A&P Term 2 - Chapter 6: Urinary System

Brief function of the kidneys

Filtration of blood, and urine production

Brief function of the ureters

Transportation of urine from the kidneys to the urinary bladder

Brief function of the urinary bladder

Temporary storage reservoir for urine

Brief function of the urethra

Transportation of urine out of the body

Which gland sits atop each kidney?

Adrenal gland

Functions of the kidneys

* Waste excretion
* Blood ion, pH, and osmolarity regulation
* Blood volume and pressure regulation
* Hormone production (ex. erythropoietin)
* Blood glucose regulation

Word for blood glucose regulation (specifically, within the kidneys)

Gluconeogenesis

Explain the products which are subject to waste excretion of nitrogenous wastes, particularly within the kidneys.

* Urea and ammonia (amino acid)
* Creatinine (creatine phosphate)
* Uric acid (nucleic acids)
* Urobilin (hemoglobin)
* Drugs and environmental toxins

Explain why the kidneys are not at the same level/height, retroperitoneally

The right kidney is lower than the left, because it is crowded by the liver.

How many layers of supportive tissue make up the kidneys?

Three layers

Explain the layers of supportive tissue surrounding the kidneys

* Renal fascia: Outer layer of dense fibrous connective tissue that anchors kidney and adrenal gland to surrounding structures.
* Perirenal fat capsule: Surrounds and cushions the kidney.
* Fibrous capsule: Transparent capsule that protects from surrounding infections and trauma.

Explain the three distinct regions of the kidney

* Renal cortex (outermost)
* Renal medulla (contains renal pyramids)
* Renal pelvis (funnel-shaped tube continuous with the ureter).
* Major and minor calyces collect urine and empty it into the renal pelvis.

How much (%) of resting cardiac output is delivered to the kidney?

20% - 25%

What does filtrate (urine formation) contain?

Almost everything found in plasma (except proteins)

* Processed to produce urine, which contains unneeded substances, such as excess salts, and metabolic wastes.

Explain arterial blood flow to the kidney

* Blood enters each kidney through one renal artery
* Blood flows to the cortex through progressively smaller arteries.
* Afferent arterioles give rise to microscopic vasculature that is the key element of kidney function.

Renal nerve plexus

Regulates renal blood flow by adjusting the diameter of renal arterioles, and influencing urine formation by the nephron.

Briefly explain what a nephron is

The structural and functional units that form urine

Around how many nephrons are there per kidney?

Around 1 million

Explain the main function of the renal corpuscle, and components that make it up:

Renal corpuscle: Plasma filtration.

* Glomerulus: Tuft of fenestrated capillaries.
* Glomerular (Bowman’s capsule): Double-walled epithelial cup surrounding the glomerulus.

Explain the main function of the renal tubules, and the order or components that make it up:

Renal tubules: Filtrate regulation

* Proximal convoluted tubule → nephron loop (loop of Henle) → distal convoluted tubule → collecting duct (collects filtrate from several nephron distal convoluted tubules).

Glomerulus:

Fenestrated capillaries increase permeability, allowing a solute-rich, but protein-free filtrate.

Layers of the glomerular (Bowman’s) capsule:

* Parietal layer: Contributes to capsular structure.
* Visceral layer: Associated with the glomerular capillaries, consisting of podocytes.

Podocytes:

Within the visceral layer of the Bowman’s capsule.

* Allow filtrate to pass into the space within the glomerular (Bowman’s) capsule.

Proximal convoluted tubule

The wall has dense microvilli to increase surface area for absorption from, and secretion to, the urine.

Nephron loop (loop of Henle):

* Thin descending limb
* Ascending limb has both thin (simple squamous epithelium) and thick (simple cuboidal epithelium) segments.

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Distal convoluted tubule

Similar to the proximal convoluted tubule, except the cells almost entirely lack microvilli.

Types of cells making up collecting ducts

* Principal cells: With Na+ and K+ channels and aquaporins to reabsorb water.
* Intercalated cells: Control pH by secreting or absorbing H+ or HCO3-.

Function of collecting ducts, and direction

Collect filtrate from many nephrons, and extend through the renal pyramid to the renal papilla, where they empty into the minor calyx.

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What gives pyramids their striped appearance?

