Urinary System and Fluid Balance

Urinary System and Fluid Balance

• The urinary system is the major excretory system
• It controls the composition and volume of body fluids

Urinary System Functions

• Excretion of waste products
• Regulation of blood volume and pressure
• Regulation of the concentration of solutes in the blood
• Regulation of extracellular fluid pH
• Regulation of red blood cell synthesis
• Regulation of vitamin D synthesis

Kidneys

• Bean-shaped organs located retroperitoneally
• Found on either side of the vertebral column
• The kidneys are the major excretory organ of urinary system
• Divided into two major regions: outer cortex and inner medulla
• The two regions surround the renal sinus

Renal Capsule

• Layer of connective tissue
• Surrounds each kidney
• A thick layer of adipose tissue around it protects the kidney from mechanical shock

Hilum

• Medial area
• Renal artery and nerves enter, as well as renal veins, ureter, and lymphatic vessels exiting the kidney

Renal Sinus

• Cavity containing blood vessels
• Part of the system for collecting urine

Renal Pyramids

• Cone-shaped
• Bases are located at boundary between cortex and medulla
• Tips project towards the center of kidney

Calyx

• Funnel-shaped structure
• Surrounds the tip of each pyramid

Renal Pelvis

• Large funnel
• Results from joining of all the calyces

Ureter

• Small tube
• Comes from narrowed renal pelvis exiting the kidney and connecting to urinary bladder

Urine Flow

• Tips of renal pyramid → calyces → renal pelvis → ureter → urinary bladder

Nephron

• Functional unit of the kidney

Renal Corpuscle

• Houses the filtration portion of nephrons

Bowman Capsule

• Enlarged end of nephron
• Forms a double-walled chamber
• Cavities open into proximal convoluted tubule
• The proximal convoluted tubule carries fluid away from the capsule

Bowman Capsule's Inner Layer

• Consists of podocytes
• Specialized cells wrapped around the glomerular capillaries
• Contains filtration membrane consisting of endothelium of glomerular capillaries, podocytes, and basement membrane

Filtrate

• Fluid forced across the filtration membrane

Glomerulus

• A tuft of capillaries
• Lies within the indentation of Bowman capsule

Juxtaglomerular Apparatus

• Vital regulatory structure located next to glomerulus
• Responsible for the secretion of enzyme renin

Juxtaglomerular Cells

• Cuff of specialized smooth muscle cells
• Located at the point where the afferent arteriole enters the renal corpuscle

Macula Densa

• Group of specialized cells
• Located at the part of the distal convoluted tubule
• Lies between the afferent and efferent arterioles

Proximal Convoluted Tubule

• Primary site for reabsorption of solutes and water

Loop of Henle - Descending Limb

• Extends toward the renal sinus
• Critical site for water reabsorption via osmosis

Loop of Henle - Ascending Limb

• Extends toward the cortex
• Filtered fluid flows to the distal convoluted tubule

Distal Convoluted Tubule

• Empties fluid into a single collecting duct
• Multiple ducts empty into a single papillary duct

Types of Nephrons - Juxtamedullary

• Next to medulla
• Approximately 15% of nephrons

Types of Nephrons - Cortical

• Do not extend deep into medulla

Blood Supply Through The Kidney

• Renal Artery → Interlobar Arteries → Arcuate Arteries → Interlobular Arteries → Afferent Arteriole → Glomerulus → Efferent Arteriole → Peritubular Capillaries (including Vasa Recta) → Interlobular Veins → Arcuate Veins → Interlobar Veins → Renal Vein

Major Processes in Urine Production - Filtration

• Movement of materials across the filtration membrane into the Bowman capsule to form filtrate
• The driving force is blood pressure

Filtration Pressure

• Pressure gradient that forces fluid from glomerular capillary across filtration membrane into the Bowman capsule
• Influenced by three factors: glomerular capillary blood pressure, blood protein concentration, pressure in Bowman capsule

Glomerular Capillary Blood Pressure

• Normally higher than blood pressure in other capillaries

Blood Protein Concentration

• Encourages the movement of water by osmosis back into glomerular capillaries
• This reduces overall filtration pressure
• Decreased blood protein concentration means increased overall filtration process

Pressure in Bowman Capsule

• Resists movement of fluid into the space, decreasing it increases overall filtration process

Major Processes in Urine Production

• Solutes and water reabsorbed from proximal convoluted tubule enter the peritubular capillaries (65% of filtrate volume)
• To descending limb, reabsorbed water and solutes then to the vasa recta (15% of filtrate volume)
• To ascending limb which dilutes the filtrate by removing solutes
• Solutes diffuse out of the nephron (Na+ via active transport; K+ & Cl- via cotransport)
• Highly concentrated filtrate converts to dilute solution as it travels
• Enters interstitial fluid of medulla and helps concentration of solutes high
• To distal convoluted tubule & collecting duct which remove water & additional solutes (19% of filtrate volume)
• Na+ & Cl- are the principal reabsorbed solutes

