BIO 201 Urinary and Reproductive Systems

Urinary System

Organs of excretion: composed of a pair of kidneys and the urinary tract.

• Bean-Like

• Left kidney is positioned between T₁₂ - L₃

• Right kidney is slightly inferior

• 11th and 12th ribs provide some protection for both kidneys

• Adrenal glands sit on top of the kidneys

Urinary Tract

Composed of:

• Pair of ureters

• Urinary bladder

• Single urethra

Pathway:

• Urine exits kidneys → ureters → urinary bladder (urine is stored) → urethra → exits the body

Urinary System Functions

1. Removal of metabolic wastes

2. Regulate fluid & electrolyte balance

3. Regulate acid-base balance

4. Maintenance of blood pressure

5. Regulation of erythropoiesis

6. Performing other metabolic functions

Removal of Metabolic Wastes

A function of the urinary system.

• Waste products in the blood are eliminated via urine

• Ex. urea, uric acid, creatinine, salts, etc

Regulate Fluid & Electrolyte Balance

A function of the urinary system.

• Regulate osmolarity

  • Osmolarity - number of particles in a solution

• Either conserve or eliminate water & electrolytes (Na⁺, K⁺, Ca²⁺)

Regulate Acid-Base Balance

A function of the urinary system.

• Assists in keeping blood neutral

• Long term regulation of blood pH by conserving or eliminating hydrogen (H⁺) & bicarbonate (HCO₃^-) ions

Maintenance of Blood Pressure

Function of the urinary system.

• Directly influenced by controlling blood volume and sodium excretion

• Secrete an enzyme (renin) that influences blood volume and peripheral resistance via the RAAS (renin-angiotensin-aldosterone-system)

Regulation of Erythropoiesis

Function of the urinary system

• Regulation of red blood cell production

• Release hormone erythropoietin (stimulates RBC production)

Other Metabolic Functions of the Urinary System

• Detoxifying substances in the blood

• Activating vitamin D via calcitriol production (promotes bone growth)

• Making new glucose (gluconeogenesis)

External Layers of the Kidneys

Each kidney held in place and protected by three layers of connective tissue, from superficial to deep:

• Renal Fascia

• Adipose Capsule

• Renal Capsule

Renal Fascia

Most superficial external layer of the kidneys

• Made of dense connective tissue

• Anchors each kidney to the peritoneum & muscles

Adipose Capsule

Middle external layer of the kidneys.

• Thickest layer

• Wedges each kidney in place

• Shields them from physical shock

Renal Capsule

Deepest external layer of the kidneys.

• Thin layer of dense irregular connective tissue

• Covers exterior of each kidney

• Protects kidneys from infection and physical trauma

Hilum

Opening on the medial surface of the kidneys where the renal artery, vein, nerves, and ureters enter and exit.

Renal Sinus

Cavity lined by the renal capsule.

• Anchors the ureter, blood vessels, & nerves in place

• Filled with urine-draining structures and adipose tissue

Internal Anatomy of the Kidneys

Three distinct regions of the renal sinus.

• Renal Cortex

• Renal Medulla

• Renal Pelvis

Renal Medulla

Region of the renal sinus within the kidneys.

• Houses cone-shaped renal pyramids

• Darker in color with fewer blood vessels

Nephrons

Functional unit of the kidneys.

• Over one million filtering apparatuses

• Found within the cortex and medulla

• Two main components:

  • Globe-shaped renal corpuscle

  • Long Renal Tubule

Renal Pyramids

Housed in the medulla of the kidney.

