Excretory System and Kidneys

1. Excretion: process of removing wastes and excess water from body
• essential process in all forms of life
• ammonia, uric acid, and urea are removed through urine
• urine produced in kidney
• also necessary to remove excess water, salts, and other substances from body
• kidneys are main organs of excretion of wastes, but several other organs are also involved in excretion
• large intestine: eliminates solid wastes that remain after digestion of food in gastrointestinal tract
• liver: breaks down excess amino acids in blood to form urea (also breaks down alcohol and drugs) 
• skin: eliminates water and salts in sweat
• lungs: exhale water vapor and carbon dioxide
2. Urinary system
• includes kidneys, ureters, bladder, and urethra
• main function is to filter waste products and excess water from blood and remove it from the body
• kidneys filter the blood and form urine 
• urine is liquid waste product of body that is excreted by urinary system
• from the kidneys, urine enters the ureters
• carries it to bladder
• ureter is a muscular tube about 25 centimeters long
• peristaltic movements of muscles in ureter sends urine to bladder in small spurts
• bladder
• hollow organ that stores urine
• can stretch to hold up to 500 mL
• when the bladder is half full, the stretching of the bladder sends impulse to sphincter
• the sphincter that controls the opening to the urethra relaxes and lets urine flow into urethra
• urethra
• muscular tube that carries urine out of the body
• urine leaves body through another sphincter
• sphincter and process of urination are normally under conscious control
3. Kidneys
• participate in whole body homeostasis
• primary roles of kidney is to remove nitrogenous wastes
• pair of bean-shapes, reddish brown organs about the size of a fist
• located above waist at back of the abdominal cavity, on either side of the spine
• protected by ribcage
• on top of each kidney is an adrenal gland
• secretes several hormones
• helps regulate kidney functions
• has three layers
• renal cortex, renal medulla, and renal pelvis
• renal pelvis is where the renal artery enters kidney, and where the renal vein exits the kidney
• renal artery carries blood to kidney to be filtered
• renal vein carries filtered blood away from kidney
• nephrons extend from cortex down into medulla
4. Nephron
• structural and functional units of the kidneys
• a single kidney may have more than a million nephrons
• main structures: glomerulus, Bowman's capsule, and renal tubule
• glomerulus: cluster of arteries that filters substances out of the blood
• Bowman's capsule: cup-shaped structure around the glomerulus that collects the filtered substances
• renal tubule: long, narrow tube surrounded by capillaries that reabsorbs many of the filtered substances and secretes other substances
5. Filtration, reabsorption, and secretion
• renal arteries branch into capillaries of glomerulus of nephron
• pressure of blood moving through arteries forces some water and dissolves substances through capillary walls
• goes into Bowman's capsule
• Bowman's capsule is composed of layers
• space between layers (Bowman's space) fills with filtered substances
• filtering substances from blood into glomerulus is called filtration
• fluid in Bowman's space is called filtrate
• composed of water, salts, glucose, amino acids, and urea
• protein molecules, blood cells, and platelets are too big to pass into Bowman's space, so they return to main circulation
• filtrate passes into renal tubule
• reabsorption is main function
• reabsorption: return of needed substances back into bloodstream
• necessary because some substances removed by filtration are needed by body
• 75% of substances are reabsorbed in renal tube
• renal tube divided into three parts: proximal tubule, Loop of Henle, and distal tubule
• filtrate enters the proximal tubule 
• most absorption takes place here
• microvilli line the proximal tubule and increase surface area for reabsorption
• filtrate passes through loop of Henle
• loop of Henle carries filtrate from cortex down into medulla and then back up to the cortex
• primary purpose: to reabsorb water and salt from fluid
• remaining fluid  enters distal tubule
• distal tubule carries fluid (tubular fluid) from loop of Henle to collecting duct
• distal tubule also reabsorbs or secrets substances such as calcium and sodium when it transports the fluid
• process of secreting substances into tubular fluid is called secretion
6. Urine formation
• collecting ducts are at the site of urine formation
• process is crucial for water conservation in the body
• collecting ducts reabsorb water from tubular fluid and return it to blood
• remaining fluid (urine) has smaller volume and greater concentration than tubular fluid
• urine enters a ureter and eventually excreted from body
• reabsorption of water is controlled by negative feedback mechanism
• involves a hormone secreted by pituitary gland, called antidiuretic hormone (ADH) 
• ADH makes collecting ducts more permeable to water
• allows more water to be reabsorbed from tubular fluid
• not enough water in blood, more ADH is secreted, so more water is reabsorbed from tubular fluid, and less water excreted in urine
• opposite happens when there is too much water in blood
7. Kidneys and homeostasis
• kidneys are responsible for maintaining balance in blood in other ways
