Urinary System & Fluid Balance

• renal artery

• segmental artery

• interlobar artery

• arcuate artery

• interlobar artery (again)

• afferent arteriole

• glomerulus

• efferent arteriole

• peritubular capillaries OR vasa recta

• interlobar vein

• arcuate vein

• interlobar vein

• renal vein

What is the pathway of renal circulation?

filtration: glomerulus

exchange: vasa recta and peritubular capillaries

Between the following capillaries: glomerulus, vasa recta, and peritubular capillaries, which are for filtration and which are for exchange (reabsorption and secretion)?

liquid that only contains water and plasma (minus plasma proteins)

filtrate is the fluid that is filtered through the glomerulus and enters the nephron

What is filtrate?

blood contains formed elements and plasma, filtrate only contains water and plasma

blood is found in the glomerulus and filtrate is found in the glomerular capsule

What is the difference between blood and filtrate (in composition and where it’s found in the renal corpuscle)?

• renal corpuscle (containing glomerulus and glomerular capsule)

• proximal convoluted tubule (PCT)

• nephron loop

• distal convoluted tubule (DCT)

• collecting duct

List the parts of the nephron

• blood flows through the glomerulus (water and solutes move across wall of glomerular capillaries into capsular space)

• filtrate enters the PCT, then the:

  • nephron loop

  • DCT

  • collecting ducts

  • papillary duct in renal papilla (now considered urine)

  • urine goes through the minor calyx, major calyx, renal pelvis, ureter, and then bladder

How does filtrate and urine flow through the nephron?

nephrons that contain long loops that extend deep into the renal medulla

this establishes a salt concentration gradient in the medulla that concentrates the urine and allows the body to conserve water (water exits tubule and enters the interstitial fluid)

What are juxtamedullary nephrons?

most common nephron, has short loops mainly found in the renal cortex

performs excretory and regulatory functions (like filtering blood and maintaining blood pressure)

What are cortical nephrons?

sympathetic division

• nerves extend to the afferent and efferent arterioles and the juxtaglomerular apparatus, which decreases urine production

parasympathetic division

• effects are unknown

The kidneys are innervated by both divisions of the ANS. Explain the effects each division has on the kidneys.

a structure in the kidney responsible for regulating systemic blood pressure and filtrate formation (GFR)

contains:

• granular cells

  • in afferent arteriole, they contract the arteriole and synthesize, store, and release renin

• macula densa

  • epi cells in the DCT that detect changes in NaCl concentration of tubular fluid and signals granular cells to release renin

What is the juxtaglomerular apparatus?

1. glomerular filtration

   1. some water and dissolved solutes from the blood are separated and enter the capsular space of renal corpuscle

2. tubular reabsorption

   1. some components of the tubular fluid move from the tubules and collecting ducts back into the blood

   2. all vital solutes and most water are reabsorbed, excess solutes and waste remain in tubules

3. tubular secretion

   1. movement of solutes out of the blood from the peritubular capillaries and vasa recta into the tubular fluid to be excreted in urine

water and secreted substances are eventually excreted from the body as urine

What are the three steps of urine formation?

• endothelium

  • fenestrated, allows plasma to pass through (no large substances though)

• basement membrane of glomerulus

  • restricts passage of large plasma proteins

• visceral layer of glomerular capsule

  • filtration slits restrict the passage of most small proteins

*diagram on page 960

What are the three parts of the filtration membrane (in the glomerulus)?

CAN

• water, glucose, amino acids, ions, urea, some hormones, ketones, and very small amounts of proteins

CANNOT

• leukocytes, erythrocytes, platelets, and large proteins

What materials can and cannot pass through the glomerular filtration membrane?

• glomerular hydrostatic pressure (HPg)

  • the blood pressure WITHIN the glomerulus

  • pushes water and some solutes out of glomerulus into the capsular space, high pressure

• blood colloid pressure (OPg)

  • osmotic pressure exerted by dissolved solutes, draws fluid BACK into glomerulus

• capsular hydrostatic pressure (HPc)

  • pressure in capsule due to filtrate, STOPS the movement of additional fluid

remember: OPg and HPc are AGAINST HPg

What are the three pressures affecting filtration at the glomerular filtration membrane?

concept that filtration occurs when pressures PROMOTING filtration are greater than those OPPOSING filtration

HPg - (OPg + HPc) = NFP

What is net filtration pressure (NFP)?

• glomerular filtration rate (GFR)

• solutes and water remaining in tubular fluid

• and amount of substances in urine

What three things increase due to increased NFP?

filtrate reabsorption

What is the one thing that decreases due to increased net filtration pressure (NFP)?

the rate of the volume of filtrate that is formed (volume/unit of time)

influenced by:

• diameter of the afferent arteriole lumen

• the surface area of the filtration membrane

What is glomerular filtration rate (GFR) and what two things influence it?

• when components within the tubular fluid move across the tubules and into the peritubular capillaries or vasa recta, returning to the blood

  • water and vital solutes are reabsorbed

• where: primarily in the PCT (but can also happen everywhere else)

What is tubular reabsorption and where on the nephron does this occur?