• Smooth muscle within walls of calyces and renal pelvis propel urine towards the ureter

• Tapers into a slender papilla → minor calyx → major calyx → renal pelvis → ureter

Blood Supply to Kidneys

Blood travels from the abdominal aorta → left and right renal arteries → kidneys

• Kidneys receive about ¼ of total cardiac output (25%)

  • ~1200 ml of blood per minute

• Renal arteries fan out into ever-smaller vessels

• Blood flows from the largest to the smallest arteries

  • 1. Renal Artery →

  • 2. Segmental Artery →

  • 3. Interlobar Artery →

  • 4. Arcuate Artery →

  • 5. Interlobular Artery →

  • 6. Afferent Arterioles →

  • 7. Glomerulus →

  • 8. Efferent Arterioles →

  • 9. Peritubular Capillaries (Second Capillary Bed) →

  • 10. Interlobular Veins →

  • 11. Arcuate Vein →

  • 12. Interlobar Vein →

  • 13. Renal Vein →

  • Inferior Vena Cava

Capillary Bed System of the Kidneys

Unusual system where arterioles both feed and drain capillaries.

Peritubular Capillaries

Form a network or plexus surrounding the renal tubule of each nephron.

Renal Corpuscle

Round-shaped structures found on nephrons.

• Responsible for filtering blood

• Composed of:

  • Glomerulus

  • Glomerular Capsule (Bowman’s Capsule)

Glomerulus

Group of looping fenestrated capillaries; extremely “leaky” or permeable

• Housed within the renal corpuscle

• Surrounded by Bowman’s capsule

Glomerular Capsule (Bowman’s Capsule)

Double-layered sheath of epithelial tissue surrounding the glomerulus.

• Consists of outer parietal layer and inner visceral layer

Podocytes

Have special extensions called foot processes, or pedicels.

• Surround glomerular capillaries, forming filtration slits.

Renal Tubule

The pipes of the nephron that collect and modify the filtrate (“proto-urine”).

• Three structurally and functionally distinct regions:

  • Proximal Tubule

  • Nephron Loop

  • Distal Tubule

Proximal Tubule

First and longest segment of the renal tubule.

• Walls made of simple cuboidal epithelial cells with prominent microvilli

  • Project into the tubule lumen to form the brush border

  • Greatly increases surface area of this region

Nephron Loop (Loop of Henle)

Middle segment that filtrate enters after the proximal tubule.

• Only part of renal tubule to dip into the renal medulla

• Consists of a descending and an ascending limb

Descending Limb

Thin limb of nephron loop that travels towards the renal medulla.

• Made of simple squamous epithelial cells

Ascending Limb

Once descending limb turns 180 degrees.

• Some nephrons have a thin ascending limb at the bend because the region is made of thin simple squamous epithelial cells

• Majority is made of simple cuboidal epithelial cells, and referred to as the thick ascending limb

Distal Tubule

The last segment of the renal tubule that filtrate passes through.

• Made of simple cuboidal epithelium without a brush border and very few microvili

• Filtrate will be known as proper urine after passing through this part

Juxtaglomerular Apparatus (JGA)

Composed of both macula densa and juxtaglomerular (JG) cells.

• Found at transition point between ascending limb of nephron loop and distal tubule

• Closely associated with the afferent/efferent arterioles

• Regulates blood pressure and glomerular filatration rate

Collecting Tubule

Where urine in nephrons drains into.

• Multiple of these empty into larger collecting ducts

Collecting Ducts

Numerous collecting tubules drain urine into these.

• Multiple of these structures drain into the renal papilla

Urine Drainage

• Renal Corpuscle →

• Proximal Convoluted Tubule →

• Nephron Loop (Loop of Henle) →

• Distal Convoluted Tubule →

• Collecting Tubule →

• Collecting Duct →

• Renal Papilla →

• Minor Calyx →

• Major Calyx →

• Renal Pelvis →

• Ureter →

• Urinary Bladder →

• Excreted through the Urethra

Types of Nephrons

• Cortical Nephrons

• Juxtamedullary Nephrons

• Classified based on:

