Urinary System Functional Anatomy

Urinary System Function

• excretion: sifting out metabolic wastes, water soluble toxins and drugs

• regulation of blood volume and osmolarity

  • osmolarity → concentration of solutes in molarity

  • regulate water and salt excretion

• regulation of blood ions

• regulation of long-term blood pH

• endocrine secretions

  • kidney erythropoietin → stimulate hematopoiseis → production of RBC

• conversion of vitamin D into an active form

Gross Anatomy of the Kidney

• paired, bean shaped organs that are located retroperitoneally in the superior lumbar region with hilum proximal to spine

• right kidney sits more inferiorly than left

Kidney’s coverings

• protected by three connective tissue layers:

  • Renal fascia (outermost): anchors kidney to surrounding structures.

  • Perirenal fat capsule: cushions the kidney.

  • Fibrous (renal) capsule (innermost): thin connective tissue surrounding the kidney.

• Parenchyma (tissues) within kidney has three distinct regions

  • cortex: outermost

    • contains renal corpuscles where blood filtration occurs

    • base of pyramid

  • Medulla: middle

    • rich in tubules where filtrate processing occurs

    • apex of pyramid

    • mature urine exits at the tip of the pyramid

  • Calyx: innermost

    • urine is collected and passed onto the ureters

    • tip of pyramid

    • lined with submucous to facilitate urine flow

• Pyramids seperated by renal column of bertin

4 processes

• Filtration of blood solutes to form filtrate

• Reabsorption of select solutes

• secretion of unwanted solutes

• excretion of formed urine from the filtrate

Hilum

medial indentation where vessels, nerves and ureter enter/exit

Blood vasculature and nerves

• sympathetic afferents and effertent used to mod vascular tone and BP via integration in hypothalamus

• rich vascularture and high blood flow to facilitate functions

• from abdominal aorta and drained by inferior vena cava

• 3 major renal processes

  • fine control of filtration pressure within renal corpuscle

  • first capillary bed is part of the filter

  • second capillary bed is in cortex or deep in medulla

Flow of Blood

1. Aorta

2. Renal Artery

3. Segmental Artery

4. Interlobar Artery

5. Arcuate Artery (Carry blood to glomeruli → filtering parts of nephrons) at corticomedullary junction

6. Cortical Radiate artery

7. Afferent arteriole

8. Glomerus capillaries

   1. ball of fenestrated capillaries

   2. filters blood plasma → urine formation

   3. cells + large proteins remain in blood but water ions enter the filtrate

9. Efferent Arterioles drains blood from the glomerular capillaries (BLUE FROM HERE)

10. Peritubular Capillaries or Vasa Recta

    1. Peritubular Capillaries: surround convoluted tubules, important for reabsorption and secretion

    2. Vasa Recta: long, straight capillaries that run from the cortex into the deep medulla than back to the cortex

11. Corticate Radial Veins

12. Arcuate Veins

13. Interlobar Veins

14. Renal Vein

15. Blood is then drained into the inferior vena cava (deoxygenated blood)

Four layers of ureter, bladder and urethra

• Mucosa → transitional epithelium

• Submucosa → connective tissue for support

• Muscularis → smooth muscle with multiple layers

• Adventitia → connective tissue connecting it in to suround tissue

Functional Anatomy of the Ureters, Bladder, and Urethra

Ureters

• muscular tubes connecting the renal pelvis to the bladder

• uses peristaltic waves to transport urine

• Olibquely enter the bladder to prevent backflow

Bladder

• hollow muscular sac, stores 500 mL of urine

• lined with transitional epithelium and detrusor muscle

  • smooth muscle in the bladder wall that contracts to expel urine during urination and relaxes to allow the bladder to hold urine

• Trigone: a triangular area between the ureteral openings and the urethra; a common infection site

• Empty → thick with mucus and submucus (due to rugae with contractions)

• Full → thin wall, no rugae, and mucus with 2-3 layers of thin epithelium

Urethra

• Conveys urine from bladder to outside.

