Unit 11-renal system

functions of the renal/urinary system

• removal of waste products from body fluids= excretion

• elimination of organ waste materials from body(nitrogenous wastes-urea, uric acid, creatinine)

homeostatic regulation in the urinary system

• regulating blood volume and blood pressure by adjusting water lost in the urine

• works to preserve water/concentration urine

• hormonal response(aldosterone, ADH, renin, erythropoietin)

• regulating plasma ion concentrations(NA+, K+, Cl-, H+, etc)

• regulation of blood pH

• preservation of valuable nutrients

the kidneys-external anatomy

• at level T12-L3

• left kidney higher than right due to location of liver

  3 layers of supportive tissue around each kidney:

• inner fibrous capsule

• middle adipose tissue layer(cushions and protects)

• outer renal fascia(anchors to surrounding structures)

• the adrenal gland is on the superior surface of each kidney(aka suprarenal gland)

hilum

area where blood vessels, lymphatics, nerve fibers enter the kidney

• runs at the proximal end of the ureters

• blood supply:

• renal arteries and veins

• kidneys receive 20% of cardiac output, liver receives 25%

renal medulla

divided into 6-19 renal pyramids separated by renal columns

• base of each pyramid at the apex(renal papilla) projections into renal sinus

• at the renal papilla are calyces(singular calyx)= site of urine collection

• urine moves from renal medulla→ minor calyx→ major calyx→ renal pelvis → ureter

kidney lobe

blood vessels run around the pyramid

• includes a renal pyramid

• this is where urine is made

blood flow in the kidneys

blood travels to the kidneys via Left and right renal arteries

1. renal artery

   • branches to

2. segmental arteries

   • branches to

3. interlobar arteries

4. arcuate arteries

5. radiate arteries

6. afferent arterioles that feed capillaries supplying individual nephrons

the nephron

it is the functional unit of the kidney

2 kinds

• cortical nephron(85%)→ in the outer cortex of the kidney

• juxtamedullary nephron→ deeper down in the kidneys(concentrate liver)

Each nephron has a renal corpuscle and renal tubule

renal corpsule

includes the glomerulus(capillary network)capsule aka bowman’s capsule

• where filtrate is produced

renal tubule

includes the proximal convoluted tubule(PCT), nephron loop-loop of henle,distal convoluted tubule(DCT)

• From the glomerular capsule to collecting tube

Functions:

• reabsorption and secretion of water and solutes to maintain homeostasis and produce urine

• reabsorption and secretion varies along the renal tubule due to variation in tissues and membrane permeability

ureters

pair of muscular tubules extending from renal pelvis of kidney to urinary bladder(about 30mcm)

• path of ureters depend on presence of reproductive organs

• enter posterior wall of bladder at an oblique angle(slit-like opening called ureteric orifices

• ureteric orifices→ prevent backflow from the bladder if it is overfilled

• peristaltic contractions sweep across ureters every 30 secs→ force urine toward the bladder

urethra

extends from the internal urethral sphincter and carries urine out of body

• 5x longer in males than females

• male urethras have 3 sections prostatic, membranous, and spongy urethra

• female urethras are shorter and more prone to infection

• external urtheral sphincter is under voluntary control

• resting tone of the external urtheral sphincter must be voluntarily relaxed to allow urination

glomerulus

includes 50 intertwined capillaries(within the glomerular capsule)

• receives blood from afferent arterioles and blood exits via efferent arterioles

• the outer layer is made up of simple squamous epithelium

• extraglomerular mesangial cells→ muscular cells that contract to decrease the size of the vessel lumen(only in juxtamedullary nephrons)

  • they impact filtration and change constriction

filtration membrane

filtration slits at the visceral membrane created by gaps between unique podocyte cells

• glomerular capillaries are fenestrated capillaries meaning they have pores that allow the rapid movement of more fluids and solutes

• filtration happens in the renal corpuscles with water and small solutes being pushes out of glomerular capillaries into the capsular space

• the end product of filtration=filtrate

• filtrate leaves the glomerular capsule and enters the renal tubule(starting at the proximal convoluted tubule)

  3 types:

• macula densa cells

• juxtaglomerular cells

• mesangial cell

  s

juxtaglomerular complex(JGC)

has a critical role in the regulation of blood pressure and the formation of filtrate