  • Relative position of renal corpuscle in the cortex

  • Length of the nephron loop

Cortical Nephrons

• Oriented with renal corpuscles near the outer surface of the renal cortex

• Short nephron loop barely penetrates the renal medulla

• 85% of nephrons

Juxtamedullary Nephrons

• Renal corpuscles located near the border between the renal cortex and medulla

• Long nephron loop extends deep into the renal medulla

• Help establish salt concentration gradient in interstitial space

• Allows for regulation of urine concentration by antidiuretic hormone (ADH)

Capillary Types

Cortical Portion of Nephron Tubules

• Surrounded by peritubular capillary branches

Nephron Loop

• Surrounded by a ladder-like network of capillaries called the vasa recta

• Unique capillary structural arrangement allows juxtamedullary nephrons to control volume and concentration of urine

Physiological Proccesses Overview of the Kidneys

• Glomerular Filtration

• Tubular Reabsorption

• Tubular Secretion

Glomerular Filtration

Initial process of nephrons used to filter blood

• Circulating blood → glomerular capsule → proximal tubule

• The glomerular capillary filters blood content based on size

  • Large cells and proteins cannot enter

  • Smaller substances can enter from blood into the glomerular capsule, and become components of the filtrate

    • Water

    • Acids and Bases (H⁺ and HCO₃^- Ions)

    • Electrolytes (Na⁺ and K⁺ Ions)

    • Organic Molecules

    • Metabolic Wastes

• Filtration rate is controlled by pressure

Tubular Reabsorption

Substances are reabsorbed/reclaimed from filtrate back into the blood (in peritubular capillaries)

• Water, Glucose, Amino Acids, and Electrolytes

• The nephron can reabsorb the majority of filtered water and solutes from the proximal tubule and nephron loop

Tubular Secretion

Peritubular capillaries add substances to the filtrate in the tubule for excretion from the body.

• Occurs along the entire tubule

• Helps maintain

  • Electrolyte Homeostasis

  • Acid-Base Homeostasis

• Removes toxins from the blood that did not enter tubular fluid by filtration

Gomerular Filtration Membrane

Consists of three layers that collectively create a selectively permeable barrier

1. Fenestrated (leaky) glomerular capillary endothelial cells

   1. Prevent formed elements from passing through

2. Basal Lamina (extracellular contact between endothelial cells and podocytes)

   1. Prevent most proteins from passing through

3. Podocytes

   1. Prevent smaller molecules from passing through

Nitrogenous Wastes

Group of small substances that are readily filtered

• Urea and ammonium ions (NH₄⁺) from protein metabolism

• Creatinine - prodiced by enzyme creatine kinase in muscle

• Uric Acid - product of nucleic acid metabolism

Filtration Fraction

Percentage of plasma that becomes filtrate in capsular space

• Average about 20% - 1/5th of the plasma that enters the glomerulus becomes filtrate

Glomerular Filtration Rate (GFR)

Amount of filtrate formed by both kidneys in one minute

125 ml/min or equivalent of filtering all 3 liters of blood plasma about 60 times per day

Fluid Movement Driven By:

• Pressure Gradient

  • Difference between blood pressure in the glomerulus vs. fluid pressure of filtrate in the glomerular capsule

• Two Forces Generate Filtration Pressures:

  • Hydrostatic Pressure

  • Colloid Osmotic Pressure

Hydrostatic Pressure (HP)

The force of a fluid pressing on the wall of a container

• Equals to the blood pressure → tends to push water out of capillary into the interstitial space

Colloid Osmotic Pressure (COP)

Pressure generated by solutes, primarily albumin, in plasma.

• Osmotic gradient pulls water into capillaries by osmosis

  • The side with more solutes will pull water towards it

  • Blood plasma contains a high concentration of proteins

Net Filtration Pressure (NFP)

The interaction between hydrostatic and colloid osmotic pressures determines this and the direction in which water will flow.

