Urinary System Overview

Overview of the Urinary System

• The lecture will cover general functions, components, kidney structure, nephron function, renal processes, physiological events, ADH action, and micturition.

Functions and Components

• Kidneys perform multiple physiological functions.
• Ureters transport urine from kidneys to the bladder.
• The bladder stores urine until voided and The urethra carries urine from the bladder to the outside of the body.

Kidney Functions

• Kidneys regulate blood plasma volume, blood pressure (MAP), and plasma osmolarity.
• They regulate pH, produce hormones/enzymes (erythropoietin and renin), and excrete wastes.
• Water volume affects blood pressure and osmolarity, which initiates homeostatic control.
• Volume and osmolarity values are interrelated.

Kidney Location

• Kidneys are located on either side of the vertebral column and are retroperitoneal, supported by connective tissue.
• The peritoneum covers the abdominal cavity and organs, and a fibrous capsule covers each kidney.
• Each capsule is surrounded by adipose tissue.

Kidney Blood Supply

• Kidneys receive 20-25% of cardiac output at rest.
• Blood flow starts with the renal artery and exits through the renal vein.

Kidney Structure

• The kidney has two layers: a renal cortex (outer) layer and a renal medulla (inner) layer.
• Renal medulla contains cone-shaped renal pyramids.
• The tip of these pyramids is called renal papilla, which projects into the renal pelvis.

Nephron Anatomy

• The nephron consists of a renal corpuscle and renal tubules.
• The renal corpuscle includes the Glomerulus and Bowman's capsule where filtration occurs.
• Renal tubules include the Proximal convoluted tubule, Loop of Henle, distal convoluted tubule, and collecting duct.
• Segments have different transport mechanisms; Nephrons are supplied by peritubular capillaries.

Three Renal Processes

• Glomerular filtration is a bulk-flow passive process driven by hydrostatic pressure, where water and low-molecular-weight substances are filtered.
• Tubular secretion involves movement of substances from peritubular capillaries into the tubular lumen.
• Tubular reabsorption: Substances move from tubular lumen/epithelial cells into peritubular capillaries.

Renal Processes Dynamics

• Blood filtrate is refined into urine by reabsorption and secretion after glomerular filtration.

Glomerular Filtration Details

• Afferent arterioles supply glomeruli, while efferent arterioles drain them.
• Glomeruli are capillaries inside Bowman's capsules that yield filtrate that enters nephron tubules.
• Filtration occurs at the glomerulus and filters plasma (minus proteins and blood cells) into Bowman's capsule.
• Valuable nutrients, ions, and water are included

The 3 Filtration Barriers

• Capillary endothelium
• Basement membrane or basal lamina
• Epithelium of Bowman's capsule or parts of podocyte

Glomerular Filtration Pressures

• Glomerular Filtration is affected by glomerular capillary blood pressure (PGC), Fluid pressure in Bowman's space (PBS) , and Osmotic force due to protein in plasma (πGC).

Glomerular Filtration Rate (GFR)

• GFR is as an index of renal function that measures filtrate produced per minute, averaging 125 mL/min or 180 L/day.
• GFR depends on blood flow to the kidney (20-25% of cardiac output) and net filtration pressures (NFP); renal blood flow and GFR are affected by changes in renal arterioles resistance.
• Vasoconstriction of the afferent arteriole increases resistance and decreases renal blood flow, PGC and GFR.
• Vasoconstriction of the efferent arteriole decreases renal blood flow but increases PGC and GFR.

Excretion Dynamics

• Amount excreted equals amount filtered, minus amount reabsorbed, plus amount secreted.
• The filtered load is GFR multiplied by the substance's plasma concentration. The filtered load equals the amount of any substance filtered from renal glomerular capillaries into the Bowman's capsule.

Tubular Reabsorption

• Reabsorption begins in the proximal tubule and extends to the collecting duct.
• Transport can occur transcellularly (through cells) or paracellularly (across tight junctions).
• Transport can be active (requiring ATP) or passive (no ATP required).

Proximal Tubule (PCT) Reabsorption.

