Renal/Urinary System Functions

• The renal/urinary system has four basic functions: excretory, endocrine, cardiovascular, and respiratory.
• Excretory function involves the creation and concentration of water-soluble wastes, while non-water soluble wastes are removed via feces.
• The endocrine function includes the secretion of and response to hormonal signals.
• Cardiovascular function regulates blood pressure via regulation of blood volume and tonicity.
• The respiratory function removes dissolved gases in the form of bicarbonate and H+ ions.
• Retroperitoneal refers to the kidneys' location posterior to the peritoneum of the abdomen, with the peritoneum anchoring the kidneys to the paired quadratus lumborum muscles.

Functions of the Urinary Structures

• Ureters: Tubes extending from the kidneys to the bladder, they carry urine from the hilum of the kidney along the posterior abdominal wall, into the posterior aspect of the bladder
• Bladder: A hollow, distensible muscular pouch in the pelvis that holds urine until micturition. Located just posterior to the pubic symphysis in both males and females.
• Urethra: The exit point of the renal system, which allows urine to exit the bladder.

Anterior and Posterior Protective Structures of the Kidney

• Anterior protection is provided by the viscera (organs) of the abdomen.
• Posterior protection includes the 11th and 12th ribs for superior protection and the quadratus lumborum muscles for inferior protection.
• Kidney stones are most commonly lodged in the renal pelvis as it narrows into the ureter, as the ureter passes over the common iliac artery, and as the ureter empties into the bladder.

Bladder Structure and Function

• Rugae: Wrinkles of the inner mucosa that allow for the expansion of the bladder as it fills with urine
• Detrusor Muscle: Three layers of smooth muscle that make up the muscularis layer of the bladder
• Urethral Openings: Curtain-like openings allowing urine to enter the bladder from the bilateral ureters.
• Trigone: Smooth area of the bladder mucosa with an opening at each point of the triangle.
• Internal Urethral Sphincter: Keeps urine in the bladder.
• Urethra: Provides a path of urine from the bladder to the outside world.
• External Urethral Sphincter: Prevents urinary incontinence.

Nervous System Control of Urethral Sphincters

• Internal Urethral Sphincter: Smooth muscle under autonomic (involuntary) control.
• External Urethral Sphincter: Skeletal muscle under somatic (voluntary) control.
• Micturition: Excretion of urine from the bladder into the urethra.

Nervous System Control of Micturition

• Parasympathetic stimulation results in contraction of the detrusor muscle and relaxation of 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 within the mucosa of the bladder send signals to the sacral region of the spinal cord.
• A reflexive response from the spinal cord causes relaxation of the internal urethral sphincter (smooth muscle).
• Somatic response from the CNS travels down the spinal cord and causes relaxation of the external urethral sphincter (skeletal muscle).
• Toilet training aims to learn to control the external urethral sphincter muscle in order to maintain urinary continence.

Clinical Relevance of Urethra Length

• The male urethra is much longer than the female urethra (20 cm vs 4 cm).
• The shorter female urethra allows pathogens to easily move up the urethral lumen, into the bladder, causing a Urinary Tract Infection (UTI).

Clinical Relevance of Urethral Segments in Males

• The three urethral segments in the male, from proximal to distal, are the prostatic urethra, membranous urethra, and spongy urethra.
• During catheterization in the male, the middle segment (membranous urethra) is most likely to be damaged due to its location and structure.
• The structures that pass through the hilum of the kidney are: Ureter, nerves, blood vessels, and lymphatic vessels.

Connective Tissue Layers Encapsulating the Kidney

• From superficial to deep: Renal Fascia - Adipose Capsule - Renal Capsule.
• The triple layering of connective tissue layers provides the required additional protection to these highly vascular organs due to slight skeletal protection.
• Pathway of urine from the renal papillae to the bladder: Renal Papillae - Minor Calyx - Major Calyx - Renal Pelvis - Ureter - Bladder.
• Branching pattern of renal blood supply beginning at the abdominal aorta: Abdominal Aorta - Renal Artery - Segmental Arteries - Interlobar Arteries - Arcuate Arteries - Cortical Arteries - Afferent Arterioles - Glomerular Capillary.

Nephron Structure Functions

• Afferent Arteriole: branch of cortical artery carrying oxygen rich blood to the glomerulus
• Glomerulus: fenestrated capillary allowing for adequate filtration of substances from the plasma of the blood
• Efferent Arteriole: carry oxygen rich blood out of the glomerulus, to the peritubular capillary
• Peritubular Capillaries: capillary bed surrounding portions of the nephron where gas & nutrient exchange occurs
• The blood flow within the kidney is unique because there are two capillary beds in series.
• The nephron carefully controls the composition of the urine with regards to water and ion concentrations.

Parts of the Nephron

• Proximal convoluted tubule.
• Glomerulus and capsule.
• Distal convoluted tubule.
• Descending limb of nephron loop.
• Ascending limb of nephron loop.
• Collecting duct.

Three Principle Actions of the Nephron

• Glomerular Filtration: substances move from the blood into the glomerulus to produce the initial filtrate.
• Tubular Reabsorption: fluid & solutes are reabsorbed from the filtrate, returning to the blood.
• Tubular Secretion: substances move from the blood into the filtrate (occurs in segments of the nephron other than the glomerulus).
• Proteins are never filtered from the blood in the glomerulus.
• Glucose is filtered from the blood into the glomerulus but is always completely reabsorbed.
• Presence of either glucose or proteins in the urine are signs of pathology (diabetes for glucose & renal failure for proteins).

