Unit 4: Homeostasis

Unit 4: Homeostasis Chapter 10: Excretion and the Interaction of Systems

Chapter 10: Excretion and the Interaction of Systems • Overview • The excretory system • Functions • Organs • Urine formation in the nephron • Other functions and disorders of the excretory system • Regulating water-salt balance • Maintaining blood pH • Disorders

Section 10.1: Overview of the Excretory System • Excretion is the process of separating wastes from body fluids, then eliminating the wastes from the body • Several body systems perform this function • The respiratory system excretes carbon dioxide and small amounts of other gases, including water vapour • The skin excretes water, salts, and some urea in perspiration • The digestive system excretes water, salts, lipids, and a variety of chemical compounds • Note that the elimination of food residue (feces) is not considered to be a process of excretion

Section 10.1: Overview of the Excretory System • Most metabolic wastes are dissolved or suspended in solution and are excreted by the excretory system (also called the urinary system) • The excretory system produces urine and conducts it outside the body • As the kidneys produce urine, they carry out four functions that contribute to homeostasis

Functions of the Excretory System • Excretion of metabolic wastes • The kidneys excrete metabolic wastes, notably nitrogenous (nitrogen-containing) wastes • Include ammonia, urea, and uric acid • Ammonia is highly toxic, but it converted in the liver to the less toxic compound urea • Urea makes up the majority of nitrogenous waste in the body • About half of it is eliminated in urine • Uric acid is present in much lower concentrations, and is contained in urine

Functions of the Excretory System • Maintenance of water-salt balance • Kidneys maintain the appropriate balance of water and salt in the blood • Blood volume is closely tied to the salt balance of the body • By regulating salts in the blood, the kidneys are closely involved in regulating blood pressure • Kidneys also help maintain the appropriate level of potassium (K+), bicarbonate (HCO3-) and calcium (Ca2+) in the blood

Functions of the Excretory System • Maintenance of acid-base balance • Kidneys monitor and help keep the blood pH at about 7.4, mainly by excreting hydrogen ions (H+) and reabsorbing bicarbonate ions (HCO3-) as needed • Human urine usually has a pH of 6 or lower because our diet often contains acidic foods

Functions of the Excretory System • Secretion of hormones • Kidneys secrete two hormones: • Calcitriol • Active form of Vitamin D • Promotes calcium absorption from the digestive tract • Erythropoietin • Stimulates the production of red blood cells • Released in response to increased oxygen demand or reduced oxygen-carrying capacity of the blood • Kidneys also secrete renin • Leads to the secretion of the hormone aldosterone from the adrenal cortex

The Organs of the Excretory System • The human excretory system consists of: • Two kidneys • Two ureters • The urinary bladder • The urethra

The Organs of the Excretory System • Two fist-sized kidneys are located in the area of the lower back on each side of the spine • A large cushion of fat usually surrounds them • Offers protection • Although most people have two kidneys, humans are capable of functioning with only one • If one kidney ceases to work, or one is removed due to disease, the remaining kidney will increase in size to handle the increased workload

The Organs of the Excretory System • The kidneys release urine into two muscular, 28-cm-long tubes called ureters • From the ureters, urine is moved by the peristaltic actions of smooth muscle tissue to the muscular urinary bladder where it is temporarily stored • Drainage from the bladder is controlled by two rings of muscles called sphincters • Both sphincters must relax before urine can drain from the bladder • The innermost sphincter is involuntarily controlled by the brain • During childhood we learn to voluntarily control relaxation of the other sphincter

The Organs of the Excretory System • Urine exits the bladder and the body through a tube called the urethra • In males, the urethra is approximately 20cm long and merges with the ductus deferens of the reproductive tract to form a single passageway to the external environment • In females, the urethra is about 4 cm long and the reproductive and urinary tracts have separate openings

The Kidneys • The kidneys are bean shaped and reddish-brown in colour • The concave side of each kidney has a depression where a renal artery enters and a renal vein and a ureter exit the kidney • Many branches of the renal artery renal vein reach inside the kidney

The Kidneys • A kidney has 3 regions • The renal cortex is an outer layer that dips down into an inner layer called the renal medulla • The renal medulla contains cone-shaped tissue masses • The renal pelvis is a central space, or cavity, that is continuous with the ureter

