Chapter 25: The Urinary System

includes Chapter 19: Blood Pressure and Regulation

nephron

primary structural and functional unit of kidneys where urine formation occurs

* > 1 million per kidney


* where they filter blood plasma into filtrate
* renal corpuscle
* cortical or juxtamedullary

filtrate

plasma-derived fluid that renal tubules process to form urine

* blood plasma minus the proteins (free from cells so liquid electrolytes)
* filtered about 60 times a day by the kidneys
* 180 L produces about 1.5 L of urine
* less formation if sum of HPcs and OPgc > outward
* more formation if sum of HPcs and OPgc < outward

kidneys

major excretory organs that maintain that body’s internal environment by

* regulating total water volume and solute concentration (degree to which body gets rid of H2O) - water retention
* regulating ion concentrations in ECF (Na, K, Cl)
* ensuring long-term acid-base balance
* excreting metabolic wastes, toxins, and drugs through urine
* producing erythropoeitin and renin
* activating vitamin D
* carrying out glucogenesis if needed
* at rest, consumes about 25% of the oxygen used by the body

erythropoeitin

secreted by the kidneys and regulates blood pressure

renin

secreted by the granular cells of the kidneys and regulates RBC production

ureter

tube that transports urine from the kidneys to the urinary bladder

urinary bladder

very elastic structure with smooth muscle to stretch and expand that serves as a temporary storage reservoir for urine

urethra

transports urine out of the body

* sphincters

renal cortex

granular-appearing superficial region of the kidneys

renal medulla

deep to the renal cortex of the kidneys, composed of cone-shaped medullary (renal) pyramids

* broad base of pyramid faces cortex
* papilla
* renal pyramids separated by renal columns


* lobe

papilla

the tip of the renal pyramids that points internally

renal columns

separate the renal pyramids; inward extensions of cortical tissue

renal lobe

medullary pyramid and surrounding renal cortex tissue (8 per kidney)

renal pelvis

funnel-shaped tube continuous with the ureter

* fed by major calyces fed by minor calyces

minor calyces

cup-shaped areas of the kidneys that collect urine draining from the pyramidal papillae

major calyces

ares of the kidneys that collect urine from the minor calyces and empty it into the renal pelvis

urine flow

renal pyramid to minor calyx to major calyx to renal pelvis to ureter

renal corpuscle

the part of the kidney consisting of the glomerulus and glomerular capsule

glomerulus

tuft of very porous capillaries composed of fenestrated endothelium which allows for efficient filtration formation systems

* filtrate

glomerular capsule

“Bowman’s capsule”; cup-shaped hollow structure surrounding the glomerulus

* parietal layer of simple squamous epithelium
* visceral layer that clings to glomerular capillaries and has branching epithelial podocytes

podocytes

branch off visceral layer of glomerular capsule and terminate in foot processes that create filtration slits and cling to to the basement membrane

filtration slits

formed by foot processes of podocytes and allow filtrate to pass into capsular space

* control net filration (increased by increasing size of slits and vice versa)
* macrmolecules can get stick so glomerular mesangeal cells engulf them

renal tubule

\~ 3 cm long, made up of a single layer of epithelial cells with 3 regions of unique histology and function

* PCT
* nephron loop
* DCT

proximal convoluted tubule

PCT; are of the renal tubule that helps with reabsorption (65% of sodium, water, and uric acid) and secretion

* cuboidal cells with large mitochondria and dense microvilli that form brush-border, increasing surface area
* confined to cortex
* regulated by ADH, aldosterone, ANP, PTH

nephron loop

“loop of Henley”; U-shaped part of the renal tubule with 2 limbs

* descending: water can leave but solutes cannot
* ascending: water cannot leave but solutes can

descending nephron loop

thin; proximal part is continuous with the PCT and distal part is made of simple squamous epithelium

ascending nephron loop

thick; made of cuboidal or columnar cells

* thin in some nephrons

distal convoluted tubule

DCT; part of the renal tubule that is important for secretion

* confined to renal cortex
* made of cuboidal cells with few microvilli

collecting ducts

the part of the kidneys that receive filtrate from multiple nephrons and run through medullary pyramids, giving them their striped appearance, fusing together to deliver urine through papillae into minor calyces