• the movement of solutes out of the blood within the peritubular capillaries and vasa recta and into the tubular fluid

  • selective

• where: primarily in the PCT and DCT

What is tubular secretion and where on the nephron does it occur?

• 98%-100% is reabsorbed

• two ways reabsorption happens:

  • Na⁺ moves down its concentration gradient from the tubule lumen into the tubule cells

    • (cells lining the tubules, Na⁺ eventually leaves the cells via Na⁺/K⁺ pumps and into the blood)

  • hormonal reabsorption

    • aldosterone and ANP

How much Na⁺ is reabsorbed in the PCT and what are the two ways reabsorption happens?

aldosterone

• stimulates synthesis of Na⁺ channels and Na⁺/K⁺ pumps

  • this increases Na⁺ reabsorption

• aldosterone maintains blood pressure and volume by decreasing urine output

ANP

• inhibits Na+ reabsorption in the PCT and collecting tubules

  • this stops the release of aldosterone (results in increased water and Na⁺ secretion)

• ANP decreases blood volume and pressure

How does aldosterone and ANP regulate Na⁺ reabsorption?

how much:

• 65% reabsorbed in the PCT

• 10% reabsorbed in the nephron loop

• DCT and collecting ducts: controlled by hormones

how water is reabsorbed:

• water moves via osmosis and is reabsorbed by paracellular transport between principal cells of the tubular walls

• can also move out via aquaporins

How much water is reabsorbed along the nephron and how does it happen?

ADH

• binds to principal cells in the collecting tubules and ducts, providing extra aquaporins to increase water reabsorption

aldosterone

• increases the number of Na+/K+ pumps (Na+ reabsorbs, water follows)

How does ADH and aldosterone increase water reabsorption?

• 60%-80% is reabsorbed, 10%-20% happens in the thick segment of the nephron loop

• K+ reabsorption depends on Na+ movement:

  • if Na+ is reabsorbed, water follows

  • this increases the concentration of solutes in the tubular fluid, creating a gradient between the tubule and interstitial fluid

  • K+ moves down the gradient from the tubular fluid and into the interstitial fluid (then the blood)

How much potassium is reabsorbed along the nephron and how does it happen?

• 60% Ca²⁺ and 90%-95% PO₄^3-

• regulated by parathyroid hormone (PTH)

  • inhibits phosphate reabsorption in the PCT, but stimulates Ca²⁺ reabsorption

  • this causes blood Ca²⁺ to rise

What percentage of calcium and phosphate is secreted and how does it happen?

• bicarbonate: yes

  • moves freely, but does not get reabsorbed (to prevent acidic blood)

• H⁺: no

  • most remains in blood

Is bicarbonate and H⁺ filtered in the renal corpuscle?

• causes vasoconstriction of the afferent and efferent arterioles

  • this causes a decrease in surface area of the filtration membrane, decreasing GFR

  • decreased GFR = less filtrate/tubular fluid in nephron, less reabsorption

How does angiotensin II affect tubular reabsorption?

the maximum rate of substances that can be reabsorbed (or secreted) across tubule epithelium per a certain time

• depends on the number of transport proteins in tubule membrane

What is transport maximum?

effect:

• causes blood to become hypertonic (high in solutes, water leaves cells and causes them to shrivel)

how to fix it:

• more water is reabsorbed from filtrate in the nephron and ADH is also released to further increase water reabsorption

  • both cause decreased urine output

How does dehydration affect blood osmolarity, and how does the body fix it?

• composition

  • 95% water, 5% solutes (salt, nitrogenous waste, some hormones and drugs)

• volume

  • average 1-2 L per day

  • variations due to fluid intake, bp, temperature, diuretics, etc.

• pH

  • normally between 4.5 and 8.0

  • more acidic with more protein in diet, more alkaline with veggies and fruit

• specific gravity

  • density of a substance compared to density of water

  • slightly higher than water (due to solutes)

• color

  • ranges from clear to dark yellow (depends on concentration of urobilin)

Describe the characteristics of urine (composition, volume, pH, specific gravity, and color)

• continuous sympathetic stimulation causes:

  • relaxation of the detrusor muscle of bladder to accommodate urine

  • constriction of the internal urethral sphincter to retain urine

• the external urethral sphincter is continuously stimulated to remain contracted

Describe the storage reflex for storing urine

1. volume of the bladder reaches 200-300 mL

2. baroreceptors (responds to stretch) signal sensory neurons

3. micturition center of brain sends nerve signals

4. parasympathetic stimulation causes detrusor muscles to contract and the internal urethral sphincter to relax

   1. remember, the external urethral sphincter is consciously controlled

Describe the micturition reflex for excreting urine

MALE

• bladder

  • anterior to rectum

• urethra

  • functions as a passageway for urine and sperm

  • longer and made of three parts

FEMALE

• bladder

  • anteroinferior to uterus

• urethra

  • only function is to transport urine, shorter than male

What are the differences between the male and female urinary tract?

age

• infants have the highest percentage of fluid

• elderly people have the lowest percentage

• children, young adults, and adults are between

sex

• male adults have about 60% body fluid

• female adults have about 55% body fluid

How does body water percentage vary based on age and sex?