The Kidneys • Embedded within the renal cortex and extending into the renal medulla are more than a million microscopic structures called nephrons • A network of blood vessels is closely associated with these nephrons • Nephrons are responsible for filtering various substances from blood, transforming it into urine • To perform this function, each nephron is organized into 3 main regions: • A filter • A tubule • A collecting duct

The Nephron • A Filter • The filter structure at the top of each nephron is a cap-like formation called the Bowman’s capsule • Within each capsule, the renal artery enters and splits into a find network of capillaries called a glomerulus(means “little ball” in Latin) • The walls of the glomerulus act as a filtration device • Impermeable to proteins, other large molecules, and red blood cells, so these remain within the blood • Water, small molecules, ions, and urea (the main waste products of metabolism) pass through the walls and proceed further into the nephron • The filtered fluid that proceeds from the glomerulus into the Bowman’s capsule of the nephron is referred to as filtrate

The Nephron • A Tubule • The Bowman’s capsule is connected is connected to a small, long, narrow tubule that is twisted back on itself to form a loop • This long hairpin loop is a reabsorption device • The tubule has 3 sections: • The proximal tubule • The loop of Henle • The distal tubule • The tubule absorbs substances that are useful to the body, such as glucose and a variety of ions, from the filtrate passing through it • Also secretes substances into the tissues surrounding it

The Nephron • A Duct • The tubule empties into a larger pipe-like channel called a collecting duct • Functions as a water-conservation device, reclaiming water from the filtrate passing through it so that very little water is lost from the body • The filtrate that remains in the collecting duct is a suspension of water and various solutes and particles • It is now called urine • Its composition is distinctly different from the fluid that entered the Bowman’s capsule • The solutes and water reclaimed during reabsorption are returned to the body via the renal vein

Section 10.2: Urine Formation in the Nephron • Nephrons are surrounded by the tissues of the renal cortex and the renal medulla • Also closely associated with a network of blood vessels that spreads throughout the surrounding tissue • Thus, any substances secreted by the nephrons enter the surrounding tissue of the kidney • Most of these substances return to the bloodstream through the network of blood vessels • The remainder leave the body in the form of urine

How Urine Forms • 4 processes are crucial to the formation of urine • Glomerular filtration • Moves water and solutes, except proteins, from blood plasma into the nephron (recall this filtered fluid is called filtrate) • Tubular reabsorption • Removes useful substances such as sodium from the filtrate and returns them into the blood for reuse by body systems • Tubular secretion • Moves additional wastes and excess substances from the blood into the filtrate • Water reabsorption • Removes water from the filtrate and returns it to the blood for reuse by body systems

Glomerular Filtration Filters Blood • The formation of urine starts with glomerular filtration • This process forces some of the water and dissolved substances in blood plasma from the glomerulus into the Bowman’s capsule • This occurs in millions of nephrons at the same time

Glomerular Filtration Filters Blood • Two factors contribute to filtration • The first factor is the permeability of the capillaries of the glomerulus • Capillaries of the glomerulus have many pores in their tissue walls • The pores are large enough to allow water and most dissolved substances in the blood plasma to pass easily through the capillaries and into the Bowman’s capsule • The pores are small enough to prevent proteins and blood cells from entering • The second factor is blood pressure • Blood pressure within the glomerulus is about 4X greater than it is in capillaries elsewhere in the body • The great rush of blood through the golmerulus provides the force for filtration

Glomerular Filtration Filters Blood • Each day, 1600L – 2000L of blood pass through the kidneys, producing about 180L of glomerular filtrate • This filtrate is almost chemically identical to blood plasma, minus proteins and blood cells • If the composition of urine were the same as that of glomerular filtrate, the body would continually lose water, salts, and nutrients • Therefore, the composition of the filtrate must change as this fluid passes through the remainder of the tubule

Tubular Reabsorption: Recovery of Substances in the Proximal Tubule • About 65% of the filtrate that passes through the entire length of the proximal tubule (including the loop of Henle) is reabsorbed and returned to the body • This process of reabsorption involves both active and passive transport mechanisms • The cells of the proximal tubule contain many mitochondria, which use the energy-releasing power of ATP to drive the active transport of sodium ions, glucose, and other solutes back into the blood • Negatively charged ions tag along passively, attracted by the electrical charge on the transported substances • Water follows the ions by osmosis, so it, too, is reabsorbed into the blood flowing through the capillaries