* principal cells
* intercalated cells
* regulated by ADH, aldosterone, ANP, PTH

intercalated cells

cells that help to maintain the body’s acid-base balance

* cuboidal cells with many microvilli

principal cells

cells that help to maintain water/sodium balance

* sparse with short microvilli

cortical nephrons

account for the majority of nephrons (85%) and are mostly found in the renal cortex

juxtamedullary nephrons

long nephron loops that deeply invade the medulla and are important in the production of very concentrated urine

* ascending limbs have thick and thin segments
* vaso recta

glomerular capillaries

capillaries specialized for filtration and fed/drained by high-resistance arterioles

* afferent arterioles
* efferent arterioles

afferent arterioles

arise from cortical radiate arteries and deliver blood/fluid to glomerular capillaries

efferent arterioles

feed into either peritubular capillaries or vaso recta; blood/fluid leaves glomerular capillaries through these

peritubular capillaries

low-pressure porous capillaries adapted for absorption of water and solutes

* arise from efferent arterioles
* empty into venules
* cling to adjacent renal tubules in cortex

vaso recta

low-pressure, porous capillaries adapted for the absorption of water and solutes (very permeable to them which allows for the maintenance of urine concentration gradient)

* associated with juxtamedullary nephrons
* arise from efferent arterioles
* empty into venules
* cling to adjacent renal tubules in cortex

juxtaglomerular complex

JGC; modified distal portion of ascending limb of nephron loops and afferent (and sometime efferent) arteriole

* important in regulating the rate of filtrate formation and blood pressure


* 1 per nephron
* macula densa
* granular cells
* extraglomerular mesangial cells

macula densa

tall, closely packed cells of ascending limbs of nephron loops of JGC that contain chemoreceptors which sense NaCl content of filtrate

granular cells

juxtaglomerular/JG cells; enlarged, smooth muscle cells of arterioles that act as mechanoreceptors which sense blood pressure in afferent arterioles

* contain secretory granules containing renin enzyme

extraglomerular mesangeal cells

cells between arteriole and tubule cells that are interconnected by gap junctions that pass signals between macula densa and JG cells

glomerular filtration

the process by the kidneys that produces filtrate of metabolic waste products and unneeded substances

* passive process so does not require metabolic energy
* filtration slits
* within capillaries, pores increase permeability
* filtration membrane
* no reabsorption

tubular reabsorption

kidney process that occurs in the renal tubules and quickly reabsorbs 99% of materials filtered out, returning them to the blood (almost all organic nutrients but H2O and ions are hormonally regulated and dynamically adjusted)

* transcellular route or paracellular route


1. Na+ transport across basal lateral membrane (most abundant in cation in filtrate)
2. Na+ transported again, across basal lateral membrane through primary active transport into interstitial space by sodium-potassium ATPase pump and into blood (in peritubular capillaries)
3. enters cell through apical membrane via secondary active transport (co-trsansported) or facilitated diffusion with glucose, amino acids, ions, and vitamins


1. high blood glucose levels (from some types of diabetes) lead to more urine production because glucose is co-transported with sodium (and higher levels of glucose mean more is being transported) and water follows

tubular secretion

selective movement of substances from blood to filtrate (from peritubular capillaries, through tubule cells, out into filtrate, into blood, into urine, out of the body)

* occurs almost entirely in the PCT
* K+, H+, NH4+, and creatine (bicarbonate (HCO3) is also created and secreted here) can regulate blood pH levels by either retaining bicarb. or H+ - depending)


* most important for getting rid of substances (drugs or metabolites - anything bound to plasma proteins) to get rid of the aldosterone effect (high Ca/K levels)

fenestrations

pores within capillaries that increase permeability for glomerular filtration

hydrostatic pressure

pressure that is exerted by fluid in a space that regulates filtration membranes for glomerular filtration

net filtration

controlled by filtration slits and the filtration membrane; normally no cells can get through (due to hydrostatic pressure) but allows water and solutes smaller than the plasma proteins to pass through

* glucose, H2O, amino acids, nitrogenous waste products

filtration membrane

effects net filtration and is controlled by hydrostatic pressure

* fenestrated endothelium with fenestrations
* basement membrane anchors
* foot processes with podocytes formed in between fenestration slits

outward pressure

pressure that promotes filtrate formation

* hydrostatic pressure in glomerular capillaries; HPgc (glomerular BP)
* increased BP leads to increased filtrate formation
* driving force for pushing water and solutes out of the blood because the feeding arteriole has a large diameter and greater flow than efferent arteriole

inward pressure

pressure that inhibits filtrate formation

* hydrostatic pressure in capsular space; HPcs
* colloidal osmotic pressure in capillaries; OPgc: the pressure required to stop net movement across membrane

net filtration pressure

the sum of pressures responsible for filtrate formation

glomerular filtration rate

GFR; the amount of filtrate formed per minute by both kidneys (about 120-125 mL per minute)