• Internal: smooth muscle, involuntary

• External: skeletal muscle, voluntary

Male Urethra

Male

• three regions + semen and urine

• Prostatic urethra → passes thru prostate gland, receives semen from ejaculatory ducts, can be compressed into benign prostatic hyperplasia

• Membranous Urethra → connects prostatic to penilthe

  • short and passes through the urogenital diaphragm

  • surrounded by external sphincter

• Penile Urethra

  • runs through the corpus spongiosum of the penis

  • ends at the external urethral orifice at the tip of the penis

  • conveys both semen and urine out of the body

Female Urethra

• much shorter + urine only

• only carries urine

• opens anteriorly to vaginal opening

Micturition and Neural Control

• urination

• controlled by both involuntary and voluntary mechanism

• Involuntary (autonomic):

  • Parasympathetic: stimulates detrusor muscle contraction, internal sphincter relaxation.

  • Sympathetic: inhibits urination.

• Voluntary (somatic):

  • Involves external sphincter control via CNS.

• Stretch receptors in bladder from the detrusor muscle send signals to sacral spinal cord to expand

• Somatric response sent to pon and higher CNS centers for processing

  • Pons had two nuclei in control

    • storage and micturition center

    • storage → inhibit urination through autonomic and somatic efferents

    • micturition → activates urination through autonomic and somatic efferents

• Higher brain centers can override this reflex

Anatomy of Nephron

• Nephron: Functional Unit of Kidney

1. Renal Corpuscle → base of pyramid/cortex, filtration

2. Nephron → back and forth through pyramid, reabsorption and secretion

3. Collecting Duct → base to apex of pyramid, reabsorption and excretion

FLOW

1. Bowman's capsule (surrounds glomerulus)

2. Proximal convoluted tubule (PCT)

3. Descending thick

4. descending thin limb

5. loop of Henle

6. Ascending limb of loop

7. thin tubule

8. thick tubule

9. Distal convoluted tubule (DCT)

10. Collecting duct

• Types:

  • Cortical nephrons: short loops, mainly in cortex.

  • Juxtamedullary nephrons: long loops into medulla, important for urine concentration.

Functional Anatomy of Epithelial Cells in the Nephron

All are simple epithelium, specialized for transport:

• PCT: cuboidal with microvilli, high mitochondria → active reabsorption.

• Descending thin limb: squamous → passive water movement.

• Ascending thick limb & DCT: cuboidal, fewer microvilli, Na+/K+

• Transport mechanisms:

  • Primary active: Na+/K+ pumps.

  • Secondary active: symporters (Na+ with glucose, etc.).

  • Facilitated diffusion: via channels like aquaporins.

  • Paracellular transport: between cells via tight junctions.

Functional Anatomy of the Renal Corpuscle

renal corpuscle is the filtration unit of the nephron, located in the renal cortex. It is made of two main parts:

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1. Structure of the Renal Corpuscle

• It has two poles:

  • Vascular pole: Where the afferent arteriole enters and the efferent arteriole exits.

  • Urinary pole: Where the filtrate (urine) leaves the corpuscle and enters the proximal convoluted tubule.

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2. Glomerulus

• A tight ball of fenestrated capillaries.

• Blood enters via the afferent arteriole and exits via the efferent arteriole.

• The capillary walls are fenestrated (have pores) to allow plasma (minus cells and proteins) to pass through.

• These capillaries are covered in a diaphragm-like filtration barrier for size-selective filtration.

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3. Bowman’s (Glomerular) Capsule

Surrounds the glomerulus like a balloon and collects the filtrate.

It has two layers:

• Visceral Layer (inner):

  • Made of specialized cells called podocytes.

  • Podocytes have foot-like projections called pedicels that wrap around the glomerular capillaries.

  • The spaces between pedicels are called filtration slits.

  • Filtrate passes through these slits, aided by a shared basement membrane between podocytes and capillaries.

• Parietal Layer (outer):

  • Made of simple squamous epithelium.

  • Forms the outer wall of the capsule and plays a structural role.

• Capsular (urinary) space:

  • The space between the two layers.

  • This is where the filtrate collects before entering the nephron tubule.

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4. Mesangial Cells

• Found inside the glomerulus, between capillaries.

• Act like macrophages: they clean the basement membrane and help maintain filtration surface.

• May also help regulate capillary blood flow by contracting.

Functionally, what happens here?

• Filtration of blood begins in the glomerulus.

• Plasma (water + small solutes like glucose, Na⁺, urea) filters through:

  1. Fenestrated endothelium of glomerular capillaries.

  2. Basement membrane (shared between endothelium and podocytes).

  3. Filtration slits between podocyte pedicels.

• Cells and large proteins are too big to pass through → they stay in the blood.

• The resulting fluid, filtrate, enters the capsular space and moves to the proximal convoluted tubule.