Juxtamedullary vs. Cortical Nephrons

• Juxtamedullary Nephrons: Glomerulus close to boundary of renal cortex & medulla, long nephron loops, supplied by unique capillaries called the vasa recta, maintains osmotic gradient in the medulla.
• Cortical Nephrons: Primarily in the renal cortex, short nephron loops, supplied by peritubular capillaries, most common nephron in human kidneys.

Significance of Histology in the Nephron

• The types of cells present in each segment of the nephron directly corresponds to the function of that segment.
• Glomerular Capsule: filtration of the blood w/ simple squamous epithelium (permits diffusion) & podocytes (form filtration slits), and mesangial cells (provide support & regulate filtration).
• PCT: reabsorption of water & nutrients w/ cuboidal cells & brush border microvilli.
• Descending nephron loop: reabsorption of water; simple squamous epithelium.
• Ascending nephron loop: reabsorption of Na+ & Cl- w/ cuboidal/columnar cells (permeable to salts).
• DCT: selective secretion & reabsorption of ions w/ principal cells & intercalated discs.
• Collecting duct: reabsorbs solutes & water w/ principal cells & intercalated discs.
• The juxtaglomerular apparatus is located at the transition point between the ascending limb of the nephron loop & the distal tubule and permits regulation of blood pressure and GFR.

Juxtaglomerular Apparatus: Cell Types

• Macula Densa Cells: sensory cells; detects salinity of the filtrate
• Juxtaglomerular Cells: modified smooth muscle cells; receive signals, secrete renin, and control blood entering the glomerular capillary.
• Filtration is the first process of the kidney and occurs within the glomerulus.
• The fluid in the nephron is the filtrate, remaining blood leaves via the efferent arteriole.
• Glomerular filtration is influenced by Blood Hydrostatic Pressure (favors), Blood Osmotic Pressure (opposes), and Capsular Hydrostatic Pressure (opposes).
• Net Filtration Pressure indicates the amount of filtration occuring and is influenced by afferent arteriole diameter.
• The three elements of the glomerular membrane that control filtration are fenestrated squamous cells, basal lamina, and filtration slits overlying podocytes.

Features that Allow for Filtration of Large Volumes of Fluid

• Large Surface Area allows for sufficient filtration opportunity.
• Thin Endothelial Cells allow for diffusion and osmosis.
• Diameter of Afferent Arteriole > Efferent Arteriole increases blood hydrostatic pressure.
• The substances that should be in the urine are ions, protons, urea, and some hormones.
• The substances that should NOT be in the urine are glucose, insulin, cholesterol, CO2, blood, ketones, bile pigments, and proteins.
• Glomerular Filtration Rate (GFR): the amount of filtrate formed per minute by the combined function of both kidneys.
• The GFR is responsible for the adjustment of the glomerular blood pressure.
• GFR must be regulated to maintain water & solute balance within the blood.
• Local reflex loop in which nephrons adjust their blood flow.

Three Mechanisms of GFR Control

• Renal Autoregulation: the nephrons adjust the blood flow they allow to their glomerulus.
• Neural Regulation: sympathetic stimulation reduces blood flow to the kidneys to decrease the GFR.
• Hormonal Regulation: cascade triggered by changes in blood pressure.
• GFR changes during dehydration, hemorrhage and exercise.
• Aldosterone: stimulated by a decrease in blood pressure, and it regulates sodium levels in the blood by increasing its retention in the kidneys.
• Antidiuretic Hormone (ADH- aka Vasopressin): stimulated by the increased tonicity of blood, and it decreases blood tonicity by increasing water/Na+ retention
• Atrial Natriuretic Peptide (ANP): stimulated by increased mechanical stretch of the heart; decreases blood pressure by increasing water excretion.

Renin Angiotensin Aldosterone System (RAAS) steps

• Systemic blood pressure decreases, causing GFR to decline.
• Juxtaglomerular cells release renin and Macula densa cells detect the decrease in blood flow through the afferent arteriole.
• Renin converts Angiotensinogen to Angiotensin I.
• ACE converts Angiotensin I to Angiotensin II.
• The functions of Angiotensin II include: vasoconstriction of the efferent arteriole, vasoconstriction of systemic blood vessels, reabsorption of Na+ and Cl- ions from the Proximal Tubule, promotes aldosterone release, and stimulates thirst.
• ACE converts Angiotensin I to Angiotensin II.
• RAAS is stimulated by drop in BP & restores blood pressure by reducing water loss.
• High blood pressure = large blood volume; low blood pressure = small blood volume.
• ANP will be released when blood pressure is too high.
• ADH and Aldosterone will be released when blood pressure is too low.

Blood Chemistry & Urine Production

• Increased CO2 partial pressure = Increased H+ ion concentration, meaning decreased pH value.
• The bicarbonate buffer system is responsible for maintain the pH of blood and the kidneys excrete involved ions in order to shift the equilibrium.
• High CO2 in the blood = acidic blood, kidneys increase excretion of excess H+ ions & retain HCO3- to increase pH.
• Low CO2 in the blood = basic blood, kidneys increase retention of H+ ions & excrete excess HCO3- to decrease pH.
• Reabsorption moves into blood and secretion moves into filtrate from blood.
• The sections of the nephron that are responsible for secreting nothing are the proximal and descending tubules.
• The overall function of the countercurrent flow is to conserve water when needed to maintain homeostasis.
• In the ascending limb of the nephron loop, the filtrate entering is hypertonic.
• The different permeabilities of the ascending vs descending nephron loops contribute to maintenance of the concentration gradient.