Tubular Secretion

• Solutes are secreted across the wall of the nephron into the filtrate, either through active mechanisms or passive mechanisms

Tubular Secretion - Active Mechanism

• Solutes actively transported into the nephron, including H+, K+, creatinine, histamine, and penicillin

Tubular Secretion - Passive Mechanisms

• Solutes diffuse into the lumen of nephron like ammonia secretion

Regulation of Urine Concentration and Volume

• Increased body fluid concentration = increased water reabsorption = concentrated urine (eliminates increased solute and conserves water)
• Decreased body fluid concentration = decreased water reabsorption = diluted urine (eliminates increased water and conserves solutes)
• Increased blood volume = increased blood pressure = large volume of urine
• Decreased blood volume = decreased blood pressure = small volume of urine

Hormonal Mechanisms

• Renin-Angiotensin-Aldosterone (RAA) Mechanism
• Antidiuretic Hormone (ADH) Mechanism
• Atrial Natriuretic Hormone (ANH) Mechanism

Urinary Bladder

• Hollow, muscular container
• Stores urine
• Can hold to a maximum of about 1000mL

Trigone

• Triangle-shaped portion of urinary bladder
• Located between the opening of ureters and urethra

Internal Urethral Sphincter

• Smooth muscles
• At the junction of urinary bladder and urethra
• Prevents urine leakage from the urinary bladder

External Urethral Sphincter

• Skeletal muscle
• Allows a person to voluntarily start or stop the flow of urine

Micturition Reflex

• Activated by stretch of urinary bladder wall
• Automatic reflex
• Can be inhibited or stimulated by higher centers in brain

Urine Movement

• Renal Pelvis → Ureters → Urinary Bladder

Process

• The bladder reaches a volume (few hundred mL), and increases bladder pressure
• This activates stretch receptors leading to micturition reflex
• Conduction of action potential goes to spinal cord through pelvic nerves
• The parasympathetic response causes urinary bladder to contract
• It decreases somatic motor nerves to external urethral sphincter causing it to relax
• Results in urine flow out

Body Fluid Compartments

• Intracellular Fluid Compartment – the fluid inside all the cells of the body

Extracellular Fluid Compartment

• Fluid outside the cells, including interstitial fluid, plasma within blood vessels, fluid in lymphatic vessels
• Specialized fluids such as aqueous & vitreous humor of the eye, CSF, synovial fluid, serous fluid in body cavities, fluid secreted by glands, renal filtrate, & urine in bladder

pH

• Concentration of H+ in body fluids
• Normal value: 7.35 – 7.45

Buffers

• Chemicals that resist a change in the pH of a solution
• Contain salts of either weak acids or weak bases
• Binds to H+ when H+ increases, release H+ when H+ decreases
• Major buffers in body fluids are proteins, PO43- buffer system, and the HCO3- buffer system

Respiratory System

• Responds rapidly to a change in pH
• Neurons in respiratory center of the brain are sensitive to CO2 levels and pH
• Increased CO2 levels & low pH = elevated rate & depth of respiration to eliminate CO2 & pH rises back
• Decreased CO2 levels & high pH = reduced rate & depth of respiration to conserve CO2 & pH goes down

Kidneys

• Nephrons secrete H+ into the urine
• Regulate the pH of body fluids directly
• Decreased blood pH = increased secretion of H+ in distal convoluted tubules and increased reabsorption of HCO3- resulting in increased blood pH
• Increased blood pH = decreased secretion of H+ in distal convoluted tubules and increased loss of HCO3- in urine resulting in decreased blood pH

Acidosis

• pH level below 7.35
• Respiratory factor is the inability to eliminate adequate amounts of CO2
• Metabolic factor is the excess in acidic substances (lactic acid & ketone bodies) due to increased metabolism and decreased ability eliminate H+ in the urine
• Manifestations include CNS malfunctions and disorientation

Alkalosis

• pH level above 7.45
• Acid-base imbalance where results from hyperventilation as response to stress
• Can arise from rapid elimination of H+ (severe vomiting) or when excess aldosterone is secreted by adrenal cortex
• Results in hyperexcitability of nervous system, spasms, tetanic contractions, extreme nervousness and convulsions, can cause tetany

Cystitis

• Inflammation of urinary bladder
• Usually results from bacterial infection

Kidney Stone/Renal Calculus

• Consists mainly of calcium oxylate
• Causes excruciating and debilitating discomfort in lateral abdominal region