Kidney Function and Filtration

Urinary System and Osmotic Balance

• This lecture covers the urinary system and osmotic balance in animals
• The lecture explains the relationships between physiology and anatomy in the context of larger animal orders
• The lecture includes a discussion of acid/base regulation
• The lecture explains kidney function
• The lecture explains water and salt balance in larger animal orders
• The lecture includes an explanation of the anatomical and physiological adaptations animals develop to live in aquatic environments
• The lecture covers the functions of excretory systems in larger animal orders

Water and Salt Balance

Kidney

• The kidney refers to tubular structures that discharge directly or indirectly to the outside world
• Kidneys produce and eliminate aqueous solutions derived from blood plasma or other extracellular body fluids
• Kidneys excrete nitrogenous waste products
• They regulate the composition and volume of blood plasma and other extracellular body fluids via controlled excretion of solutes and water
• Organs like kidneys include gills, salt glands, and malpighian tubules

Urine

• Urine refers to a complex solution containing multiple inorganic and organic solutes
• All the constituents of urine, including water, are drawn from blood plasma via osmosis

Production of Primary Urine

• Insects use Malpighian tubules for secretion and reabsorption
• A secretion-reabsorption kidney uses active and passive transport
• Vertebrate kidneys use filtration and reabsorption

Kidney - Mammals

• The internal anatomy includes the renal cortex, renal medulla, renal pyramids, renal veins, renal arteries
• Also included are segmental arteries, interlobar veins, interlobar arteries
• Arcuate veins, arcuate arteries, cortical radiate veins, cortical radiate arteries are involved
• Renal pelvis, major and minor calyxes are anatomical structures
• The nephron, kidney lobe, renal papilla, ureter, renal columns, and fibrous capsule are structures in the kidney

Kidney - Mammals - Nephron

• The functional unit of the mammalian kidney is the nephron
• Blood plasma filtration results in ultrafiltrate in the Bowman’s Capsule lumen
• Tubular reabsorption reabsorbs about 99% of water and most electrolytes, urea remains
• Tubular secretion involves the secretion of various substances via active transport

Nephron Anatomy

• Afferent arterioles bring blood to the glomerulus while efferent arterioles take blood away
• The nephron is composed of the proximal convoluted tubule, loop of Henle, and distal convoluted tubule which then connects to the collecting system

Glomerular Filtration

• Renal corpuscle = Bowman's Capsule + Glomerulus
• Capsular fluid is primary urine formed by ultrafiltration from blood plasma

Features in a Renal Corpuscle

• Complexly porous barrier between blood and capsular fluid
• Capillary endothelium is perforated with fenestrations
• The Bowman's capsule consists of podocytes with interdigitated processes forming slits

Colloid Osmotic Pressure (or Oncotic Pressure) = PCOP

• Formed due to ultrafiltration, accumulation of plasma proteins, and other macromolecules in glomerular capillaries
• Draws water into the capillaries
• Filtration of fluid from blood plasma along the capillaries increases the colloidal osmotic pressure in the glomerular capillaries
• Pcop increases non-linearly along glomerular capillary

Glomerular Filtration - Starling Forces

• Glomerular hydrostatic pressure (GHP) in the glomerular capillaries pushes water and solute molecules into the filtrate and averages 50 mm Hg
• Blood colloid osmotic pressure (BCOP) draws water out of the filtrate and into the plasma and averages about 25 mm Hg
• Net filtration pressure (NFP) represents the sum of hydrostatic and colloid osmotic pressures for an average of 10 mm Hg
• Capsular hydrostatic pressure (CsHP) opposes GHP, pushing water and solutes out of the filtrate and into the plasma

Kidney Facts

• Human kidneys form 180 L of filtrate per day
• Humans excrete about 1 L of Urine per day
• All plasma water in a human body is filtered every 30 minutes

Glomerular Filtration Rate (GFR)

• GFR (glomerular filtration rate) is the amount of filtrate the kidneys produce each minute

GFR factors varying with species

• Permeability of glomerular capillaries for water
• Net filtration pressure
• Mammals generally have higher arterial blood pressure compared to non-mammalian vertebrates (fish, amphibians, reptiles, birds)
• Size of the glomerulus

GFR in Non-Mammals

• Non-mammalian vertebrates can increase or decrease the amounts of their nephrons to regulate filtration
• Affected by internal changes in flood pressure and environmental factors that affect fluid and salt balance
• In aquatic non mammals, GFR is usually higher for animals acclimatized to different salinities, relative to those in freshwater

Auto-Regulation of GFR in Mammals and Birds

• Relatively independent of blood pressure changes due to adaptation in the kidneys
• Afferent arterioles primarily responsible
• Includes a rapid renal myogenic response of vasoconstriction and slower tubulo-glomerular feedback mechanism
• Protects against high blood pressure, such as during disease or training

Macula Densa

• Located in the distal tubular epithelium
• It releases vasoactive substances that control vascular tone
• It controls renin excretion from granules in the walls of afferent arterioles

Renin-Angiotensin-Aldosterone System (RAAS)

• The RAAS detects decreasing blood flow to the kidneys and acts, in part, by directing aldosterone to increase Na+ and water retention