• controls acid-base balance in the blood
• secreting hydrogen ions into tubular fluid and reabsorbing bicarbonate ions from tubular fluid
• also regulates blood concentrations of many other ions (sodium, potassium, calcium, and magnesium) by controlling amounts that are excreted in urine
• controlling hormones
• ADH is produced in hypothalamus of brain
• stores in and released from posterior pituitary gland
1. positioned just below hypothalamus
• osmoreceptor cells monitor osmolarity of blood
1. rises above 300 mosm/L, more ADH is released into bloodstream and reaches kidney
• main targets of ADH are distal tubules and collecting ducts of kidney
• hormone increases permeability of epithelium to water
• amplifies water reabsorption, reducing urine volume and helping prevent further increase of blood osmolarity above set point
• subsiding osmolarity of blood reduces activity of osmoreceptor cells
1. less ADH is secreted
• gain of additional water in food and drink can only bring osmolarity back down to 300 mosm/L
• if large intake of water reduced blood osmolarity below set point, very little ADH is released
• decreases permeability of distal tubules and collecting ducts, so water reabsorption is reduced
1. increased discharge of dilute urine
• alcohol can disturb water balance by inhibiting release of ADH
• causes excessive urinary water loss and dehydration
• blood osmolarity, ADH release, and water reabsorption in kidney are linked to feedback loop that contributes to homeostasis
• secondary regulatory mechanism involves specialized tissue
• juxtaglomerular apparatus (JGA) 
• located near afferent arteriole that supplies blood to the glomerulus
• when blood pressure or blood volume in afferent arteriole drops, enzyme renin initiates chemical reactions
• converts plasma protein (angiotensinogen) to peptide called angiotensin 2
• raises blood pressure by constricting arterioles and decreasing blood flow to many capillaries
• angiotensin 2 also stimulates proximal tubules of nephrons t reabsorb more NaCl and water
• reduces amount of salt and water excreted in urine and raises blood volume and pressure
• stimulation of adrenal glands to release hormone called aldosterone
• acts on nephrons distal tubules, making them reabsorb more sodium and water
• increases blood volume and pressure
8. Secreting hormones
• kidneys secrete various hormones to help with homeostasis
• erythropoietin: secreted when blood does not have enough red blood cells to carry sufficient oxygen
• stimulates production of red blood cells by the bone marrow
• rennin: secreted when blood pressure falls
• stimulates secretion of aldosterone by adrenal gland
• aldosterone stimulates kidneys to absorb more sodium ions and water
• increases volume of blood, causing increase in blood pressure
9. Kidney diseases and dialysis
• person can live normal life with only one kidney
• however, that kidney has to function properly to maintain life
• diseases include kidney stones, infections, and diabetes
• kidney stones
• crystals of dissolved minerals that form in urine inside kidneys
• can grow to be as large as a grapefruit
• may be one large stone or many small ones
• small kidney stones often pass through urinary tract and out of the body in urine
• kidney stones may grow large enough before passing to block a ureter
• causes buildup or urine above blockage and severe pain
• large kidney stones can be broken down into smaller pieces that wash out of the urinary tract
1. broken down by using high-intensity sound waves focused on them from outside the body
• can also remove them surgically
• infections
• bacterial infections are very common
• urinary tract infections (UTI) are second most common type of bacterial infections seen 
• most common type of UTIs are bladder infections
• can be treated with antibiotics prescribed by doctor
• if left untreated, infection can spread to kidney and cause a kidney infection (pyelonephritis) 
• most serious kind of UTI
• can damage kidney and interfere with normal kidney function
• diabetes insipidus
• disease characterized by the inability to concentrate to urinate
• person with this disease typically produces  many liters of dilute urine each day
• can be caused by deficiency of ADH, or by kidneys failing to respond to ADH
• if cause can be treated, the disease can be cured
• diabetes mellitus
• kidneys try to reduce high glucose level in blood by excreting more glucose in urine
• causes frequent urination and increased thirst
• blood glucose levels may damage capillaries of glomerulus and interfere with kidney's ability to filter blood if left alone
• high glucose levels may lead to kidney failure, so kidney function is greatly reduced
• kidney failure leads to high levels of urea and other wastes in the blood
• may require treatment with dialysis
• dialysis (medical procedure) 
• blood is filtered with the help of a machine 
• blood from patient's vein enters dialysis machine through tube
• excess water, wastes, and other unneeded substances are filtered from the blood
• filtered blood then returned to patient's vein through another tube
• usually lasts 3-4 hours and is repeated 3 times a week
• generally performed on patients who have kidney failure
• helps them stay alive, but does not cure their failing kidneys
• only cure is kidney transplant