• About 65% of glomerular filtrate volume is reabsorbed at the PCT.
• All glucose, amino acids, and organic nutrients are reabsorbed at the PCT.
• A large amount of ions , such as Na+, K+, Ca2+, Mg2+, HCO3-, are also reabsorbed at the PCT.
• Some ionic reabsoprtion are hormonally regulated, like Ca2+ by parathyroid hormone (PTH), and Na+ and K+ by aldosterone, etc.

Sodium Reabsorption in PCT

• Na+ ions are reabsorbed using cotransport mechanisms, and transport maximum of glucose determines the maximal amount of glucose can be transported by the carrier transporter.
• Glucose is reabsorbed from filtrate, normally it is 100%.
• Glucose transporters don't saturate under normal conditions but do become saturated in diabetes; Na+ and water reabsorption levels are coupled.

The Loop of Henle.

• About 25% of the filtrate are reabsorbed in the Loop of Henle.
• In the descending limb, fluid flows with renal pelvic.
• The descending limb is permeable to water, but relatively impermeable to solutes
• The tubule turns 180 degrees.
• In the ascending limb, the fluid follows toward the renal cortex.
• Th ascending limb is permeable to solutes, but relatively impermeable to water.
• The interstitial tissue fluid of the renal medulla has unusually high solute concentrations (NaCl).

Descending loop details

• The descending loop is permeable to water only while ascending loop NaCl is actively pumped out.
• Water is reabsorbed from the descending limb water impermeable to salt by the surrounding area.
• The deep regions of the medulla contain 1400mOsm and water diffuses out of filtrate.
• This water is then reabsorbed by capillaries.

Ascending Loop details

• NaCl is actively pumped out of the ascending limb and has a thin segment in depths of medulla with thick areas going to cortex.
• It is water proof but permeable to salt.
• Thick portion ATs salt out of filtrate.
• This causes filtrate to become dilute (100 mOsm) by the end

The Distal Convoluted Tubule

• This is involved in active secretion of ions, acids, drugs, and toxins (e.g. hydrogen ions or potassium ions)
• The Loop of Henle allows for selective reabsorption of sodium due to the action of aldosterone

The Collecting Duct (CD)

• Many distal convoluted tubules drain into one collecting duct.
• The collecting duct adjusts final filtrate composition.
• This is done via final osmotic concentration, the final urine volume, and acts as important sites of aldosterone as well as ADH.

Hormonal Control of the Collecting Duct

• Water is is reabsorbed due to ADH, and sodium due to aldoserone.

Anti-diuretic Hormone (ADH)

• ADH, also known as vasopressin, acts as a potent vasoconstrictor at higher concentrations and is secreted by the posterior pituitary in response to dehydration.
• Aquaporins ,which are water channels, are inserted in the apical side of the tubules plasma membrane.
• When ADH is high, H₂O is extracted from collecting duct by high osmolality of interstitial fluid; aquaporins are virtually absent in collecting ducts without ADH.
• ADH facilitates water reabsorption which is maintained by the high osmotic gradient found in the medullary interstatitum. This achieved by the active transportation of NaCl from the Henle loops.

Renal Secretion Dynamics

• Reabsorption in reverse occurs in different regions of the nephron tubules and involves selected substance.
• K+, H+, NH4+, creatinine, organic acids/bases move from peritubular capillaries through tubule cells into filtrate.
• Substances that are synthesized as tubule cells, with and are also secreted , are e.g., HCO3-.

Micturition

• The event is also known as urination or voiding.
• Three simultaneous events must occur:
  • Contraction of detrusor muscle by ANS.
  • Opening of internal urethral sphincter by ANS
  • Opening of external urethral sphincter by somatic nervous system.

Urine Properties

• Once filtrate leaves the collecting duct there is no further reabsoprtion. Fluid is now considered urine.
• Concentration and composition will vary based on hydration, metabolic, and the body's hormonal activities. Urine is temporarily stored in the urinary bladder until full.
• Void volume reached initiates The micturition or urination reflex.

Excreted Waste

• Metabolic wastes must be excreted to maintain homeostasis.
• Urea is removed from amino acid breakdown.
• Creatinine is removed from break down of high-energy compound from creatinine phosphate in skeletal muscle.
• Uric acid id removed from the breakdown of RNA