Urinary System (Pt 2)

Ch 24

Filtrate, tubular fluid, and urine flow (pt 1)

• Filtrate

• Blood flows through glomerulus

• Both water and solutes filtered from blood plasma

• Moves across wall of glomerular capillaries and into capsular space

• Forms filtrate

The first step of urine formation is filtration

LAYERS OF THE FILTRATION BARRIER

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

Overview of urine formation (pt 1)

• Urine formed through 3 interrelated processes

• Filtration, reabsorption, and secretion

• Steps of urine formation:

1. Glomerular filtration

• 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

Substances filtered by filtration membrane

Filtration is driven by pressure differences

GBHP = glomerular blood hydrostatic pressure

• Blood pressure w/in the 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)

Pressures associated with glomerular filtration

• Determining net filtration pressure

• If pressures promoting filtration are greater than pressures opposing

  • Difference is net filtration pressure (NFP)

    • HP_g - (OP_g + HP_c) = NFP

    • 60 mm Hg - (32 mm Hg + 18 mm Hg) = NFP

    • 60 mm - 50 mm Hg = 10 mm Hg

Glomerular filtration rate (GFR) and regulation of filtration

• Glomerular filtration rate = 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 is eliminated by the kidneys

• A substance w/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 inulin is equal to GFR. Based on the values below, calculate GFR.

Inulin concentration in urine = 50 mg/mL

Inulin concentration in blood plasma = 1 mg/mL

Rate of urine formation = 2 mL/min

A: 50 mL/min

B: 25 mL/min

C: 200 mL/min

D: 100 mL/min

D: 100 mL/min

Filtrate, tubular fluid, and urine flow (pt 2)

• Tubular fluid

• New name for filtrate when enters PCT

• Flows through

1. PCT

2. Nephron loop

3. DCT

4. Enters collecting tubules

5. Empties into collecting ducts

6. Enters papillary duct w/in renal papilla; now called urine

Overview of urine formation (pt 2)

• Steps of urine formation (continued)

2. Tubular reabsorption

• Movement of components w/in tubular fluid

• Move by diffusion, osmosis, or active diffusion

• Move from lumen of tubules and collecting ducts across walls

• Return to blood w/in 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⁺ reabsorbed by primary active transport

• Glucose, amino acids, proteins, vitamins reabsorbed by secondary active transport

• HCO₃^-, Ca²⁺, Mg²⁺, PO₄^3-, K⁺ also actively reabsorbed

• Water and other ions passively reabsorbed by osmosis

Ascending and descending loops:

• Majority of remaining water, Na⁺, Cl^- and K⁺ is reabsorbed

• Opposing permeability: descending loop is permeable to water, ascending loop is permeable to solutes

Overview of urine formation (pt 3)

• Steps of urine formation (continued)

3. Tubular secretion

• Movement of solutes, usually by active transport

• Move out of blood w/in peritubular and vasa recta capillaries

• Move into tubular fluid

• Materials moved selectively into tubules to be excreted

Overview of the processes of urine formation

Filtrate, tubular fluid, and urine flow (pt 3)

• Urine

• Enters papillary duct located w/in 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 transport processes (pt 1)

• Overview of structures and conditions that influence reabsorption and secretion

• Simple epithelium of tubule wall = transport barrier

• Paracellular transport

  • Movement of substances between epithelial cells

• Transcellular transport

  • Movement of substances across epithelial cells

  • Must cross luminal membrane in contact w/fluid

  • Must cross basolateral membrane on basement membrane

  • Order depends on whether being reabsorbed or secreted

Overview of transport processes (pt 2)

• Transport proteins embedded w/in luminal and basolateral membranes

  • Ctrl 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

Convoluted tubules and peritubular capillaries

Transport maximum and renal threshold

• Transport maximum (T_m)

• 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 all reabsorbed

    • If greater than 375 mg/min, excess glucose excreted in urine

• Renal threshold

• Max plasma concentration of a substance that can be transported in the blood w/o appearing in the urine

• Renal threshold for glucose = 180 mg/dl

If an ion moves between 2 renal tubule cells to enter the PCT, it is using

A: Autocrine transport

B: Transcellular transport

C: Paracellular transport

D: Symport

C: Paracellular transport

If transport maximum for a molecule is exceeded, that molecule will

A: Be excreted into the urine

B: Be broken down

C: Be absorbed into the Vasa Recta

D: Be kept in tubule cells

A: Be excreted into the urine

Substances eliminated as waste products (pt 1)

• Elimination of nitrogenous waste

• Nitrogenous waste: metabolic waste containing nitrogen

• Main nitrogenous waste products

  • 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 liver

    • Both reabsorbed and secreted

  • Creatinine, produced from creatinine metabolism in muscle

    • Only secreted

Substances eliminated as waste products (pt 2)

• Elimination of drugs and bioactive substances

• Most secretion occurring in PCT

• Certain drugs

  • Ex. penicillin, sulfonamides, aspirin

• Other metabolic wastes

  • Ex. urobilin, hormone metabolites

• Some hormones

  • Ex. human chorionic gonadotropin, epinephrine

Substances eliminated as waste products (pt 3)

• 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 w/in tubular fluid until it reaches collecting duct

• Urea “cycled” between collecting tubule and nephron loop

Ion imbalance can have negative effects on the body (pt 1)

Hyponatremia

• Low plasma Na⁺

• Renal disease, congestive heart failure, Addison’s disease

• Symptoms are all CNS dysfunction

Hypernatremia

• High plasma Na⁺

• Dehydration, vomiting, diarrhea

• Symptoms are all CNS dysfunction

Ion imbalance can have negative effects on the body (pt 2)

Hypokalemia

• Low plasma K⁺

• Vomiting, diarrhea, Cushing’s disease

• Muscle weakness

Hyperkalemia

• High plasma K⁺

• Renal failure, Addison’s disease

• Muscle fatigue, heart abnormalities

Ion imbalance can have negative effects on the body (pt 3)

Hypocalcemia

• Low plasma Ca²⁺

• Muscle stiffness, spasms

• Hypotension, heart failure, arrhythmia

Hypercalcemia

• High plasma Ca²⁺

• Frequent urination, nausea, vomiting

• Muscle weakness, heart abnormalities