* directly proportional to net filtration pressure, total surface area available for filtration, and filtration membrane permeability (very permeable)

intrinsic auto-regulation

local smooth muscle of kidneys contracts when stretched to maintain tension

* myogenic mechanism
* tubular glomerular feedback mechanism

myogenic mechanism

intrinsic auto-regulation mechanism where drop of BP leads to drop in filtration rate and vasodilation (due to drop in stretch and contraction) ending in an increase in filtration rate to compensate for low BP

tubular glomerular feedback mechanism

intrinsic auto-regulation mechanism that is flow-dependent

* directs macula densa cells towards NaCl leading to increase in GFR and flow, so there’s less time for reabsorption, leading to increased levels of NaCl in filtrate, triggering net constriction of afferent arteries causing a drop in BP and a drop in GFR

neural control

extrinsic auto-regulation of kidneys by the sympathetic nervous system

* normally, blood vessels are dilated
* unusually (low ECF or low BP), leads to a release of Ne and epi. causing systemic vasoconstriction, leading to a net increase in BP but a drop of BP in renal arterioles, causing decreased HPgc

renin angiotensin aldosterone mechanism

RAAS; the main mechanism for BP regulation through


1. direct stimulation of granular cells by the sympathetic nervous system (releasing renin)
2. stimulation by activated macula densa cells (when filtrate of NaCl is low)
3. reduced stretch of granular cells leads to a drop in BP, so granular cells of JG complex in kidney release renin causing metabolic cascade and formation of angiotensin II which stimulates release of aldosterone by the adrenal cortex and vasoconstriction of systemic arterioles to increase resistance and increase BP

aldosterone

this hormone increases sodium retention and water follows so blood volumes increases and so does BP

* promotes synthesis of Na/K channels in apical membrane and Na/K ATPase pumps in basement membrane, increasing re-absorptive rate of sodium and therefore, water
* helps get rid of high K levels

transcellular route

pathway through which tubular reabsorption occurs where solute enters into the apical membrane and exits through the basement membrane, eventually entering the blood through the endothelium of peritubular capillaries

paracellular route

pathway through which tubular reabsorption occurs where water and solutes can selectively pass through tight junctions, renaming them “leaky junctions” of the PCT

antidiuretic hormone

ADH; hormone released by the posterior pituitary gland that increases water retention by the collecting ducts and distant convoluted tubule

atrial natriuretic peptide

ANP; hormone released in response to high blood volume and BP that decreases Na+ retention by excreting Na+ and H2O, leading to blood volume and BP dropping back down

parathyroid hormone

hormone that acts on the distal convoluted tubule to increase calcium reabsorption

urine concentration

the amount of solute/kg of water in osmolality (mOsm)

* countercurrent mechanisms

countercurrent mechanisms

mechanisms that maintain an average milliosmolality of the blood

* fluid flows in opposite directions within a single segment of the hairpin turn of the nephron loops, creating a urine concentration gradient of about 900 mOsm
* countercurrent multiplier
* countercurrent exchanger

countercurrent multiplier

helps to establish the urine concentration gradient; the interaction of filtrate flow between the ascending and descending limbs of nephron

* primarily within juxtamedullary nephron

countercurrent exchanger

blood flow in the ascending and descending limbs of the vaso recta helps to maintain urine concentration gradient

medullary gradient

the 900 mOsm difference in urine concentration which can be used to form dilute or concentrated urine

* concentration of different solutes caused urine color to change, depending
* overhydration
* dehydration

overhydration

produces large volumes of very dilute urine that is light in color and is only around 100 mOsm

* reduce production of ADH to get rid of excess water
* producing aldosterone leads to re-uptake of sodium, which water follows (some but not as much because of the high, high amounts of water and not as much Na+)

dehydration

very low water intake can lead to small volumes of very concentrated urine around 1200 mOsm

* produce a lot of ADH to conserve water which is pulled from filtrate and dispersed back into the blood
* in severe cases, body can reabsorb almost 99% of water in urine
* alcohol inhibits ADH production, leading to a loss of water and dehydration

diuretics

any kind of chemical that can enhance urinary output (whereas antidiuretics prevent it)

