Urine Formation

Joanna I. AlafagAdv. Animal PhysiologyFeb. 13, 2020

HUMAN URINARY SYSTEM

group of organs concerned with filtering out excess fluid and other substances from the bloodstream

works with the other systems of the body to help maintain homeostasis.

Parts of the Urinary SytemI. KIDNEYS

Functions of the Kidneys

1. Regulation of blood ionic composition.

2. Maintenance of blood osmolarity.

3. Regulation of blood volume.

4. Regulation of blood pressure.

5. Regulation of blood pH

6. Release of hormones

7. Regulation of blood glucose level

8. Excretion of wastes and foreign

substances

Functions of the Kidneys

External Anatomy of the Kidney:

1. Size length: 4-5 inwidth: 2-3 inthickness: 1 inmass: 145 -150 g

 

2. Renal hilus- deep vertical fissure through which the ureters, vessels and nerves leave the kidney

 

3. Tissue layers 

a. Renal capsule - smooth, transparent, fibrous membrane- barrier against trauma and helps maintain the shape of the kidney

 

b. Adipose capsule- fatty tissue- protects the kidney from trauma and holds it firmly in place

c. Renal fascia- thin, dense layer of irregular connective tissue -anchors the kidney

 Internal Anatomy of the Kidney

Blood and Nerve Supply of the Kidneys

Renal vein

Segmental veins

Interlobar veins

Arcuate veins

Interlobar veins

Peritubular capillaries and / or vasa recta

Efferent arterioles

Glomerular capillaries

Affferent arterioles

Interlobar arteries

Arcuate arteries

Interlobar arteries

Segmental arteries

Renal artery

NEPHRONfunctional unit of the kidney

3 basic functions: filters blood returns useful substances to the

blood removes wastes and other

substances maintain homeostasis of blood

urine is produced

Parts of the Nephron

A. Renal corpuscle-where plasma is filtered

1. Glomerulus- a tangled, rounded mass of capillary network

2. Glomerular (Bowman’s) capsule- double-walled epithelial cup that surrounds the glomerulus- 2 layers: a. visceral layer *podocytes - modified simple squamous epithelial cells

b.parietal layer- simple squamous epithelium and forms outer wall of capsule

* capsular (Bowman’s) space- space between the 2 layers of the glomerular capsule

B. Renal tubule- where filtered fluid passes

Region Description

1. Proximal convolutedtubule (PCT)

Simple cuboidal epithelial cells with prominent brush borders of microvilli

2. Loop of Henle (LOH)

Loop of Henle: descendinglimb and thin ascending limb

Simple squamous epithelial cells

Loop of Henle: thick ascendinglimb

Simple cuboidal to low columnar epithelial cells

3. Most distal convoluted tubule (DCT)

Simple cuboidal epithelial cells

4. Collecting duct (CD)Final portion of DCT; collectingduct (CD)

Simple cuboidal epithelium consisting of principal cells and intercalated cells in last part of DCT and collecting duct

2 Types of Nephrons

1. Flow of Fluid through a Juxtamedullary Nephron

Glomerular (Bowman’s) capsule

Proximal convoluted tubule

Descending limb of the loop of Henle

Thin ascending limb of the loop of Henle

Thick ascending limb of the loof of Henle

Distal convoluted tubule(drains into collecting duct)

Glomerular (Bowman’s) capsule

Proximal convoluted tubule

Descending limb of the loop of Henle

Ascending limb of the loop of Henle

Distal convoluted tubule(drains into collecting

duct)

2. Flow of Fluid through a Cortical Nephron

URINE FORMATION

I. GLOMERULAR FILTRATIONsubstances in blood are filtered at the endothelial-capsular membrane and the filtrate enters the PCT

Filtration fraction- fraction of plasma in the afferent arterioles of the kidneys that becomes:

Glomerular filtrate- fluid that enters the capsular space

average daily glomerular filtrate in adults:

99% of the glomerular filtrate returns to the bloodstream via tubular reabsorption

1-2L are excreted as urine

♀female – 150L♂male – 180L

Filtration Membrane or Endothelial –Capsular Membrane

- sandwich like assembly of endothelial cells of glomerular capillaries and podocytes

- permits water and small solutes

- prevents most plasma proteins, blood cells, and platelets

3 Components:

1. glomerular endothelial cells- large fenestration (70-100nm)

-permits all solutes in blood plasma to exit glomerular capillaries

-prevents filtration of blood cells and platelets.

mesangial cells - contractile cells that help regulate glomerular filtration

relaxation: maximal surface area high glomerular

filtration

contraction: reduced surface area low glomerular

filtration

2. basal lamina - acellular material between the endothelium and the podocytes

- fibrils in a glycoprotein matrix

- prevents filtration of larger plasma proteins

3. pedicels - foot-like processes from each podocyte filtration slits - spaces between pedicelsslit membrane- permits the passage of molecules having a diameter smaller than 6-7 nm (water, glucose, urea, and ions)

 

Why is there larger volume of fluid filtered in renal corpuscle than in other capillaries?

