Kidney Functions and Urinary Structures

Kidney Functions

• Kidneys perform excretory, endocrine, cardiovascular, and respiratory functions for the body
• Excretory function involves creating and concentrating water-soluble wastes, with non-water soluble wastes removed via feces
• Endocrine function involves secretion of and response to hormonal signals
• Cardiovascular function regulates blood pressure through blood volume and tonicity
• Respiratory function removes dissolved gases in the form of bicarbonate and H+ ions

Retroperitoneal Kidneys

• Retroperitoneal describes the kidneys' location posterior to the abdomen's peritoneum
• The peritoneum anchors the kidneys to the paired quadratus lumborum muscles

Functions of Urinary Structures

• Ureters are tubes extending from the kidneys to the bladder
• Ureters transport urine from the kidney hilum along the posterior abdominal wall to the bladder's posterior aspect
• The bladder is a hollow, distensible muscular pouch in the pelvis that holds urine until micturition
• Bladder location is posterior to the pubic symphysis in both sexes
• The urethra is the exit point of the renal system, through which urine passes to exit the bladder

Kidney Protective Structures

• Anteriorly, the kidneys are protected by the viscera (organs) of the abdomen
• Posteriorly, superior protection is provided by ribs 11 and 12, with inferior protection from the quadratus lumborum muscles

Common Kidney Stone Locations

• Kidney stones are commonly lodged where the renal pelvis narrows into the ureter when exiting the kidney
• Kidney stones may also be found where the ureter passes over the common iliac artery
• Another common location is where the ureter empties into the bladder

Bladder Structures and Functions

• Rugae are wrinkles of the inner mucosa that allow bladder expansion as it fills
• The detrusor muscle consists of three smooth muscle layers forming the muscularis layer of the bladder
• Urethral openings are curtain-like and allow urine to enter the bladder from the bilateral ureters
• The trigone is a smooth area of bladder mucosa with an opening at each point of the triangle
• The internal urethral sphincter keeps urine in the bladder
• The urethra provides a path for urine from the bladder to the outside world
• The external urethral sphincter prevents urinary incontinence

Nervous Control of Urethral Sphincters

• The internal urethral sphincter is smooth muscle under autonomic (involuntary) control
• The external urethral sphincter is skeletal muscle under somatic (voluntary) control

Micturition

• Micturition is the excretion of urine from the bladder into the urethra

Nervous Control of Micturition

• Parasympathetic stimulation contracts the detrusor muscle and relaxes the internal urethral sphincter
• Absence of somatic neural impulse results in relaxation of the external urethral sphincter

Sacral Spinal Reflex Pathway of Micturition

• Stretch receptors in the bladder mucosa send signals to the sacral region of the spinal cord
• The spinal cord's reflexive response causes relaxation of the internal urethral sphincter (smooth muscle)
• A somatic response from the CNS travels down the spinal cord, relaxing the external urethral sphincter (skeletal muscle)

Goal of Toilet Training

• Toilet training aims to control the voluntary external urethral sphincter to maintain urinary continence

Clinical Relevance of Urethra Length

• The male urethra (20 cm) is much longer than the female urethra (4 cm)
• A shorter female urethra allows pathogens to move up the urethral lumen, causing UTIs

Clinical Relevance of Urethral Segments in Males

• Male urethral segments (proximal to distal): prostatic, membranous, and spongy urethra
• The membranous urethra is most likely to be damaged during catheterization due to its location and structure

Kidney Hilum Structures

• The ureter, nerves, blood vessels, and lymphatic vessels pass through the hilum of the kidney

Layers of Connective Tissue Surrounding the Kidney

• Renal Fascia, Adipose Capsule, and Renal Capsule from superficial to deep
• The triple layering of connective tissue provides additional protection to these highly vascular organs
• The posterior aspect of the kidneys has little skeletal protection

Urine Pathway

• Urine travels from the renal papillae to the bladder
• The pathway is Renal Papillae, Minor Calyx, Major Calyx, Renal Pelvis, Ureter, and then the Bladder

Renal Blood Supply

• Blood is supplied to the kidneys beginning with the abdominal aorta, it follows this course; Renal Artery, Segmental Arteries, Interlobar Arteries, Arcuate Arteries, Cortical Arteries, Afferent Arterioles and then the Glomerular Capillary