• Water moves out of the capillary when HP is higher than the COP

• Water moves into the capillary when COP is higher than HP

Net Filtration Pressure (NFP) at Glomerulus

Overall pressure that drives fluid out of the blood and into the glomerular capsule. Determined by three forces:

• Glomerular Hydrostatic Pressure (GHP)

  • Tendency for fluid to be pushed out of the glomerular blood vessels

• Glomerular Colloid Osmotic Pressure (GCOP)

  • Tendency for fluid to be sucked into the capsule

• Capsular Hydrostatic Pressure (CHP)

  • Tendency for fluid in the capsule to be pushed back into the blood vessels

Affects of Glomerular Filtration Rate

• Internal Factors:

  • Autoregulation

• External Factors:

  • Hormonal

  • Neuronal

Autoregulation

Internal kidney mechanisms that work together to maintain GFR within a normal range.

• GFR is fairly constant despite normal blood pressure fluctuations

• Consists of two negative feedback processes:

  • Myogenic Mechanism

  • Tubuloglomerular Feedback

Myogenic Mechanism

One of the negative feedback processes of autoregulation of GFR.

• Contraction or relaxation of the smooth muscle of the afferent arteriole in response to stretch

  • Increased systemic blood pressure → stretches afferent arteriole → increases GFR → smooth muscle responds by constricting the afferent arteriole → reduces blood flow through the glomerulus → returns GFR back to normal

  • Decreased systemic blood pressure → stretches afferent arteriole less → reducing GFR → smooth muscle responds by relaxing → increasing blood flow through the glomerulus → causing an increase in GFR back toward normal range

Works best for systemic blood pressure changes between 80 and 180 mm Hg to rapidly restore GFR to normal

Tubuloglomerular Feedback (NaCl)

Another autoregulation mechanism of GFR.

• Involves the juxtaglomerular apparatus

  • Part of the negative feedback loop that controls pressure in the glomerulus in response to the NaCl (sodium chloride) concentration of filtrate

Tubuloglomerular Feedback Response to Increased GFR

As the glomerular filtration rate (GFR) increases → volume of the filtrate flowing through the renal tubule increases → more Na⁺ and Cl^- ions are absorbed into the macula densa

Macula Densa

• Cells release chemicals that tell the afferent arteriole to constrict

• Also signals juxtaglomerular cells to reduce the release of the hormone renin → reduction of angiotensin II → dilation of the efferent arteriole → a decrease in GFR towards normal

Tuboglomerular Feedback Response to Decreased GFR

Decreases in GFR → reduce Na⁺ and Cl^- ions absorbed by macula densa → triggers dilation of afferent arteriole and constriction of efferent arteriole → increases glomerular hydrostatic pressure to restore GFR

Hormonal Control of GFR

Hormonal effects on GFR are part of a larger system that involves regulation of systemic blood pressure and includes:

• Angiotensin II

• Natriuretic Peptides

Renin-Angiotensin-Aldosterone System (RAAS)

A system that primarily functions to stabilize overall systemic blood pressure

• By doing so, it also stabilizes GFR

• Responds to three conditions:

  • 1. Stimulation by the sympathetic nervous system

  • 2. Low glomerular hydrostatic pressure

  • 3. Stimulation from the macula densa

Atrial Natriuretic Peptide (ANP)

A hormone released by heart cells in the atria in response to increasing fluid volume → lowers blood volume and blood pressure to reduce the workload of the heart

• ANP increases → dilates afferent arterioles and constricts efferent arterioles → increases glomerular hydrostatic pressure → blood volume reduction by favoring fluid loss from the kidneys → reduces systemic blood pressure

Neural Regulation of GFR

Primarily involves the sympathetic division of the NS and the hormone norepinephrine (NE)

• Works with a larger system to control systemic blood pressure

• Increased sympathetic activity → increases release of NE → constriction of afferent arterioles like most systemic blood vessels → increases systemic blood pressure

Control of GFR

• Autoregulation

  • Myogenic Mechanism

    • Decreases GFR

  • Tuboglomerular Feedback

    • Decreases GFR

Hormonal

• Renin-Angiotensin-Aldosterone System (RAAS)