1. glomerular capillaries are long and extensive – greater surface area

2. filtration membrane is thin and porous

3. glomerular capillary blood pressure is high

Net Filtration Pressure (NFP)- total pressure that promotes filtration

1. Glomerular blood hydrostatic pressure (GBHP) - promotes filtration- forces water and solutes in blood plasma- about 55mm Hg

2. Capsular hydrostatic pressure (CHP)

- opposes filtration- pressure exerted against the filtration membrane by fluid present capsular space and renal tubule- about 15mm Hg

3. Blood colloid osmotic pressure (BCOP)

- opposes filtration - due to proteins such as albumins, globulins, and fibrinogen in blood plasma- about 30 mm Hg

NFP = GBHP – CHP - BCOP= 55 mmHg - 15mmHg - 30mm

Hg= 10mm Hg

pressure that causes a normal amount of plasma to filter from glomerulus into capsular space 

Glomerular Filtration Rate (GFR)

- amount of filtrate formed in all renal corpuscles of both kidney each minute

- average adult GFRfemale: 105 ml/minmale: 125 ml/min

Type of Regulation Major Stimulus Mechanism and Site of Action Effect on GFR

Renal Autoregulation Myogenic mechanism Increased stretching of smooth

muscle fibers in afferent arteriole walls due to increased blood pressure.

Stretched smooth muscles fibers contract, thereby narrowing the lumen of the afferent arterioles.

Decrease

Tubuloglomerular feedback

Rapid delivery of Na+ and Cl- to the macula densa due to high systemic blood pressure.

Increased release of a vasoconstrictor by the juxtaglomerular apparatus causes constriction of afferent arterioles.

Decrease

Neural regulation Increase in level of activity of renal sympathetic nerves releases norepinephrine.

Constriction of afferent arterioles through activation of α1 receptors and increased rennin.

Decrease

Hormonal RegulationAngiotensin II

Atrial natriuretic peptide (ANP)

Decreased blood volume or blood pressure stimulates production of angiotensin II.

Stretching of the heart stimulates secretion of ANP.

Constriction of both afferent and efferent arterioles.

Relaxation of mesangial cells in glomerulus increase capillary surface area available for filtration.

Decrease

Increase

Regulation of Glomerular Filtration Rate (GFR)

Renal Corpuscle

Glomerular filtration rate:105-125 ml/min of fluid that is isotonic to blood

Filtered substances: water and all solutes present in blood (except proteins) including ions, glucose, amino acids, creatinine, uric acid

Substance Total Amount in Plasma

Filtered (Enters glomerular capsule

per day)

Reabsorbed (Returned to blood

per day)

Urine (excreted per day)

Water 3 L 180 L 178-179 L 1-2 L

Proteins 200g 2 g 1.9 g 0.1 g

Na+ ions 9.7 g 579 g 575 g 4 g

Cl- ions 10.7 g 640 g 633.7 g 6.3 g

HCO3- ions 4.6 g 275 g 275 g 0.03 g

Glucose 2.7 g 162 g 162 g 0

Urea 0.9 g 54 g 27 g 27 g

K+ ions 0.5 g 29.6 g 29.6 g 2 g

Uric acid 0.15 g 8.5 g 7.7 g 0.8 g

Creatinine 0.03 1.6g 0 1.6 g

Substances in plasma and amounts filtered, reabsorbed and excreted in urine

 II. SELECTIVE TUBULAR REABSORPTION

- selective process that reclaims materials from tubular fluid and returns them to the bloodstream- epithelial cells all along the renal tubule and duct carry out reabsorption

- PCT cells make the largest contribution

Reabsorption Routes:

1. Paracellular Reabsorption- water and solutes move between tubule cells

- up to 50 % of the reabsorption of certain ions and the water that osmotically accompanies them

2. Transcellular Reabsorption- substance passes through the apical

membrane of a tubule cell, cytosol, basolateral membrane and into interstitial fluid

Transport Mechanisms:

1. Primary active Transport- energy derived from hydrolysis of ATP

is used to pump a substance across a membrane

- used by the Na+ pumps

2. Secondary Active Transport- the energy stored in an ion’s

electrochemical gradient drives substance across a membrane

- symporters: membrane proteins that move 2 or more substances in the same direction

- antiporters: membrane proteins that move 2 or more substances in the opposite direction

- transport maximum (Tm): maximum amount of substance that can be reabsorbed per unit time

3. Osmosisa. obligatory water reabsorption: - water reabsorbed together with

solutes in tubular fluid

- occurs in the PCT and descending limb of Henle due to their permeability to water

- 90% of reabsorption of water filtered by the kidneys occurs together with Na+, Cl- and glucose

 

b. facultative water reabsorption- occurs mainly in collecting ducts and is regulated by antidiuretic hormone

- 10% of reabsorption of water

III. TUBULAR SECRETION

- removes a substance from the blood by transferring materials from the blood and tubule cells into tubular fluid

