A&P Unit 4

Functions of the urinary system

Maintains homeostasis by managing the volume and composition of fluid reservoirs, primarily blood

List the major organs of the urinary system

kidneys, ureters, bladder, and urethra

Homeostatic Kidney Function

• Regulation of blood ionic composition- Na+, K+, Cl-

• Regulation of blood pH-, H+, HCO₃-

• Regulation of blood volume - H₂O

• Regulation of blood pressure

• Maintenance of blood osmolarity

• Production of hormones: calcitrol and erythropoietin

• Excretion of metabolic wastes and foreign substances (drugs or toxins)

• Regulation of blood glucose level

Renal/Kidney anatomy

retroperitoneal, partially protected by the lower ribs

External layers of connective tissue (superficial to deep) of the kidney

• Renal Fascia: anchors to other structures

• Adipose capsule: protects and anchors

• Renal capsule: continuous with ureter

Renal hilum

• Indented area of kidney

• Entrance for 

  • Renal artery

  • Renal vein

  • Ureter

  • Nerves

  • Lymphatics 

Internal renal anatomy

• Renal cortex: outer layer

• Renal medulla: inner region

• Renal pyramids: secreting apparatus and tubules

• Renal columns: anchor the cortex

Discuss the function of nephrons

• Functional unit of the kidney

• A tiny funnel with a long winding stem

• Over a million in each kidney

  • Filter 45 gallons of blood a day

Renal corpuscle

1. Glomerulus

2. The glomerular (Bowman’s) capsule

Glomerulus

a mass of capillaries that is fed by the afferent arteriole and drains into the efferent arteriole

glomerular (Bowman’s) capsule

1. Has a visceral layer of podocytes, which wrap around the capillaries

   1. Filtrate is collected between the visceral and parietal layers

   2. The glomerular endothelial cells have large pores (fenestrations) and are leaky

   3. Basal lamina lies between endothelium and podocytes

   4. Podocytes form pedicels, between which are filtration slits

Cortical nephrons

• 80-85% of nephrons

• Renal corpuscle in outer portion of cortex

• Short loops of Henle extend only into outer region of medulla

• Create urine with osmolarity similar to blood

Juxtamedullary nephrons

• 15% of nephrons

• Renal corpuscle deep in cortex with long nephron loops

• Receive blood from peritubular capillaries and vasa recta

• Ascending limb has thick and thin regions

• Enable kidney to secrete very concentrated urine

Juxtaglomerular Apparatus

• The ascending loop contacts the afferent arteriole at the macular dense

• The wall of the arteriole contains smooth muscle cells called juxtaglomerular cells

  • The apparatus regulates blood pressure in the kidney

• When blood pressure is low, juxtaglomerular (JG) cells release renin

  • Activates the renin-angiotensin-aldosterone (RAAS) mechanism which increases BP bodywide

Urine formation

• Nephron increases the surface area for urine processing and allows time for the different stages of urine production

  • Filtration

  • Reabsorption

  • Secretion 

Urine formation formula

Excretion of urine = glomerular filtration + secretion - reabsorption

Glomerular filtration

• Driven by blood pressure’opposed by capsular hydrostatic pressure and blood colloid osmotic pressure

  • Glomerular blood hydrostatic pressure is higher than capsular hydrostatic pressure and blood colloid osmotic pressure leading to an overall positive net filtration pressure (GBHP-CHP-BCOP)

• Water and small molecules move out of the glomerulus

• In one day, 150-180 liters of water pass out into the glomerular capsule

Glomerular filtration rate

Amount of filtrate formed by both kidneys each minute

Homeostasis requires kidneys to maintain a relatively constant GFR

• Too high - substances pass too quickly and are not reabsorbed

• Too low - nearly all reabsorbed and some waste products not adequately excreted

How is glomerular filtration rate regulated

• Controlled by renal autoregulation

  • Myogenic mechanism: increased BP stretches the afferent arteriole

    • Stretched smooth muscle in the wall contracts causing vasoconstriction of afferent arteriole

    • Less blood enters the glomerulus

    • GFR decreases

  • Tubuloglomerular feedback: rapid delivery of electrolytes due to high blood pressure

    • Macula densa detects high NaCl

    • Signals afferent arteriole to constrict

    • Blood flow into glomerulus decreases

    • GFR decreases back to normal

Tubular reabsorption

Much of the filtrate is reabsorbed, especially water, glucose, amino acids, and ions

Secretion

Secretion helps to manage pH and ride the body of toxic and foreign substances

Where does filtration occur?