  • Primarily increases GFR

  • Decreases GFR at high levels of activation

• Atrial Natriuretic Peptide

  • Increases GFR

• Neural

  • Sympathetic Nervous System

    • Primarily decreases GFR

    • Increases at low levels of activation

Reabsorption in the Proximal Tubule

• Large % of electrolytes: sodium, potassium, chloride, sulfate, and phosphate ions

  • Ensures electrolyte homeostasis

• Almost 100% of nutrients, including glucose, amino acids, & other organic substances (ex., water-soluble vitamins, small proteins, and lactic acid)

  • Vital for nutrition purposes

• 90% of bicarbonate ions

  • Ensures acid-base homeostasis

• About 65% of filtered water

  • Ensures the body's fluid homeostasis

Secretion in the Proximal Tubule

• Hydrogen Ions (H⁺)

• Ammonium Ions (NH₄⁺)

• Creatinine (Metabolic Byproduct)

• Uric Acid (how humans get rid of most nitrogenous waste)

  • First half of tubule, most uric acid is reabsorbed

  • Nearly all of it is secreted back into the filtrate in the second half of the tubule

• Small amounts of Urea

  • Alternative means of getting rid of nitrogenous waste

• Drugs

  • Including Penicillin and Morphine

Gout (Clinical Connections)

A type of arthritis that is caused by the deposition of uric acid crystals in the joints

• Causes inflammation, pain, and immobility

• Kidneys play a crucial role in regulating the level of uric acid in the blood

• Risk Factors include

  • Obesity

  • High BP

  • Diabetes

  • Diet high in purines

  • Diet rich in red meat and organ meat

• Treatment

  • Medications

  • Lifestyle changes to reduce inflammation and prevent future attacks

• Left Untreated

  • Joint damage

  • Kidney damage

  • Development of tophi (swollen joints)

Reabsorption in the Nephron Loop

• 20% of H₂O

• 25% of Na⁺ & Cl^- ions

Filtrate throughout the Nephron

Proximal Tubule:

• Filtrate concentration is = interstitial fluid concentration

Nephron Loop:

• Water reabsorbed → filtrate becomes more concentrated

Distal Tubule:

• NaCl secreted into tubule cells → draws water into filtrate via osmosis → filtrate becomes less concentrated

Reabsorption in the Distal Tubule and Collecting System

• Cells with hormone receptors receive hormone signals to regulate water, electrolyte, and acid-base balance depending on the body’s needs

• 85% of H₂O is reabsorbed

• 90% of Na⁺ ions have been reabsorbed

Renal Failure

When renal function is greatly diminished or absent.

• Often from chronic disease affecting the glomerulus or small blood vessels

Dialysis

A treatment that can help remove waste products and excess fluid from the body when the kidneys are unable to do so on their own.

Kidney Transplant

In some cases, it may be a treatment option for renal failure.

• Involves surgically placing a healthy kidney from a donor into the recipient’s body to replace the failed kidney

Ureters

Part of the urinary tract.

• Contain smooth muscle cells that contract rhythmically (peristalsis) to propel urine toward the urinary bladder

• Begin at L2 → Travel behind the peritoneum → Empty into the urinary bladder

Urinary Bladder

A hollow, distensible organ found on the pelvic cavity floor that stores urine.

• Held in place by the parietal peritoneum

• Collapses when empty

• Pear-shaped when full

  • Holds 700-800 ml of urine in males and slightly less in females

Trigone

Triangular region on the bladder floor made of

• Two ureter openings with mucosal flaps

• Internal urethral orifice

Urethra

Final segment of the urinary tract.

• Histology is similar to the ureters, with the following exceptions:

  • The opening is surrounded by an internal urethral sphincter

  • The external urethral sphincter is formed by skeletal muscle

Internal Urethral Sphincter

Surrounds the opening of the urethra.

• Only opens when urine is passing through

External Urethral Sphincter

Allows for voluntary control of urination.

• Formed by skeletal muscle

Micturition

Also known as urination or voiding.