Collecting ducts

Two classes of nephrons, and the amount (%) they make up

* Cortical nephrons (85%)
* Juxtamedullary nephrons (15%)

Cortical nephrons:

Located almost entirely within the cortex, and have a short nephron loop

Juxtamedullary nephrons:

Located near the cortex-medulla junction, and have a long nephron loop

The renal tubule is closely associated with which two capillary beds?

* Glomerular
* Peritubular

Explain the glomerulus in context of nephron capillary beds:

Specialized for filtration.

* Fed and drained by afferent and efferent arterioles.
* Maintains high pressure in the glomerulus for filtration.

Explain what efferent arterioles divide to form, in context of nephron capillary beds:

* Peritubular capillaries:
* Surround cortical tubule.
* Low-pressure, porous capillaries that surround adjacent renal tubules to absorb solutes and water from tubule cells.
* Vasa recta:
* Surround medulla tubule.
* Arise from the efferent arterioles near the juxtamedullary nephrons, and run parallel to the longest nephron loops.

Juxtaglomerular complex

A structural arrangement between the afferent arteriole and distal convoluted tubule.

Main cells which make up the juxtaglomerular complex:

* Macula densa cells (in distal convoluted tubule).
* Juxtaglomerular cells (in walls of afferent arterioles).

Macula densa cells:

* In distal convoluted tubule.
* Use chemoreceptors to monitor sodium content of filtrate entering DCT.
* Release paracrine to juxtaglomerular cells.

Juxtaglomerular cells

* In walls of afferent arterioles.
* Act as mechanoreceptors that monitor blood pressure, and house secretory vesicles that contain the enzyme, renin.

Three processes for urine formation, and adjustment of blood composition:

* Glomerular filtration
* Produces cell- and protein-free filtrate.
* Tubular reabsorption
* Selectively returns 99% of substances from filtrate to blood in renal tubules, and collecting ducts.
* Tubular secretion
* Selectively moves substances from blood to filtrate in renal tubules, and collecting ducts.

How does glomerular filtration work?

* Passive (doesn’t require metabolic energy).
* Nonselective process.
* Hydrostatic pressure forces fluids through glomerular membrane.
* No reabsorption occurs within the capillaries of the glomerulus.

Explain the filtration membrane for glomerular filtration:

Filtration membrane is a porous membrane which allows free passage of water and solutes (smaller than plasma proteins).

For filtration membranes, what portion of glomerular capillaries prevents filtration of blood cells?

Fenestrated endothelium

Explain the basement membrane within the filtration membrane

Negatively charged glycoproteins inhibit filtration of negatively charged molecules (ex. plasma proteins).

Explain the role of foot processes of podocytes within filtration membranes

They form filtration slits with slit membrane.

* Prevents filtration of medium-sized proteins.

Which filtration membrane mechanism filters blood cells?

The fenestrated endothelium

Which filtration membrane mechanism filters negatively charged molecules?

The basement membrane

Which filtration membrane mechanism filters medium-sized proteins?

Food processes of podocytes

Glomerular hydrostatic pressure (HPgc)

* Essentially glomerular blood pressure.
* Primary force pushing water and solutes out of blood across the filtration membrane.

Capsular hydrostatic pressure (HPcs) and blood colloid osmotic pressure (OPgc)

Both forces oppose HPgc and try to move fluid back into the glomerulus.

In relation to the glomerulus and Bowman’s capsule, what is the direction of movement?

Glomerulus → Bowman’s capsule.

* Positive net filtration pressure (NFP)

Glomerular filtration rate (GFR)

Volume of filtrate formed each minute by all the glomeruli of the kidneys combined

Glomerular filtration rate is proportional to which three factors?

* Net filtration pressure
* Total surface area available for filtration
* Filtration membrane permeability

Which forms of homeostasis within the body can be maintained due to the regulation of glomerular filtration?

* Glomerular filtration rate
* Arterial blood pressure

Explain intrinsic controls (renal autoregulation) for the regulation of glomerular filtration:

* Maintains constant renal blood flow and glomerular filtration rate with typical fluctuations in blood pressure.
* Myogenic mechanism and tubuloglomerular feedback.

Intrinsic controls (neural and hormonal regulation) for the regulation of glomerular filtration

Indirectly regular glomerular filtration rate by maintaining systemic blood pressure through nervous and endocrine mechanisms.