Nephron Structures

• Afferent Arteriole: carries oxygen-rich blood to the glomerulus
• Glomerulus: filters substances from blood plasma
• Efferent Arteriole: carries oxygen-rich blood from the glomerulus to the peritubular capillary
• Peritubular Capillaries: the capillary bed surrounding nephron portions for gas and nutrient exchange

Kidney Blood Flow Arrangement

• The kidney has a unique blood flow arrangement with two capillary beds in series
• Blood flows through the glomerular capillary for filtrate production, then the peritubular capillary for gas exchange
• After both capillary beds, blood returns to the heart

Kidney Functional Unit

• The nephron is the kidney's functional unit
• Nephrons control urine composition for water and ion concentrations

Nephron Actions

• Glomerular Filtration: substances move from the blood into the glomerulus
• Tubular Reabsorption: fluid and solutes are reabsorbed, returning to the blood
• Tubular Secretion: substances move from the blood into the filtrate

How Nephrons Handle Proteins and Glucose

• Proteins aren't filtered from the blood in the glomerulus
• Glucose is filtered from the blood into the glomerulus and always reabsorbed completely
• The presence of glucose or proteins in urine indicates pathology

Juxtamedullary vs. Cortical Nephrons

• Juxtamedullary: glomerulus close to boundary of renal cortex & medulla, long nephron loops, supplied by vasa recta, maintains osmotic gradient in medulla
• Cortical: primarily in renal cortex, short nephron loops, supplied by peritubular capillaries, most common

Nephron Histology

• Histology in nephron segments directly corresponds to the segment's function

Cell Types and Function of the Nephron

• Glomerular Capsule: for filtration
• Simple Squamous Epithelium: permits diffusion required for filtration
• Podocytes: form filtration slits via modified epithelial cells around glomerular capillaries
• Mesangial Cells: offer structural support and regulate filtration
• Proximal Convoluted Tubule (PCT): for reabsorption of water & nutrients
• Cuboidal Cells with Brush Border Microvilli: cuboidal cells permit more filtration because the microvilli increase the surface area
• Descending Nephron Loop: reabsorption of water from filtrate
• Simple Squamous Epithelium: permeable to water
• Ascending Nephron Loop: reabsorption of Na+ and Cl- ions from filtrate
• Cuboidal (proximal) → Columnar (distal): permeable to salts
• Distal Convoluted Tubule (DCT): selective secretion & reabsorption of ions
• Principal Cells with Intercalated Discs: responds to hormones; maintain salt & water balance
• Collecting Duct: reabsorbs solutes and water to produce the final urine product
• Principal Cells with Intercalated Discs: responds to hormones; maintain salt & water balance

Juxtaglomerular Apparatus

• The juxtaglomerular apparatus is at the transition between the ascending limb of the nephron loop and the distal tubule
• Communication between these segments regulates blood pressure and GFR

Juxtaglomerular Apparatus Cell Types

• Macula Densa Cells: sensory cells that detect the filtrate's salinity
• Juxtaglomerular Cells: modified smooth muscle cells in arterioles; receive signals from macula densa cells
• Juxtaglomerular cells secrete renin and change the diameter of the afferent arteriole

Filtrate Production

• Filtration occurs in the glomerulus
• Blood flows into the glomerulus via the afferent arteriole
• Water, ions, and metabolic wastes (urea) are filtered into the renal tubule
• Filtrate is the fluid in the nephron
• Blood leaves the glomerular capillary via the efferent arteriole

Pressures Influencing Filtrate Formation

• Blood Hydrostatic Pressure: pressure created by water in the blood; favors filtrate formation; strongest pressure
• Blood Osmotic Pressure: pressure created by proteins in the blood; opposes filtrate formation
• Capsular Hydrostatic Pressure: pressure created by water in the glomerular capsule; opposes filtrate formation

Afferent Arteriole Diameter and Filtrate Formation

• Net Filtration Pressure overall pressure in glomerular capsule indicates the amount of filtration occurring
• Dilation of the afferent arteriole increases blood hydrostatic pressure, leading to a higher volume of filtrate
• Constriction of the afferent arteriole decreases blood hydrostatic pressure, leading to a small volume of filtrate
• More blood into the glomerular capillary = more filtrate created

Elements That Control Filtration

• Fenestrated Squamous Cells: prevent filtration of large formed elements
• Basal Lamina: prevents the filtration of moderate-sized elements
• Filtration Slits Overlying Podocytes: creates the space through which filtration occurs