Tubular Reabsorption: Recovery of Substances in the Proximal Tubule

Focusing on the Loop of Henle in the Proximal Tubule • The function of the loop of Henleis to reabsorb water and ions from the glomerular filtrate • As the descending limb of the loop of Henle plunges deeper into the medulla region, it encounters an increasingly salty environment • The cells of the descending limb are permeable to water and only slightly permeable to ions • As a result of the salty environment of the medulla, and the permeability of the descending limb, water diffuses from the filtrate to the capillaries by osmosis • As water moving through the descending limb leaves the filtrate, the concentration of sodium ions inside the tubule increases, reaching its maximum concentration at the bottom of the loop

Focusing on the Loop of Henle in the Proximal Tubule

Focusing on the Loop of Henle in the Proximal Tubule • As the filtrate continues around the bend of the loop of Henle and into the ascending limb, the permeability of the nephron tubule changes • Near the bend, the thin portion of the ascending tubule is now impermeable to water and slightly permeable to solutes • Sodium ions diffuse from the filtrate along their concentration gradient and pass into nearby blood vessels

Focusing on the Loop of Henle in the Proximal Tubule • At the thick-walled portion of the ascending limb of the loop of Henle, sodium ions are moved out of the filtrate by active transport • This transport of Na+ out of the filtrate has 2 consequences: • It helps replenish the salty environment of the medulla, which aids in the absorption of water from filtrate in the descending limb • The removal of sodium ions from the filtrate in the thick-walled portion of the tubule makes the filtrate less concentrated than the tissues and blood in the surrounding cortex tissue • By now, about two thirds of the Na+ and water from the filtrate has been reabsorbed

Tubular Reabsorption and Secretion in the Distal Tubule • The active reabsorption of sodium ions from the filtrate into the capillaries depends on the needs of the body • Passive reabsorption of negative ions such as chloride occurs by electrical attraction • The reabsorption of ions decreases the concentration of the filtrate, which causes water to be reabsorbed by osmosis

Tubular Reabsorption and Secretion in the Distal Tubule • Potassium ions (K+) are actively secreted into the distal tubule from the bloodstream in the capillaries • Hydrogen ions (H+) are also actively secreted from the blood into the distal tubule as necessary in order to maintain the pH of the blood • Other substances that are not normally part of the body, such as penicillin and other medications, are secreted from the blood into the distal tubule • Reabsorption and secretion in the distal tubule are under the control of hormones, as you will see in the next section

Reabsorption from the Collecting Duct • The filtrate entering the collecting duct still contains a lot of water • Because the collecting duct extends deep into the medulla, the concentration of ions along its length increases • This concentration of ions is the result of active transport of ions from the ascending limb of the loop of Henle • This causes the passive reabsorption of water from the filtrate in the collecting duct by osmosis

Reabsorption from the Collecting Duct • If blood plasma is too concentrated (for example, if a person is dehydrated) the permeability to water in the distal tubule and the collecting duct is increased • This causes more water to be reabsorbed into the surrounding capillaries in order to conserve water in the body • In the collecting duct, as in the distal tubule, hormones control reabsorption and secretion • The reabsorption of water in the collecting duct causes the filtrate to become about 4X as concentrated by the time it exits the duct • This filtrate, which is approximately 1% of the original filtrate volume, is now called urine

Section 10.3: Other Functions and Disorders of the Excretory System • Kidneys not only filter wastes from the blood, but also carry out several other important homeostatic functions including: • Maintaining the water-salt balance of the blood • Regulating blood pH • Secreting some hormones • Kidneys also play an important role in maintaining blood pressure • They can be damaged if blood pressure gets too high • Blood pressure tests, blood tests, and urinalysis are used to determine whether the kidneys are functioning properly