• INTRAcellular fluid (ICF)

  • fluid within the cells

  • 2/3 of all body fluid

• EXTRAcellular fluid (ECF)

  • fluid outside the cells, two types:

    • interstitial fluid (IF) - surrounds cells

    • blood plasma

  • 1/3 of all body fluid

What are the two fluid compartments of the body?

moves in response to changes in osmolarity

• when one compartment is hypo or hypertonic to another, water moves by osmosis from one compartment to another until the concentration is equal

example: drinking water

• water enters the blood from the GI tract, plasma osmolarity decreases, and water moves from into the interstitial fluid then into the cells

How does water move between fluid compartments?

• decreased blood volume and pressure

• increased blood osmolarity

• decreased salivary secretions

When you are dehydrated, what factors stimulate the thirst center in the brain?

• increased blood volume and pressure

• decreased blood osmolarity

• increased salivary secretions

• distension of the stomach (stretch caused by fluid entering)

What stimuli inhibits the thirst center of the brain?

urine output:

• decreases urine output to increase blood volume and pressure

it additionally stimulates the thirst center and causes vasoconstriction of systemic blood vessels (to increase systemic bp)

How does ADH regulate urine output? Also, what does it do to the thirst center of the hypothalamus?

to maintain:

• pH

  • acid-base balance

• blood pressure

• hydration

• nerve and muscle function

• heart health

Why is it important to maintain electrolyte balance?

• 99% found in ECF, 1% in ICF

• the amount lost in urine is regulated by ADH, ANP, and aldosterone

• most important electrolyte in determining blood plasma osmolarity and regulating fluid balance

Na⁺

• 98% found in ICF, 2% in ECF

  • only the ECF portion is regulated

• required for neuromuscular activities and controlling heart rhythm

• most is lost in urine

• most lethal of all electrolyte imbalances

K⁺

• most abundant anion in the ECF

• found in the stomach as HCl, participates in inhibitory postsynaptic potentials, and has a part in transporting CO₂ in the form of HCO₃^-

• associated with Na⁺

  • follows Na⁺ by electrostatic interactions

• obtained in the diet, lost in sweat, stomach secretions, and urine

Cl^-

• most abundant electrolyte in bone and teeth (99%)

• needed for muscle contraction and neurotransmitter release

• can exist bound to a protein (albumin), associated with phosphate, or in an unbound/free state

• obtained in the diet, lost in urine, feces, and sweat

Ca²⁺

• most abundant anion in bone and teeth (85%)

• component of DNA, RNA, and phospholipids

• acts as an intracellular buffer and urine buffer

• regulated in similar ways to Ca²⁺ (because majority is bound to calcium)

PO₄^3-

• primarily found in bone or within cells

• abundant ICF cation

• important for enzyme reactions, helping Na⁺/K⁺ pumps, and muscle relaxation

• obtained in the diet (via beans, peas, or leafy greens) and lost by sweat and urine

Mg²⁺

• 7.35 - 7.45

• staying in this range is critical for:

  • regulating enzyme activity

  • preventing protein denaturation

  • and ensuring oxygen is delivered to all tissues

What is the normal pH range of the body, and why is it so important to maintain pH?

• nutrients absorbed in the GI tract (from proteins and wheat)

• waste produced from cells (like lactic acid)

What are two major things that cause an increase in blood acidity (increase in H⁺ concentration)?

• antacid ingestion

• vomiting with the loss of HCl

List two things that cause an increase in blood alkalinity (loss of H+)

function: substances that prevent large changes in pH after an acid or base is added to a solution, consists of a weak acid and weak base

• protein buffering system

  • consists of proteins found in cells and blood plasma

  • amine group acts as a weak base to buffer acids, and carboxylic acid is a weak acid that buffers bases

• phosphate buffering system

  • found in ICF

  • buffers metabolic acids produced by cells

  • net result: strong acid buffered to produce a weak acid

• bicarbonate buffer system

  • most important in the body

  • weak base (HCO₃^-) and weak acid (H₂CO₃)

  • net result: strong acid buffered to produce a weak acid, and a strong base buffered to produce a weak base

Describe the function of buffers and list the 3 types of chemical buffering systems in the body

• regulates levels of carbonic acid

• body at rest:

  • CO₂ is eliminated from the lungs at the same rate body cells are producing CO₂

• body during exercise:

  • high blood levels of CO₂ is detected by chemoreceptors and is relayed to the respiration center of the brain, breathing rate and depth change

How does the respiratory system control pH?

• pH is too low (acidic)

  • all filtered HCO3- is reabsorbed along the nephron

  • more HCO3- is synthesized and absorbed and excess H⁺ is secreted into filtrate (these both increase pH)

• pH is too high (basic)

  • renal tubules do not reabsorb all filtered HCO3-

  • HCO3- is secreted into filtrate and H⁺ is reabsorbed in exchange

How do the kidneys control pH?