Exclusively in the renal corpuscle, across the filtration membrane

Where does water reabsorption occur?

Primarily along the PCT and the descending limb of the nephron loop

Where does variable water reabsorption occur?

DCT and collecting system

Where does solute reabsorption occur?

The PCT, DCT, and collecting system

Reabsorption

• PCT is the major site of reabsorption: 65-70% of the filtrate is reabsorbed in PCT

  • Glucose and amino acids about 100% reabsorbed via active transport

  • NA + reabsorbed by active transport

  • Cl-, K+ and urea reabsorbed by passive transport (diffusion)

  • H2O passive reabsorption by osmosis

    • Obligatory water reabsorption - 90%: water follows the solutes that are reabsorbed

Reabsorption in the Loop of Henle

• Descending limb - continues water reabsorption

  • The wall is highly permeable to water, water leaves the tubule by osmosis

• Ascending limb - solute reabsorption only 

  • Relatively impermeable to water, especially the thick limb

    • Little or no obligatory water reabsorption

• Na+-K+-2Cl^- symporters

  • Active transport reabsorption in thick ascending limb

Reabsorption in DCT Collecting Duct

• DCT and collecting ducts are sites of regulated and variable reabsorption in the nephron

• PTH stimulates reabsorption of Ca²⁺ 

  • It also inhibits phosphate reabsorption in the PCT, enhancing its excretion

• Aldosterone stimulates Na⁺-Cl^- symporters reabsorb ions

  • It also stimulates K+ secretion

• Facultative water reabsorption - 10%

  • Regulated by ADH

• Reabsorption and secretion of HCO₃-, secretion of H+

Urine concentration

• Fluid intake is highly variable

• Homeostasis requires maintenance of fluid volumes within specific limits

• Urine concentration varies with ADH

• High intake of fluid results in dilute urine of high volume

• Low intake of fluids results in concentrated urine of low volume

Formation of Dilute Urine

• Glomerular filtrate and blood have the same osmolarity at 300mOsm.Liter BUT

• Tubular osmolarity changes due to a concentration gradient in the medulla

• When dilute urine is formed, osmolarity in the tubule

  • Increases in the descending limb

  • Decreases in the ascending limb

  • Decreases more in the collecting duct

• Thick ascending limb

  • Low water permeability

  • Symporters actively reabsorb Na⁺, K⁺, Cl^- 

  • Solutes leave, water stays in tubule

• Collecting duct

  • Low water permeability in absence of ADH

  • Water stays in tubule, large volume of dilute urine produced

Formation of Concentrated Urine

• Juxtamedullary nephrons with long loops form concentrated urine

• Osmotic gradient is created by the countercurrent multiplier

  • Solutes pumped out of ascending limb, but water stays in tubule

  • Medulla osmolarity is increased in vertical direction

• In presence of ADH, collecting ducts become very permeable to water

  • Water leave tubules by osmosis

  • Urine becomes very concentrated

Summary of Renal Function Step 1: Glomerulus

Filtrate produced at renal corpuscle has the same composition as blood plasma (minus plasma proteins)

Summary of Renal Function Step 2: Proximal Convoluted Tubule (PCT)

• Reabsorption active removal of ions and organic substrates 

  • Produces osmotic water flow out of tubular fluid

  • Reduces volume of filtrate

Summary of Renal Function Step 3: PCT and Descending Limb

• Water moves into peritubular fluids, leaving highly concentrated tubular fluid

  • Reduction in volume occurs by obligatory water reabsorption

Summary of Renal Function Step 4: Thick Ascending Limb

Tubular cells actively transport Na+ and Cl- out of tubule

Summary of Renal Function Step 5: DCT and Collecting Ducts

• Final adjustments in composition of tubular fluid

• Exposure to ADH determines final urine concentration

Summary of Renal Function - Urine Production

Ends when fluid enters the penal pelvis

ADH

• Anti-diuretic hormone is released by the posterior pituitary

• This hormone will increase water content in the blood and decrease the water content in the urine