• found in all nephrons→ it is the structure where afferent arterioles make contact with distal convoluted tubule

• 3 types:

• macula densa cells

• juxtaglomerular cells

• mesangial cell

3 cells of the juxtaglomerular complex(JGC)

1. Macula densa- epithelial cells in distal tubule(chemoreceptors and baroreceptors)

2. juxtaglomerular cells- in wall of afferent arterioles and secrete renin- activates angiotensin I

3. mesangial cells-in the space between afferent and efferent arterioles

   • feedback station for the macula densa and juxtaglomerular

collecting system

function is to move tubular fluid from the nephron to the renal pelvis (this further adjusts its composition as needed to create the final product=urine)

• a number of nephrons will empty into one collecting duct

• several collecting ducts converge into larger papillary duct

• papillary ducts empty into minor calyx

principal cells of the collecting duct

reabsorb water and secrete potassium

• made of cuboidal epithelium without microvilli

intercalated cells

maintain acid base balance

• made of cuboidal epithelium with microvilli

vasa recta

long straight capillary running beside the nephron loop

• carries solutes and water reabsorbed in the renal medulla back to the systemic circulation(countercurrent exchange)

formation of urine-filtration

blood pressure forces fluid across the glomerulus into glomerular capsule(aka bowman’s capsule)

• the main driver of filtration is hydrostatic pressure

• if the blood pressure decreases, filtration also decreases(creates a risk of renal fialure)

• large molecules cannot fit through the glomerulus because it is selectively permeable

• eg. colloid= plasma protein that cannot fit through the glomerular membrane

formation of urine-reabsorption

moving water and solutes from the tubular fluid back into circulation

• reabsorption is selective and passive

• fluid goes from the filtrate into the peritubular fluid '

• regulated by hormones

• 90% of filtrate from glomerular capsule will be reabsorbed

formation of urine-secretion

secretion of solutes back into the tubular fluid(filtrate) before excretion

• depends on permeability and availability of other transport mechanisms

proximal convoluted tubule

reabsorption of water, ions, and all organic nutrients

distal convoluted tubule

• active secretion of ions, acids, drugs, toxins to the tubular fluid

• selective reabsorption of Na+ ions and water

• active transportation of chloride and water

• aldosterone is in control of Na+ ion channel and pumps

• some reabsorption of water, sodium ions, and calcium ions→ under hormonal control

• Plays a key role in acid-base balance and secretion of ions like potassium and hydrogen.

• Lacks a brush border, as less absorption occurs compared to the PCT

• can be influenced by ADH to reabsorb more water

nephron loop

thin descending limb:

• further reabsorption of water

• not very permeable to solutes(Na+, Cl-)

Thick ascending limb:

• reabsorption of Na+ and Cl- ions

• impermeable to water

collecting duct

some reabsorption of water

• reabsorption and or secretion of sodium, potassium, hydrogen , and bicarbonate ions

papillary duct

delivery of urine to minor calyx

glomerular filtration

the first step in kidney function mainly due to hydrostatic pressure in the glomerular capillaries

• influenced by blood colloid osmotic pressure(BCOP)

• net filtration pressure=difference between hydrostatic pressure and BCOP

• glomerular filtration rate(GFR)→ the amount of filtrate that the kidneys produce in 1 minute(average=125 mL/min)

glomerular filtration rate

• the amount of filtrate that the kidneys produce in 1 minute(average=125 mL/min)

• controlled by autoregulation, hormonal regulation, and autonomic regulation

autoregulation

control of local blood flow via vasoconstriction and vasodilation

hormonal regulation

renin-angiotensin-aldosterone system(RAAS)

• lower BP(or GFR) triggers the release of renin

• renin converts angiotensinogen into inactive angiotensin I

• Angiotensin converting enzyme(ACE) converts angiotensin I→ angiotensin II

• angiotensin II promotes vasoconstriction aldosterone production, and stimulates the sympathetic nervous system to increase blood pressure and restore GFR

• natriuretic peptides have the opposite effect

autonomic regulation

sympathetic nervous system can cause powerful vasoconstriction of afferent arterioles to decrease GFR

• slow down production of filtrate which allows the retention of fluids in response to low BP