Glomerulus Features for Filtration

• Surface Area: large surface area of glomerular capillaries for filtration
• Thin Endothelial Cells: allows simple osmosis and diffusion
• Diameter of Afferent Arteriole > Efferent Arteriole: increases hydrostatic pressure

Substances in Urine

• Should be in Urine: Ions, Protons (H+), Urea, Some hormones
• Should not be in urine: Glucose, Insulin, Cholesterol, CO2, Blood, Ketones, Bile Pigments, Proteins

Glomerular Filtration Rate

• Glomerular Filtration Rate (GFR) is the amount of filtrate formed per minute by the kidneys
• GFR must be regulated to maintain water and solute balance
• If GFR is too high, hypotonicity of the blood results. If GFR is too low, hypertonicity results
• Glomerular Blood Pressure helps to adjust the GFR

Mechanisms of GFR Control

• Renal Autoregulation: local reflex loop in which nephrons adjust blood flow to their glomerulus
• Neural Regulation: sympathetic stimulation reduces blood flow via afferent arteriole vasoconstriction
• Hormonal Regulation: biochemical cascade triggered by substantial changes in blood pressure

Situations Where GFR Would Change

• Dehydration, Hemorrhage, and Exercise

Aldosterone

• Stimulate Release: Decrease in blood pressure
• Function: regulation of Na+ level in the blood
• The secretion of aldosterone increases sodium concentration in the blood

Antidiuretic Hormone (ADH- aka Vasopressin)

• Stimulate Release: Increased tonicity of blood
• Function: Decrease tonicity of blood by increasing water/Na+ retention

Atrial Natriuretic Peptide (ANP)

• Stimulate Release: increased mechanical stretch within the atria of the heart
• Function: Decrease blood pressure by signaling kidney to increase water excretion

RAAS Steps

• Systemic blood pressure decreases, causing GFR to decline
• Juxtaglomerular cells release renin in response to decreased GFR; macula densa cells detect decreased blood flow through the afferent arteriole
• Renin converts Angiotensinogen to Angiotensin I; travels through the blood until it encounters the liver
• ACE converts Angiotensin I to the active Angiotensin II; ACE is produced in the lungs

Functions of Active Angiotensin II

• Vasoconstriction of the efferent arteriole increases blood hydrostatic pressure

Functions of Angiotensin II

• Vasoconstriction of systemic blood vessels increases blood pressure
• Reabsorption of Na+ and Cl- ions from the Proximal Tubule increasing blood volume
• Promotes aldosterone release
• Stimulates thirst

ACE and RAAS Function

• ACE removes amino acids from Angiotensin I to produce Angiotensin II
• RAAS restores blood pressure

Blood Volume and Blood Pressure

• High blood pressure = large blood volume
• Low blood pressure = small blood volume

Hormones Released

• With high blood pressure, ANP is released
• With low blood pressure, ADH and Aldosterone is released

CO2 & pH in the Blood

• Increased CO2 partial pressure = Increased H+ ion concentration = Decreased pH value = Blood more Acidic
• The bicarbonate buffer system maintains the pH of the blood.
• Kidneys excrete ions to shift the equilibrium and pH value and the concentration of H+ are inversely related

Blood Too Acidic

• Kidneys increase excretion of excess H+ ions in urine & retain HCO3-

Blood Too Basic

• Kidneys increase retention of H+ ions & excrete excess HCO3-

Production of Urine

• Tubular reabsorption & secretion processes are essential
• Reabsorption: into blood from filtrate
• Secretion: into filtrate from blood

Nephron Segment Functions

• Proximal Tubule: Reabsorbs water, glucose, amino acids, ions, Secretes H+, wastes
• Descending Limb of Nephron Loop: Reabsorbs water, Secretes nothing
• Ascending Limb of Nephron Loop: Reabsorbs salts
• Distal Tubule: Reabsorbs water, ions, Secretes ions
• Collecting Duct: Reabsorbs water, Na+, Cl-, Secretes H+

Countercurrent Flow

• The overall function conserves water to maintain homeostasis
• Water moves out of the filtrate, producing a hypertonic filtrate
• Filtrate entering the ascending limb of the nephron loop is hypertonic, causing the reabsorption of Na
• The flow allows maintenance of the concentration gradient