* ADH inhibitors
* Na+ reuptake inhibitors
* osmotic diuretics

ADH inhibitors

diuretics (like alcohol) that inhibit the production of ADH

Na+ reuptake inhibitors

diuretics that inhibit sodium and therefore, the reabsorption of water

* caffeine, hypertension drugs, edema

osmotic diuretics

any substance that cannot be reabsorbed so H2O remains in the urine

* diabetic patients with high glucose pull water from the body

renal calculi

kidney stones; crystallized calcium, magnesium, or uric acid salts with pointy edges

* mostly benign (called nephrolethiasis) but can leave the kidney and enter the ureter (now called ureterlethiosis), eventually passing through the ureter, entering the bladder, and out through the urethra (causing pain, bleeding, inflammation, difficulty urinating, etc)

urinary bladder

muscular sac that exists to temporarily store urine

* deflated ballon when empty but can grow to about 5 inches; expands, putting pressure on surroundings and triggering micturtion/urinary reflex
* normally, 500 mL
* distended, 1000 mL
* more than 1,000 mL with risk of bursting
* trigone
* layers of wall:
* mucosa layer
* muscular layer
* fibrous adventitia

trigone

triangular entrance/inlets/outlets of bladder

muscular layer

3 layers of smooth detrusor muscles (2 longitudinal, 1 circular) that allow for the contracting and ringing component that forces as much urine out of the bladder as possible when the urinary reflex is activated

internal urethral sphincter

located at the base of the trigone (urethral-bladderal junction), an involuntary smooth muscle that has to contract to open

external urethral orifice

female: normally only conveys urine

male: carries both semen and urine

prostatic urerthra

part of the male urethra that passes through the prostate gland with openings to allow fluid in

* intermediate urethra is between prostate and penis itself

spongy urethra

part of the male urethra that runs down the length of the penis with tissue that prevents it from being closed off during erectiom

urinary tract infection

UTI; usually takes place due to improper toilet habits or sexual activity

* natural bacterial flora within the reproductive organs can be damaged by physicality, spermicide, lubricants, etc. that affect pH, increasing the likelihood of more

urethritis

inflammation of the urethra

cystitis

inflammation of the urinary bladder

systematic BP

BP created through contraction of left and right ventricles

* the further from the heart, the weaker due to peripheral resistance

peripheral resistance

net resistance of vasculature of smooth and skeletal muscle contraction and respiration activities

* increases/decreases in pressure as we breathe in and out

systolic pressure

the highest pressure generated in the aorta during ventricular contraction

* normally, around 120 mmHg

diastolic pressure

the low pressure generated in the aorta during relaxation of left ventricle; the pressure the ventricle has to overcome to open the aorta

* normally, around 80 mmHg

elasticity

the ability to stretch, expand, and recoil which allows arteries to act as a pressure reservoir

* kinetic force causes bulging

pulsatile BP

near the heart

* rises and falls with each heartbeat
* progressing away from the heart, becomes more consistent and steady (mean arterial pressure)

pulse pressure

systolic minus diastolic

* normally, around 40 mmHg

pulse

throbbing of arteries due to differences in pulse pressure (high and low feelings)

mean arterial pressure

MAP; the pressure needed to propel blood to the tissues

* diastolic + 1/3 pulse pressure (heart spends more time in diastole

venous BP

small pressure gradient (about 15 mmHg) that is not enough pressure to push blood up the veins

* valves help prevent pooling with gravity

venous return

the amount of blood return to the heart through


1. muscular pump
2. respiratory pump
3. sympathetic venoconstriction

muscular pump

pathway of venous return where as we walk, skeletal muscles contract and apply pressure to the veins

* with valves closed off, blood only flows one way, to the heart

respiratory pump

pathway of venous return where as we breathe in and out, the diaphragm contracts and flattens (decreasing the volume of the abdominal cavity), increasing the pressure in the abdominal cavity (decreases in thoracic cavity)

* pulls blood from high pressure of veins in abdominal cavity into the low pressure of the thoracic cavity and compresses the veins, pumping blood towards heart

sympathetic vasoconstriction

the small tunica media of the veins can constrict and dilate (a little through this pathway of venous return)

* under sympathetic control, helps smooth muscle of veins to constrict and increase pressure, pumping blood back to the heart