Waste Excretion and the Urinary System

Waste Excretion

• Metabolic processes produce waste that must be excreted
• Waste products include carbon dioxide, nitrogenous wastes, bile salts and pigments, and salts
• Carbon dioxide is excreted through the lungs
• Nitrogenous wastes, particularly urea, are excreted from protein metabolism
• Bile salts and pigments result from red cell breakdown
• Salts derive from tissue breakdown and excess consumption

Routes of Waste Elimination

• The respiratory tract eliminates carbon dioxide and water vapor
• Sweat glands eliminate salts
• The digestive system removes bile and pigments
• The urinary system removes urea, salts, water, and soluble waste products
• The kidneys can remove drug metabolites made water-soluble by hepatic enzymes

Components of the Urinary System

• Two kidneys
• Two ureters
• One urinary bladder (absent in birds and reptiles: urine flows directly to the colon)
• One urethra

Kidney Function

• "Nephro-" and "Reno-" are parts of medical terms for kidneys
• The kidneys produce urine for excretion and play a role in homeostasis
• Kidneys regulate plasma composition, acid-base status, and fluid and electrolyte balance
• Maintaining homeostasis is the kidneys' most vital function

Homeostasis

• Blood filtration, reabsorption and secretion are essential for this
• Useful substances are returned to circulation
• Waste products are secreted from circulation into urine

Regulation of Fluids

• The amount of urine produced affects the amount of water in the body
• Diuresis removes excess fluid
• Oliguria (little urine) and anuria (no urine) are ways of conserving water
• Anuric renal failure compromises fluid balance

Primary fluid control

• Controlled by antidiuretic hormone (ADH) from the pituitary gland
• Controlled by aldosterone from the adrenal glands

Acid-Base Balance

• Kidneys remove hydrogen and bicarbonate ions which alters blood pH levels
• Hydrogen ions make blood more acidic
• Bicarbonate ions makes blood more alkaline

Hormones

• Kidneys help regulate ADH release from the posterior pituitary and aldosterone from the adrenal cortex
• Specialized renal cells produce erythropoietin
• Erythropoietin is necessary for red blood cell formation by the bone marrow
• Prostaglandin production provides protective and inflammatory functions
• Prostaglandins causes increased blood flow to the kidneys

Anatomical Location

• Kidneys are located in the dorsal abdomen in the retroperitoneal space
• Kidneys are outside the parietal peritoneum and tucked under the vertebrae
• Kidneys are surrounded by a thick layer of perirenal fat for protection
• In all domestic species except the pig, the right kidney is more cranial than the left

Right Kidney Position

• The right kidney is immediately caudal to the liver
• This results in a concave renal impression on the liver

Kidney Mobility

• The left caudal kidney is somewhat mobile
• Both kidneys move slightly caudal with each contraction of the diaphragm
• The left kidney's mobility makes it vulnerable to iatrogenic trauma

Gross Appearance

• Kidneys are "bean" shaped and brownish in color
• Cattle have nodular kidneys
• The kidney parenchyma is covered by a fibrous connective tissue capsule
• The hilus is the medial indentation where vessels and the ureter leave the kidney

Renal Pelvis

• The pelvis is a funnel-shaped area for urine collection, connecting the hilus and forming the ureter
• Cattle do not have this
• The renal cortex is the outer portion of the kidney, reddish-brown and granular
• The renal medulla is the inner portion surrounding the renal pelvis
• The medulla has a smooth appearance and dark purple exterior, sending rays into the cortex
• It features a pale, gray/red interior that extends into the renal pelvis

Medulla Structure

• Multilobular or multipyramidal structure of cattle and pigs have multiple pyramid shaped areas with the apex directed towards the pelvis or ureter
• Unilobar or unipyramidal structure of dogs, horses and cats have the medullary pyramids fused to occupy the entire inner area
• The calyx is a cuplike extension of the renal pelvis connecting to the medullary pyramids

Microscopic Anatomy

• Nephrons which are the functional units of the kidneys
• Thousands of nephrons are in each kidney
• Nephrons filter, reabsorb, and secrete

Nephron Components

• Each nephron consists of a renal corpuscle, proximal convoluted tubule, loop of Henle, and distal convoluted tubule

Renal Corpuscle

• The glomerulus and Bowman's capsule make up this structure
• The glomerulus is a tuft of capillaries for filtration with mostly fluid and without protein
• Bowman's capsule surrounds the glomerulus with an inner visceral and outer parietal layers
• The space between the layers is a capsular space continuous with the proximal convoluted tubule
• The renal corpuscle's function is to filter blood in the first stage of urine production
• Glomerular filtrate is the filtered fluid from the blood

Proximal Convoluted Tubule (PCT)

• The continuation of the capsular space
• The longest portion of the tubular part of the nephron
• It has cuboidal epithelium and a brush border to increase surface area
• This increased surface area is important for reabsorption and secretion
• As the glomerular filtrate travels through the PCT, it is called tubular filtrate

Loop of Henle

• It continues from the PCT descending into the renal medulla then returning to the cortex
• It is lined with cuboidal cells brush borders
• Thinner tube with lined simple squamous epithelial cells without a brush border
• Distal Convoluted Tubule (DCT): continuation of the ascending portion of the loop of Henle
• DCT twists through the renal cortex

Collecting Ducts

• Nephrons empty the DCT into these
• These collecting ducts carry tubular filtrate through the medulla to the renal pelvis
• These ducts affect urine volume and regulates potassium and acid-base

Renal Nerves

• Kidneys are primarily controlled by the sympathetic portion of the autonomic nervous system
• Nerves control blood flow to the glomerular capillaries