Regulating Water-Salt Balance Reabsorption of water • The force generated as water moves by osmosis is called osmotic pressure • Osmotic pressure affects many cellular activities, especially the exchange of materials between cells and blood • Osmoreceptors are cells that are sensitive to osmotic pressure • Most are located in the hypothalamus • Recall from Chap 8 that the hypothalamus regulates mechanisms that enable the body to maintain homeostasis • Ex: Hunger, thirst, blood pressure, body temperature, fluid balance, and salt balance

Regulating Water-Salt Balance • When blood plasma becomes too concentrated (ex: if you are dehydrated), osmotic pressure increases • Osmoreceptors in the hypothalamus send impulses to the pituitary gland which causes the release of antidiuretic hormone (ADH) • “Anti” means “against” or “opposed to” and “diuresis” means “increased excretion of urine” • So “antidiuresis” means “decreased excretion of urine” • ADH travels through the blood to the kidneys • It increases the permeability of the distal tubule and the collecting duct • Allows more water to be reabsorbed into the blood • This dilutes the blood and lowers osmotic pressure to normal

Regulating Water-Salt Balance • If blood plasma is too dilute (i.e. if osmotic pressure is too low) osmoreceptors in the hypothalamus stop or prevent the release of ADH • As a result, the distal tubule and the collecting duct become less permeable to water • Allows more water to be excreted in the urine, concentrating the solutes in the blood • The osmotic pressure of the plasma and tissue fluids rises to normal

Regulating Water-Salt Balance

Regulating Water-Salt Balance • In a condition called diabetes insipidus ADH activity is insufficient • Person urinates excessively (as much as 4L – 8L per day) • Thirst is intense, but water is excreted more quickly than it’s consumed, leading to severe dehydration and ion imbalances • People who have this condition may take synthetic ADH to restore the balance of water reabsorption

Regulating Water-Salt Balance • The ethanol in alcoholic beverages is a diuretic • Increases the volume of urine • Alcohol stimulates urine production partly by inhibiting the release of ADH • Decreases the permeability of the tubules and collecting ducts • Because it increases water loss to urine, drinking alcohol actually intensifies thirst and leads to dehydration • Caffeine, a substance in coffee and many carbonated drinks, is also a diuretic

Regulating Water-Salt Balance Reabsorption of salt • The kidneys regulate salt balance in the blood by controlling the excretion and reabsorption of various ions • The sodium ion (Na+) is the most abundant ion in blood plasma • Its concentration can fluctuate dramatically depending on diet and the consumption of beverages with diuretic effects

Regulating Water-Salt Balance • Hormones regulate the reabsorption of sodium at the distal tubule • Recall from Chap 9 that aldosterone is a hormone secreted by the adrenal cortex • Stimulates the excretion of potassium ions (K+) and the reabsorption of sodium ions (Na+) • The release of aldosterone is set in motion by the kidneys themselves

Regulating Water-Salt Balance • When blood volume, and therefore blood pressure, is too low to promote glomerular filtration, the kidneys secrete renin • Renin, an enzyme, starts a reaction that eventually triggers the release of aldosterone from the adrenal cortex • Aldosterone stimulates the distal tubules and collecting ducts to reabsorb Na+ • The reabsorption of Na+ is followed passively by chloride ions and water • Aldosterone has the net effect of retaining both salt and water • As a result, blood volume and blood pressure increases

Maintaining Blood pH • The normal pH of body fluids is about 7.4 • pH at which our enzymes function optimally • If homeostasis is not maintained and blood pH goes above or below 7.4, serious medical conditions can result • Many processes can alter blood pH, such as • Eating a meal • Drinking liquids • Metabolic processes (ex: cellular respiration) • Three mechanisms maintain blood pH at 7.4 • Acid-base buffer system • Respiratory center • The kidneys

Acid-Base Buffer System • This system buffers the blood • Prevents changes in pH by taking up excess hydrogen ions (H+) or excess hydroxide ions (OH-) that enter the blood • On of the key buffering reactions in the blood involves carbonic acid (H2CO3) and bicarbonate ions (HCO3-) • When hydrogen ions are added to the blood: H+ + HCO3- H2CO3 • When hydroxide ions are added to the blood: OH- + H2CO3 HCO3- + H2O • These reactions temporarily prevent changes in blood pH • A blood buffer can be overwhelmed unless some more permanent adjustment is made • The next adjustment to maintain blood pH occurs in the lungs

Unit 4: Homeostasis