• Discharge of urine from the urinary bladder to the outside of the body.

• Time and training make this a voluntary process

Micturition Reflex

Reflex arc mediated by the parasympathetic nervous system when urine fills the bladder and stretches the walls.

• At about 500-600 ml → urge to urinate becomes too strong → voluntary control is lost

• Micturition center is found in the pons

Major Functions of the Male Reproductive System

• Produce / Maintain Sperm

• Transport sperm and protective fluid (semen)

• Dischrage sperm within the female reproductive tract during sex

• Produce male sex hormones

Major Functions of the Female Reproductive System

• Produce / Maintain Eggs

• Transport egg cell

• Protect and nourish the offspring until birth

• Produce female sex hormones

Similarities Between Male & Female Reproductive Systems

• Both have gonads (primary sex organs)

• Secrete sex hormones (testosterone & estrogen)

• Produce gametes (sex cells) through meiosis

  • Male → Sperm

  • Female → Ova / Egg

• Have accessory reproductive organs

  • Contribute to the functioning of the reproductive system

Human Life Cycle

• Sperm (haploid n=23) fertilizes the Egg (haploid n=23)

  • Produces a diploid zygote (2n = 46)

Diploid

Containing 46 chromosomes (2 pairs, 2n)

Haploid

Containing 23 chromosomes (1 pair, n)

Mitosis

Cell division of somatic cells (or body cells)

• Produces two genetically identical diploid daughter cells

• For purposes of tissue growth & repair

Meiosis

Cell division of germ cells (sex cells)

• Produces 4 genetically different haploid daughter cells

• For purposes of reproduction (including recombination)

Human Somatic Cells

All have a nucleus with 46 chromosomes (23 pairs) = diploid (2n)

Male Reproductive System

Consists of the penis, testis, and scrotum

• Accessory Glands include

  • Seminal Vesicle

  • Prostate Gland

  • Bulbourethral Gland

• Ducts

  • Ejaculatory Duct

  • Urethra

  • Ductus Deferens

  • Epididymis

Testes (Testicles)

Ovoid structures located outside of the abdominopelvic cavity in a saclike structure called the scrotum.

• Contain numerous lobules that contain tightly coiled loops called seminiferous tubules

• Innervated by the autonomic nervous system, but also contains a large number of pain and temperature receptors

• Functions Include

  • Sperm Production

  • Secretion of androgen hormones, primarily testosterone

Seminiferous Tubules

Tightly coiled loops within lobules of the testes.

• Contain two cell types:

  • Spermatogenic Cells

    • Sperm-Forming Cells

  • Sustentacular Cells (Sertoli Cells)

    • Support sperm production

    • Produce testicular fluid

Interstitial Cells (Leydig Cells)

Cells found between the seminiferous tubules

• Produce and secrete androgens, primarily testosterone, into the surrounding interstitial fluid

Myoid Cells

Muscle-like cells that surround seminiferous tubules

• Contract to push sperm and testicular fluid through the tubules

Epididymis

Where sperm is stored after production.

• Contains a head, body, and tail

• Sperm exits through the Ductus (vas) Deferans

Ductus Deferens (Vas Deferens)

Duct through which sperm cells exit the epididymis/testes.

• Begins at the tail of the epididymis

• Enters the abdominopelvic cavity through a fibrous tunnel (inguinal canal) → loops over the ureter on the posterior side of the bladder → joins the seminal vesicles → enters the prostatic urethra

Testicular Artery

Branch from the abdominal aorta that supplies blood to the testes.

Pampiniform Venous Plexus

Drains blood from the testes into the testicular veins

• Helps regulate the temperature of the testes for proper sperm production

Semen

Contains both lubricating and nourishing fluids for sperm.

• Produced by the seminal vesicles, prostate, and bulbourethral glands

• Alkaline (neutralizes the acidic pH of the female reproductive tract)

  • pH also helps make the sperm fully motile and allows them to begin the process of capacitation