• As a result of water being reabsorbed, blood volume will increase

Normal Urine Composition

• Normal urine is mostly water with dissolved waste products and electrolytes

• 95% water

• Nitrogen wastes:

  • Urea-most abundant solute

  • Uric acid 

  • Creatine

• Electrolytes - H+ ions, sodium, potassium, calcium

• Toxins - from bacteria, drug traces

• Pigments - urochromes from RBC breakdown

• Hormones

Abnormal constituents of urine

• Routine urinalysis primarily evaluates for the presence of abnormalities in the urine:

  • Albumin

  • Glucose

  • Red blood cells

  • Ketone bodies

  • Microbes

Urine Transportation and Storage

• Ureters transport urine from a renal pelvis by peristaltic waves

• No anatomical valve at the opening of the ureter into bladder

  • When bladder fills, it compresses the opening and prevents backflow

• During urination:

  • Detrusor muscle contracts

  • Internal urethral sphincter relaxes (involuntary)

  • External urethral sphincter relaxes (voluntary control)

Bladder

• Distensible, muscular organ with a capacity averaging 700-800 mL

  • Responsible for storage and elimination of urine

Trace filtrate from its point of formation to the renal pelvis

• Glomerular (Bowman's) Capsule 

•  Proximal Convoluted Tubule 

•  Loop of Henle (Descending then Ascending limbs) 

•  Distal Convoluted Tubule 

•  Collecting Duct 

•  Papillary Duct 

•  Minor Calyx 

•  Major Calyx 

•  Renal Pelvis

Parts of the nephron

a renal corpuscle (glomerulus and Bowman's capsule) and a renal tubule system (proximal convoluted tubule, loop of Henle, and distal convoluted tubule)

Trace urine outflow from renal pelvis to the exterior of the body

• Renal Pelvis 

•  Ureter 

•  Urinary Bladder 

•  Urethra 

•  External Urethral Orifice

In adults, body fluids make up between ___ of total body mass

55 and 65%

Body fluids are present in two main compartments

Inside cells (2/3) and outside cells (1/3)

Cytosol

intracellular fluids

Extracellular fluid

Interstitial fluid (80%) and blood plasma (20%)

Plasma membrane of cells

Separates intracellular fluid from interstitial fluid

Blood vessel walls

Divide the interstitial fluid from blood plasma

Capillary walls

Thin enough to allow exchange of water and solutes between blood plasma and interstitial fluid

Filtration, reabsorption, diffusion, and osmosis

Allow continuous exchange of water and solutes among body fluid compartments

The balance of inorganic compounds that dissociate into ions (electrolytes) is closely related to

fluid balance

The body gains water by

Ingestion and metabolic synthesis

The body loses water via

Urination, perspiration, exhalation, and in feces

Elimination of excess water occurs through

Urine production

Main factor determining body fluid volume

the amount of urinary salt loss

The two main solutes in urine

• Sodium ions (Na) 

• Chloride ions (Cl)

Wherever solutes go

water follows

3 major hormones control homeostasis of Na and Cl and water

• Angiotensin II

• Aldosterone

• Atrial natriuretic peptide (ANP)

Antidiuretic hormone (ADH)

The major hormone that regulates water loss

Explain the regulation of water and solute homeostasis by the following hormones: ADH

• Mechanism: Promotes insertion of water0channel proteins (aquaproin02) into apical membranes of principal cells in collecting ducts of kidneys. As a result, water permeability of these cells increases and more water is reabsorbed

• Effect: Reduces loss of water in urine

Explain the regulation of water and solute homeostasis by the following hormones: aldosterone

• Mechanism: by promoting urinary reabsorption of Na, increases water reabsorption via osmosis

• Effect: reduces loss of water in urine

Explain the regulation of water and solute homeostasis by the following hormones: ANP

• Mechanism: promotes natriuresis, elevated urinary excretion of Na, accompanied by water

• Effect: increases loss of water in urine

When the extracellular fluid is ___ to the cells of the body, they do not shrink or swell

isotonic

Changes in the osmolarity (or dehydration or overhydration) of the extracellular fluids