2 outcomes of tubular secretion: Secretion of H+ helps control blood pH Secretion of other substances helps

eliminate them from the body

Proximal Convoluted Tubule

Reabsorption (into blood) of filtered:Water 65% (osmosis)Na+ 65% (Na pumps,

symporters, antiporters)K+ 65% (diffusion)Glucose 100 % (symporters, facilitated

diffusion)Amino acids 100% (symporters, facilitated

diffusion)Cl- 50 % (diffusion)HCO3

- 80-90% (facilitated diffusion)Urea 50% (diffusion)Ca2+, Mg2+ variable (diffusion)

Secretion (into urine) of:H+ variable (antiporters)NH4

+ variable, increase in acidosis (antiporters)Urea variable (diffusion)Creatinine small amountAt end of PCT, tubular fluid is still isotonic to blood (300 mOsm/L)

Loop of Henle

Reabsorption (into blood) of:Water 15% (osmosis in descending

limb)Na+ 20-30%(symporters in ascending limb)K+ 20-30%(symporters in ascending limb)Cl- 35% symporters in ascending

limb)HCO3

- 10-20% (facilitated diffusion)Urea 50% (diffusion)Ca2+, Mg2+ variable (diffusion)

Secretion (into urine) of:Urea variable (recycling from CD)

At end of LOH, tubular fluid is hypotonic (100-150 mOsm/L)

Distal Convoluted Tubule

Reabsorption (into blood) of filtered

Water 10-15% (osmosis)Na+ 5% (symporters)Cl- 5% (symporters)Ca2+ variable (stimulated by

parathyroid)

Principal Cells in Late Distal Tubule and Collecting Duct

Reabsorption (into blood) of filteredWater 5-9% (insertion of water channels stimulated by ADH)Na+ 1-4% (Na pumps)Cl- 5% (symporters)Urea variable (recycling to loop of Henle)

Secretion (into urine) of:K+ variable amount to adjust for dietary intake (leakage

channels)

Tubular fluid leaving the collecting duct is diluted when ADH level is low and concentrated when ADH level is high

Intercalated Cells in Distal Tubule and Collecting Duct

Reabsorption (into blood) of filteredHCO3

- variable amount, depends on (new) H+ secretion (antiporters)Urea variable (recycling to LOH)

Secretion (into urine) of:H+ variable amount to maintain acid-base homeostasis (H +

pumps)

Hormone Major Stimuli that Trigger Release

Mechanism and Site of Action

Effects

Angiotensin II Low blood volume or low blood pressure stimulates renin-induced production of angiotensin II.

Stimulates activity of Na+ / H+ antiporters in proximal tubule cells

Increases reabsorption of Na+, other solutes, and water, which increases blood volume

Aldosterone Increased angiotensin II level and increased level of plasma K+ promote release of aldosterone by adrenal cortex.

Enhances activity and synthesis of sodium pumps in basolateral membrane and Na+ channels in apical membrane of principal cells in collecting duct.

Increases secretion K+ and reabsorption of Na+, Cl- , and water, which increases blood volume.

Antidiuretic hormone (ADH) or vasopressin

Increased osmolarity of extracellular fluid or increased angiotensin II level promote release of ADH from the posterior pituitary gland.

Stimulates insertion of water-channel proteins- aquaporin-2, into the apical membranes of principal cells.

Increases facultative reabsorption of water, which decreases osmolarity of body fluids.

Atrial natriuretic peptide (ANP)

Stretching of atria of heart stimulates secretion of ANP.

Suppresses reabsorption of Na+ and water in proximal tubule and collecting duct; also inhibits secretion of aldosetrone and ADH.

Increases excretion of Na+ in urine (natriuresis); increases urine output (diuresis) and thus decreases blood volume.

Hormonal Regulation of Selective Tubular Reabsorption and Tubular Secretion

II. URETERS

•transport urine from the renal pelvis of kidney to the urinary bladder

•25-30 cm long; thick-walled, narrow tubes that vary in diameter from 1 mm -10 mm

•peristaltic contractions of the muscular walls of the ureters, hydrostatic pressure and gravity push urine toward the urinary bladder

3 Coats:

a. Mucosa- deepest coat

transitional epithelium - stretches to accommodate variable amount of fluidmucus - prevents cells from coming in contact with urine

b. Muscularis- intermediate coat-functions for peristalsis

c. Adventitia- superficial coat; - anchors the ureters in place

III. URINARY BLADDER- a hollow , distensible muscular organ - receives urine ureters and stores urine until it is excreted through urethra

- average capacity is 700-800ml

3 Coats of Urinary Bladder

a. Mucosa- mucous membrane

b. Muscularis / Detrusor muscle- consists of 3 layers of smooth muscle fibers•internal urethral sphincter (involuntary):•external urethral sphincter (voluntary

c. Adventitia•areolar connective tissue

Micturition Reflex discharge of urine from the urinary bladder

when the volume of urine exceeds 200-400 ml, pressure within the urinary bladder increases and stretch receptors in its wall transmit nerve impulses into the spinal cord

occurs via parasympathetic impulses contractions of the detrusor muscle and relaxation of the internal urethral sphincter muscle

IV. URETHRA

- a duct leading from the internal

urethral orifice in the floor of the

urinary bladder to the exterior of the

body that conveys urine in females

and urine and semen in males

Female Urethra

Male Urethra