Water and Salt Balance - Regulation

Anti-Diuretic Hormone (ADH)

• Vasopressin, regulates water turnover in kidneys, stimulates water absorption

Aldosterone

• Increases reabsorption of sodium

Atrial Natriuretic Peptide (ANP)

• Counteracts aldosterone, decreases reabsorption of sodium

Parathyroid Hormone

• Excreted when Ca2+ decreases

Calcitonin

• Excreted when Ca2+ increases, prevents hypercalcemia and dissolution of the skeleton

Proximal and Distal Convoluted Tubules

• Reabsorb all the useful organic nutrients in the filtrate
• Reabsorb more than 90% of the water from the filtrate
• Secrete waste products that did not pass into the filtrate at the glomerulus

Collecting Duct

• Allows for variable reabsorption of water, reabsorption or secretion of sodium, potassium, hydrogen, and bicarbonate ions

Overview of Urine Formation

• Filtration occurs in the renal corpuscle
• Water reabsorption occurs in the proximal convoluted tubule and the descending thin limb of the nephron loop
• Variable water reabsorption occurs in the distal convoluted tubule and collecting system
• Solute reabsorption occurs along the proximal convoluted tubule, thick ascending limb, distal convoluted tubule, and collecting system
• Variable solute reabsorption or secretion occurs at the proximal convoluted tubule, the distal convoluted tubule, and the collecting system

Transport Activities at the Proximal Convoluted Tubule

• Sodium enters a tubular cell from the filtrate via diffusion, cotransport, or countertransport
• Sodium is then pumped into the peritubular fluid by the sodium-potassium exchange pump
• Reabsorbed solutes are ejected into the peritubular fluid by separate active transport mechanisms
• Bicarbonate absorption is associated with sodium reabsorption and hydrogen ion secretion

Proximal Convoluted Tubules

• Actively reabsorb glucose, other simple sugars, amino acids, vitamins, and ions like sodium, potassium, calcium, magnesium, phosphate, and bicarbonate
• Passively reabsorb urea, chloride ions, and lipid-soluble materials, water
• Secrete hydrogen ions, ammonium ions, creatinine, drugs, and toxins

Proximal and Distal Convoluted Tubules

• Na+ and Cl- are reabsorbed in the proximal convoluted tubule, the tubule absorbs water leaving the tubular liquid approximately isosmotic to the blood plasma
• Walls of the distal convoluted tubule are poorly permeable to water in animals in diuresis and the tubular fluid becomes dilute as salt and chloride ions are reabsorbed

Distal Convoluted Tubules

• Reabsorption: Sodium ions (variable), chloride ions, calcium ions (variable), Water (variable)
• Secretion: Hydrogen ions, ammonium ions, Creatinine, drugs, toxins

Collecting Duct

• This is where kidneys receive tubular fluid from many nephrons
• Here, permeability is hormonally regulated via Aldosterone and Antidiuretic hormone
• Important secretions occur to control levels of pH

Loop of Henle

Processes that generate the single effect.

• the ascending-limb active transport of NaCl out dilutes the fluid and concentrates interstitial fluid
• the descending limb walls are permeable to water, so water loss through the limb concentrates fluids

The Single effect to a End Gradient-Henle

• The single gradients are created side to side
• This difference in end concentration becomes multiplies

Low Volume and Hyperosmotic Urine Production

Cortex

• High transfer, equal osmotic balance

Inner Medulla

• Some areas have a urea gradient making it very concentrated

Urine concentration and Medulla thickness

• Increased urine volume is the more renal papilla

Nitrogen Excretion

• Some classes of animal have different ways of excreting Nitrogen wastes
• Mammals use Urea
• Reptiles use Uric acid
• Teleosts use Ammonia

Avian Kidney

• The kidney is a central organ to maintain ion balance
• Uses Nephrons to actively engage in water retention

Non-Mammalian Vertebrates

• The tubules filter and reabsorb in the distal convoluted tubule
• Some animals will retain Urea

Fish Gill Processes

• The kidneys absorb NA+ and CL- from the water
• Absorb salt via the Gills tissue

Migrating Fish - Gills

• When a fish moves from freshwater to saltwater, Na+ in plasma increases, which stimulates cortisol and growth hormone excretion

Adaptations by Fish Kidneys as the migrate

• Changes in water volume are mediated by physical processes to influence osmotic balance

Amphibian Skin

• Water channels, aquaphorins, allows some permeability
• Moisture for diffusion and active transport of salts

Extracorporeal Organs in Verbrates

• Organs help with osmoregulation as a cellular mechanism in kidneys
• Cartilaginous fish have a separate rectum
• Birds and Reptiles have different skulls and internal processes to help them function

Salt Gland Secretion

• passive flux of Na+ occurs along paracellular pathways, with some water

Final thoughts - Key Points

• There are many forms of excretion and osmoregulation
• A few processes vary and are specific to animals and their adaptation

Explore the critical roles of pressure dynamics, including oncotic, hydrostatic, and net filtration pressures, in glomerular filtration. Learn about the kidney's structure, including the nephron and collecting system, and its function in regulating blood plasma composition. Discover various organs like gills and salt glands that perform similar excretory functions.