• less blood going to the glomerulus= less pee is produced

tubular reabsorption

returns nutrients from tubular fluid back to blood

• tubular secretion takes unwanted substances in the blood and assds substances from the blood to the tubular fluid

• involves mechanisms of diffusion, osmosis, channel mediated diffusion, and carrier mediated transport

• carrier mediated transport→ can be saturated

carrier mediated transport

2 characteristics

1. specificity→ what it will bind with

2. saturation: capacity of carrier protein to move substances

   • once saturated it cannot take anything else

   • saturation point of transport maximum determines renal threshold(plasma concentration at which a certain substance or ion will begin to appear in urine)

renal threshold

the plasma concentration at which a specific substance or ion will begin to appear in urine

• varies between substances

• glucose over the threshold will be seen in a urinalysis

  • eg. diabetes→ increased glucose in urine test shows that the body does not have enough insulin to bring down glucose levels

• if tubular threshold is surpassed the substance will be found in urine

tubular secretion

involves transfer of selective substances from the peritubular capillaries(blood) into tubular lumen(tubular fluid)

most import secretory systems are for:

H+:

• important in regulating acid-base balance

• secreted in proximal, distal, and collecting tubules

K+: keeps plasma K+ concentration at appropriate level to maintain normal membrane excitability in muscles and nerve

• secreted only in the distal and collecting tubules under the control of aldosterone

Organic ions:

• more efficient elimination of foreign organic compounds from the body

• secreted only in the proximal tubule

regulating urine volume and concentration

urine volume and osmotic concentration are controlled by water reabsorption

• volume is very dependent on amount of water reabsorbed in the DCT and collecting system which depends on ADH

• as ADH rises, DCT and collecting system become more permeable to water→ water reabsorption increases

• under maximum ADH, osmotic concentration of urine can equal that of the surrounding medulla

urea

a byproduct of amino acid breakdown that is the most abundant organic waste

• always have a certain level in the blood

• raised levels can indicate kidney failure

• excreted only in urine

creatinine

generated during skeletal muscle activity and is a byproduct of the breakdown of creatine phosphate

• monitored the most to detect kidney function(benchmark for kidney function)

• continually excreted in the urine only

uric acid

formed as the body recycles nitrogenous bases from RNA

urine osmality

measure of the osmotic concentration of urine(mOsm/Kg of water)

urinalysis

analysis of chemical and physical properties of urine

• looks at the color, clarity, and presence of unexpected substances

creatinine clearance

compares creatinine levels in urine with creatinine levels in the blood, estimation GFR

• lower kidney function= increased creatinine levels in the blood = lower GFR

• GFR is always an estimate but provides important info on kidney function

• GFR= creatinine excreted in urine(mg/h) /plasma concentration of creatinine

blood urea nitrogen(BUN)

measures amount of urea in the blood

• increased Blood urea nitrogen(BUN)= lower kidney function

urinary bladder

hollow muscular organ that temporarily stores urine

• rugae in the mucosal lining allow for expansion

• longitudial and circular muscles form the detrusor muscles

• when detrusor muscles contract, the bladder is compressed and urine is squeezed into the urethra

• trigone area as thick smooth mucous layer that acts to funnel urine into urethra when bladder contracts(urethral opening at the apex of the trigone)

• neck of the bladder houses the internal urethral sphincter(involuntary)

urine storage reflex

spinal reflex and pontine storage center

• stretch receptors stimulate sympathetic activity that inhibits contraction of detrusor muscles and promote contraction of internal and external sphincter

• decrease parasympathetic function

urine voiding reflex

spinal reflex and micturition center

• micturition center stimulates increased parasympathetic activity and decreases sympathetic activity

• sphincters relax and detrusor muscles contract

regulation of urination

urge to urinate or void begins when there is about 200 mL of urine in the bladder

• peristaltic contractions bring urine from kidneys to bladder

• urination involves 2 spinal reflexes:

  → urine storage reflex

  →urine voiding reflex

infants and urination

infants lack a voluntary control of urination

• necessary corticospinal connections are not yet developed

• 18-36 months the voluntary control is gained

changes in renal system in older age

• decrease in number of functional nephrons

• nephrolithiasis(formation of renal calculi)

• decreases sensitivity to ADH

• loss of muscle tone and sphincter'

• urinary retention related to prostate enlargement