Can cause the cells to shrink or swell

Functions of electrolytes

• Control osmosis of water between fluid compartments

• Help maintain the acid-base balance

• Carry electrical current

• Serve as cofactors

The concentration of ions is expressed in

Units of milliequivalents per liter (mEq/liter)

Blood plasma, interstitial fluid, and intracellular fluid

Have different concentrations of electrolytes and protein ions

Blood plasma contains ___ protein ions; interstitial fluid contains ____

Many; only a few

Sodium

• Most abundant cations in extracellular fluid

• Used for impulse transmission, muscle contraction, fluid, and electrolyte balance

• Levels are controlled by aldosterone and ANP

Chloride

• The major extracellular anion

• Helps regulate osmotic pressure between compartments

• Forms HCl in the stomach

• Regulation of Cl balance is controlled by aldosterone

Potassium

• Most abundant cation in intracellular fluid

• Involved in fluid volume, impulse conduction, muscle contraction, and regulating pH

• Mineralocorticoids (mainly aldosterone) regulate the plasma level

Bicarbonate

• Important plasma ion

• Major member of the plasma acid-base buffer system

• Kidneys reabsorb or secrete it for final acid-base balance

Calcium

• Most abundant mineral in the body

• Structural component t of bones and teeth

• Used for blood coagulation, neurotransmitter release, muscle tone, excitability of nerves and muscles

• Level in plasma regulated by parathyroid hormone

Phosphate

• Occurs as calcium phosphate salt

• Used in the buffer system

• Regulated by parathyroid hormone and calcitriol

Magnesium

• An intracellular cation

• Activates enzymes involved in carbohydrate and protein metabolism

• Used in myocardial function, transmission in the CNS, and operation of the sodium pump

The pH of arterial blood ranges from

7.35 to 7.45

Mechanisms that maintain blood pH range

• Buffer systems

• Exhalation of CO2

• Kidney excretion of H: the proximal convoluted tubiels and collecting ducts of the kidney secrete H into the tubular fluid (urine)

Acid-Base Balance: Buffer systems

Most consist of a weak acid and its salt, which functions as a weak base. They prevent drastic changes in body fluid pH

Acid-Base Balance: Proteins

The most abundant buffers in body cells and blood. Hemoglobin inside RBC is good buffer

Acid-Base Balance: Carbonic acid-bicarbonate

Important regulator of blood pH. The most abundant buffers in extracellular fluid (ECF)

Acid-Base Balance: Phosphates

Important buffers in intracellular fluid and urine

Acid-Base Balance: Respiratory system

With increased exhalation of CO2, pH rises (fewer H+). with decreased exhalation of CO2, pH falls (more H+)

Acid-Base Balance: kidneys

Renal tubules secrete H+ into urine and reabsorb HCO2 so it is not lost in urine

Acidosis

blood pH is below 7.35

Alkalosis

blood pH is above 7.45

Respiratory acidosis

Blood pH drops due to excessive retention of CO2 leading to excess H2CO3

Respiratory alkalosis

Blood pH rises due to excessive loss of CO2 as in hyperventilation

Metabolic acidosis

Arterial blood levels of H+ increases, HCO3 falls

Metabolic alkalosis

Arterial blood levels of H+ falls, HCO3 rises

Male reproductive system

• Testes

• Sperm

• Accessory sex glands

• Scrotum

Testes

• Paired organs 

• sperm-producing male gonads

• lie within the scrotum partially covered by the tunica vaginalis

• Internal to the tunica vaginalis is a connective tissue capsule, the tunica albuginea that extends inward to form septa that create compartments

Seminiferous tubules

Carry sperm produced within them (spermatogenesis) out of the testes

Sperm is delivered to the body through system of ducts:

• Epididymis

• Ductus deferens

• Ejaculatory duct

• Urethra

Male accessory sex glands

seminal glands, prostate, bulbo-urethral glands

Seminal vesicles (glands)

Secrete an alkaline viscous fluid containing fructose, prostaglandins and clotting proteins

Prostate gland

Single gland that secretes slightly acidic fluid containing citric acid, proteolytic enzymes, acid phosphatase, and seminalplasmin