Blood Supply

• Kidneys receive a large blood supply, about 25% of the heart's output
• All circulating blood flows through the kidneys every 4–5 minutes
• Renal arteries branch off the abdominal aorta and enter the kidney at the hilus
• These arteries subdivide into afferent glomerular arterioles

Glomerular Arterioles

• Blood is carried to the glomerular capillaries of the renal corpuscle
• Glomerular capillaries filter plasma where some will deposit into Bowman's capsule, creating filtrate

Blood Filtrate

• Blood leaves the glomerulus via efferent glomerular arterioles still oxygenated
• The location where blood entering and leaving the capillaries is oxygenated
• The efferent glomerular arterioles divide into a network surrounding the nephron for oxygen transfer and reabsorption
• The glomerular filtrate will be reabsorbed into circulation

Vessels

• Peritubular capillaries surround the nephron
• These merge to form venules, which merge to form the renal vein, which exits the kidney at the hilus and enters the caudal vena cava.
• Filtration, reabsorption, and secretion are the mechanisms of renal action
• Filtration occurs in the renal corpuscle with capillaries between two arterioles under high pressure
• This pressure forces plasma into Bowman's capsule through fenestrations

Glomerular Filtrate

• It is similar to plasma but lacks proteins
• Plasma proteins are too large to escape unless there is damage to the endothelium of the glomerulus
• Protein in the urine indicates glomerular disease

Glomerular Filtration Rate

• GFR measures how fast plasma is filtered
• GFR depends on blood flow, which is affected by systemic blood pressure and disease

Reabsorption

• Once plasma is filtered and enters the capsular space, it is considered outside the body proper
• Glomerular filtrate contains both waste products and substances the body needs
• Includes sodium, potassium, calcium, magnesium, glucose, amino acids, chloride, bicarbonate, and water
• Reabsorption moves these substances from the glomerular filtrate back into the peritubular capillaries

Process

• Filtrate enters the PCT and becomes tubular filtrate
• Substances move out of the tubular lumen, through epithelial cells, into the interstitial fluid, and then peritubular capillaries
• Some passively; others actively use transport
• Sodium is actively pumped out of the tubular filtrate in the PCT back into the blood along with a carrier protein
• Glucose and amino acids use passive sodium cotransport

Additional Reabsorption

• Sodium ions are reabsorbed in the ascending loop of Henle and DCT in exchange for hydrogen, ammonium, or potassium
• The adrenal cortex controls this exchange via aldosterone
• Potassium moves out of the tubular filtrate in the PCT, the ascending loop of Henle, and the DCT by influence
• Calcium moves through tubular cells under influence of vitamin D, parathyroid hormone and calcitonin
• Magnesium is reabsorbed from the PCT, ascending loop of Henle, and collecting ducts

Sodium Balance

• When sodium is pumped into the interstitial space, it creates a negative charge
• Chloride ions then diffuse out of the tubular filtrate to maintain electrical neutrality
• Sodium, glucose, amino acids, and chloride leaves, water leaves the filtrate by osmosis
• The filtrate causes the concentrations of other substances to increase, therefore diffusing back into the blood

Urea

• Urea maintains a certain blood level
• 65% of tubular resorption occurs in the PCT, reabsorbing 80% of water, sodium, chloride, and bicarbonate, and 100% of glucose and amino acids
• Loop of Henle, DCT, and collecting ducts handles the rest of reabsorption

Secretion

• In this process, substances not adequately filtered in the blood,
• Substances move from the peritubular capillaries to the interstitial fluid, then to the tubular epithelial cells and tubular filtrate in the DCT
• Hydrogen, potassium, ammonia, and drugs like penicillins are eliminated

Urine Volume Regulation

• The presence of water in tubular filtrate determines it volume
• ADH regulates urine volume by acting on the DCT and collecting ducts to promote water reabsorption
• Volume will be lost without ADH (a state called polyuria)
• Aldosterone increases sodium reabsorption into the blood in the DCT and collecting duct to increase water back into the blood
• Water needs ADH to move out of these though

Tubular Movement

• Glomerular and tubular filtrate moves through the tubules and into the renal pelvis because of the pressure gradient
• Pressure in Bowman's capsule is high, and pressure in the renal pelvis is low

Ureters

• Ureters are the tubes that leave the renal pelvis and connect to the urinary bladder
• The tubes consist of three layers: the outer fibrous layer, the inner muscular layer, and an inner epithelial layer that is lined with transitional epithelium
• Transitional epithelium is used to allow that ureter to stretch when urine passes
• Ureters move urine continuously from the renal pelvis to the urinary bladder by peristaltic contractions
• Ureters enter the bladder at an angle
• The opening collapses when the bladder is full so that urine cannot back up into the ureter when the bladder is full

Urinary Bladder

• Periodically stores urine for release from the body
• Consists of a muscular sac and the neck
• It is lined with transitional epithelium that stretches as the bladder fills
• Bundles of smooth muscle contract and urine is expelled
• The neck of the bladder is attached to the urethra
• The neck is surrounded by sphincter muscles composed of skeletal muscle that enables urine control

Bladder State

• When the bladder is empty its walls are thick
• As it fills, it turns pear shaped and extends cranially into the abdominal cavity and stretches thin

Micturition Process

• Urination involves two to three steps and also known as micturition or uresis
• Urine accumulation occurs until stretch receptors activate in the bladder wall
• Muscle contraction occurs when the trigger is reached and a spinal reflex is activated that returns movement to the bladder muscles
• Sphincter muscle is voluntary to eliminate waste