal disease, congestice heart failure, Addison’s disease
-symptoms are all CNS dysfunction
*Hypernatremia
-High plasma Na+
-Dehydration, vomiting, diarrhea
-Symptoms are all CNS dysfunction
Potassium Ion imbalance
- Hypokalemia
-Low plasma K+
-Vomiting, diarrhea, Cushing’s disease
-Muscle weakness - Hyperkalemia
-High plasma K+
-Renal Failure, Addison’s disease
-Muscle fatigue, heart abnormalities
Calcium Ion imbalance
- Hypocalemia
-Low plasma Ca2+
-Muscle stiffness, spasms
-Hypotension, heart failre, arrhythmia - Hypercalemia
-High plasma Ca2+
-Frequent urination, nausea, vomiting
-Muscle weakness, heart abnormalities
Acid-base balance
- Typical blood pH is 7.4 +/- 0.05 due to constant Co2 production during cellular metabolism
- Recall that any compond that contribute H+ is and ACID and any that accepts H+ is a base
- pH represents a balance between CO2 production and loss (respiration)
-increased CO2 drives the reaction to the right, increasing H+ concentration
-decreased CO2 drives the reaction to the left, decreasing H+ concentration - blood pH is tightly regulated in the body, even small changes can be harmful
-ACIDOSIS is a blood pH level below 7.35
-ALKALOSIS is a blood pH level above 7.45
Four major buffering mechanisms
- carbonic acid/bicarbonate
- phosphate molecules
- ammonium/ ammonia
- amino acids in proteins
- each of these reactions are reversible and driven by the concentration of the reactants
Alkalosis v.s. Acidosis in the proximal collecting duct (PCT)
- Alkalosis= Na+ secretion drives the secretion HCO3- and the reabsorption of H+ in the blood
-raise blood pH - Acidosis= NA+ is reabsorbed into the PCT epithelia in exchange for protons
-lowers blood pH
Intercalated Cells in the collecting duct also regulate pH
- Type A intercalated cells: secrete HCO3- which is reabsorbed into the blood, and actively transport H+ into the lumen
- Type B intercalated cells: actively secrete H+ which is reabsorbed by the blood and secrete HCO3- into the lumen
Clinical Acid-Base Disorders Acidosis
- ACIDOSIS
-blood pH < 7.35 - Respiratory Acidosis
-inability to lose CO2 at the lungs, commonly caused by cardiac failure, and opioid overdose - Metabolic Acidosis
-overproduction of non-volatile organic acids-caused by diabetes, exercise, starvation
-kidney damage preventing proton secretion
-severe diarrhea causing excessive bicarbonate loss
Clinical Acid-Base Disorders Alkalosis
- ALKALOSIS
-blood pH > 7.45 - Respiratory Alkalosis
-Hyperventilation and excessive CO2 loss at the lungs
-Can be acute (rare) or chronic (e.g. high altitude sickness) - Metabolic Alkalosis
-Over-secretion of stomach acid or vomiting
Changes in Plasma Volume
*Key hormonal control points
-Kidney
–Renin
*Adrenal Gland
-Angiotensin II
-Aldosterone
Regulating Blood Pressure: The Renin-Angiotensin-Aldosterone System
- the renin-angiotensin-aldosterone system is a series of reactions designed to help regulate blood pressure
- when blood pressure falls (systolic; 100 mmHg or lower), the kidneys release the enzyme renin into the bloodstream
- renin splits angiotensinogen, a large protein that circulates in the bloodstream, into pieces. One piece is angiotensin I.
- angiotensin I, which is relatively inactive, is split into pieces by angiotensin-converting enzyme (ACE). One piece is angiotensin II, a hormone, which is very active
- angiotensin II causes the muscular walls of small arteries (arterioles) to constrict, increasing blood pressure. Angiotensin II also triggers the release of the hormone aldosterone from the adrenal glands and vasopressin (antidiuretic hormone) from the pituitary gland
- aldosterone and vasopressin cause the kidneys to retain sodium (salt). Aldosterone also causes the kidneys to excrete potassium. The increased sodium causes water to be retained, thus increasing blood volume and blood pressure
Renal Ptosis and Hydronephrosis
- Renal Ptosis
-inferior movement of kidney within abdominal cavity
-due to loss of adipose in elderly or individuals with anorexia nervosa
-may kink ureter, which blocks urine flow to urinary bladder
-urine backs up, results in swelling of kidney (hydronephrosis)
–can lead to renal failure
Renal Failure
- greatly diminished or absent renal functions
- often from chronic disease affecting glomerulus or small blood vessels
- from autoimmune disease, high blood pressure, diabetes
- once destroyed will not function again
- two main treatments: dialysis or kidney transplantation
Renal Calculi
- a.k.a kidney stones
- formed from crystalline minerals building up in the kidney
- Risk Factors..
-inadequate fluid intake, reduced urinary flow
-frequent urinary tract infections
-abnormal chemical or mineral levels in urine - Small stones asymptomatic
- Larger stones obstructed in kidney, renal pelvis, ureter
-severe pain along the “loin-to-groin” region - Most pass on their own if less than 4 mm in diameter
- May require lithotripsy or ureteroscopy
Urinary Tract Infections
- UTI occurs when bacteria or fungi multiply within the urinary tract
- Women more prone due to short urethra close to anus
- Often first develops in urethra, urethritis
- If spreads to bladder, cystitis
- May spread up into kidneys, pyelonephritis
- Symptoms include..
-painful urination, dysuria
-frequent urination
-pressure in pubic region
-pyelonephritis causing flank pain, back pain, nausea - Diagnosed through urinalysis
- Treated with antibiotics
Glucosuria
- Excretion of glucose in urine
- Plasma glucose concentration above 300 mg/dL
- Glucose acts as an osmotic diuretic
- Pulls water into tubular fluid
- Causes loss of fluid in urine
- Classic symptom of diabetes, along with frequent urination and thirst
Secretion of renin activates angiotensin II, which causes ___________.
vasoconstriction
Loss of peritoneum adipose tissue and weakened connective tissue fibers is linked with ______ _______.
renal ptosis