For the intrinsic regulation’s myogenic mechanism, which type of smooth muscle responds to changes in blood pressure?

Afferent arteriole smooth muscle

For the myogenic mechanism (intrinsic regulation), what does increased local blood pressure cause?

Vascular smooth muscle stretches → contraction → afferent arteriole constriction

a) In myogenic mechanisms (intrinsic regulation), what does vasoconstriction reduce?

b) What happens afterwards?

a) Vasoconstriction reduces blood flow into the nephron, and protects glomeruli from damaging high blood pressure.

b) Blood flow is redistributed to other nephrons that are not as constricted.

a) In tubuloglomerular feedback (intrinsic regulation), what do macula densa cells respond to?

b) What does increased systemic blood pressure cause?

c) What does decreased nitric oxide cause?

a) Filtrate sodium concentration.

b) Increased systemic BP → glomerular filtration rate → increased flow rate in renal tubules → decreased sodium reabsorption → increased Na+ and Cl- in filtrate.

c) Decreased nitric oxide → constriction of afferent arteriole → decreased net filtration pressure and glomerular filtration rate.

* This allows for more time in tubules for sodium reabsorption.

What do macula densa cells inhibit the release of (tubuloglomerular feedback - intrinsic regulation)

Nitric oxide (vasodilator) from cells in the juxtaglomerular apparatus.

During periods of stress (ex. blood loss, exercise), _________________ (sympathetic/parasympathetic) activity increases:

Sympathetic

When the sympathetic nervous system (extrinsic regulation) activates alpha-1-adrenergic receptors in the afferent arteriole smooth muscle, how does this ultimately effect glomerular filtration rate?

What does this then cause?

It causes vasoconstriction which decreases glomerular filtration rate.

* Decreased urine output to increase blood volume.
* Redirects blood flow to other organs.

Hormonal mechanisms (extrinsic regulation):

Renin-angiotensin-aldosterone:

Sequence of hormones, starting with renin from the kidneys.

* Released in response to decreased BP or blood volume.
* Angiotensin II: potent vasoconstrictor.
* Constricts arterioles throughout the body, increasing systemic blood pressure.
* Afferent arteriole constriction → decreased glomerular filtration rate (decreasing renal blood flow) → decreased urine output to increase blood volume.
* Aldosterone: Increased Na+ and H2O reabsorption → increased blood volume.

Hormonal mechanisms (extrinsic regulation):

Atrial natriuretic peptide (ANP)

Released by atrial cells in response to stretch (ex. increased venous return).

* Increased capillary surface area in glomerulus → increased glomerular filtration rate → increased urine output to decrease blood volume).

True or False:

Different regions of the nephron tubules and collecting ducts secrete or absorb different molecules.

True.

What does reabsorption and secretion depend on?

Specific transport proteins and channels in the epithelial cell membranes at different parts of the nephron.

Formula to calculate amount (of urine) excreted:

Amount excreted: Amount filtered + Amount secreted - Amount reabsorbed

Tubular reabsorption

Selective transepithelial process that begins when the filtrate enters the proximal convoluted tubule

In _____________ (healthy/unhealthy) kidneys, nearly all organic nutrients (ex. glucose and amino acids) are reabsorbed, while reabsorption of water and ions is continually regulated and adjusted.

Healthy

Which two mechanisms does the tubular reabsorption process include?

* Transcellular or paracellular routes.
* Active and passive tubular reabsorption.

Two main routes of tubular reabsorption:

* Transcellular route (through cells)
* Paracellular route (between cells)

Transcellular route of tubular reabsorption

* Transport across the apical membrane
* Diffusion through cytosol
* Transport across basolateral membrane

Paracellular route of tubular reabsorption

* Solute moves between tubule cells through leaky tight junctions
* Particularly in proximal convoluted tubule: water, Ca2+, Mg2+, K+ and some Na+

Both routes of tubular reabsorption involve movement through:

Interstitial fluid (and into capillaries)

Aquaporins

Water channels

Relation of aquaporins and proximal convoluted tubule

Aquaporins are always present in the proximal convoluted tubule.

* Obligatory water reabsorption

Relation of aquaporins and the collecting duct

Inactive aquaporins are stored in the cytoplasm, and are inserted into the plasma membrane in the presence of the antidiuretic hormone (ADH)

* Facultative water reabsorption.

Solute concentration in filtrate increases as _______ is reabsorbed

Water

What does the solute concentration in filtrate increasing as water is reabsorbed create (passive tubular reabsorption of solutes)?

Concentration gradients for solutes to move into tubule cell and peritubular capillaries.

a) What do fat-soluble substances, some ions, and urea follow into peritubular capillaries down their concentration gradients (passive tubular reabsorption of solutes)?

b) Explain what effect lipid-soluble drugs and environmental pollutants could have on this process.

a) Water.

b) These drugs/pollutants can be reabsorbed, even though it isn’t desirable.

Explain transport maximums (in terms of reabsorption limitations):

These maximums exist for most substances which are reabsorbed via transport proteins.

* When the concentration of a solute in the urine exceeds the saturation point of the transporters, excess is lost in the urine.

Which tubule is most active in reabsorption?

The proximal convoluted tubule

* Nearly all glucose, amino acids, and vitamins.

What is Na+ and water permeability regulated by in the distal convoluted tubule and collecting duct?

Hormones.

* Aldosterone
* ADH
* Atrial natriuretic peptide

These hormones all fine-tune the final urine concentration.

a) Where is tubular secretion most active?

b) Despite this, where does it also occur?

a) In the proximal convoluted tubule.

b) In the collecting ducts and distal convoluted tubules.

* Functions like reabsorption in reverse.

Why does tubular secretion dispose of unwanted solutes, and eliminate unwanted, reabsorbed solutes?

* Such as protein-bound substances (ex. drugs and metabolites), urea, uric acid, excess K+ and ions (ex. H+)

To control blood pH

a) How does dehydration affect blood osmolarity?

b) Explain the urine concentration this produces:

a) Dehydration increases blood osmolarity

b) Produces small volume of concentrated urine

a) How does overhydration affect blood osmolarity?

b) Explain the urine concentration this produces:

a) Decreases blood osmolarity

b) Produces a large volume of dilute urine

Explain urine concentration and volume regulation via the countercurrent mechanism:

* Fluid flows in opposite directions in two adjacent segments of the same tube, with a hairpin turn.
* Countercurrent multiplier: Juxtamedullary nephron loop.
* Countercurrent exchanger: Vasa recta.

Explain the permeability of water and solutes in the:

* Ascending limb
* Descending limb

* Ascending limb:
* H2O cannot leave but NaCl can.
* Thin segment: Passive movement of Na+ and Cl-
* Thick segment: Active transporters move Na+ and Cl- from the filtrate to interstitial fluid (reabsorb).
* Active reabsorption of Na+ and Cl- produces an osmotic gradient to concentrate the urine - effects are magnified by the countercurrent mechanism.
* Descending limb:
* Water can leave but NaCl cannot.

Explain the countercurrent multiplier and how H2O in the descending limb can establish a positive feedback cycle:

Gradients from movement of ions in the ascending limb, along with this water in the descending limb can establish this positive feedback cycle.

What can the vasa recta do while maintaining the countercurrent multiplier’s osmotic gradient?

Absorb H2O and solutes into the circulatory system

a) Which gradient allows for the regulation of urine osmolarity?

b) What does this ultimately allow for?

a) The regulation of urine osmolarity.

b) For conserving water during dehydration.

Explain the relation (and steps) between overhydration and a large volume of dilute urine:

Overhydration: Decreased anti-diuretic hormone → decreased water reabsorption → large volume of dilute urine.

Explain the relation (and steps) between dehydration and a small volume of dilute urine:

Dehydration: Increased anti-diuretic hormone → decreased water reabsorption → small volume of concentrated uring.

Diuretic

Chemical that increases urine output

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How do diuretic increase urine output?

* Inhibiting anti-diuretic hormone release (decreased water reabsorption)
* Ex. alcohol
* Inhibiting Na+ and obligatory water reabsorption
* Ex. caffeine, hypertension drugs

Osmotic diuretic

Substance that isn’t reabsorbed, and carries water with it

Diabetes mellitus

An osmotic diuretic.

* High glucose concentration.
* Remains in urine.
* Can act as an osmotic diuretic which pulls water from the body.

Urobilin (urochrome)

Pigment from hemoglobin breakdown that gives urine its yellow colour