Excretion and the Interaction of Systems - Notes

9.1 The Structures and Function of the Excretory System

• Each kidney processes blood to form urine, which drains through the ureter into the urinary bladder for excretion.
• Each kidney contains over one million nephrons that process blood to form urine.

9.2 Urine Formation in the Nephron

• The functional unit of the kidney is the nephron.
• Each nephron filters blood, reabsorbs substances such as sodium and glucose for reuse in the body, and secretes excess or toxic substances such as urea to produce urine.

9.3 Excretory System Health

• Antidiuretic hormone (ADH) regulates the amount of water reabsorbed in the distal tubule.
• Aldosterone regulates the amount of salt that is reabsorbed or secreted.
• The acid-base balance of the blood is adjusted by the secretion of hydrogen ions and reabsorption of bicarbonate ions.
• Various technologies are used to solve problems involving dysfunctions and disorders of the excretory system.

Urine Examination

• The first recorded evidence of people examining urine as a clue to the health of the body’s internal environment appeared over two thousand years ago in Greece.
• In the 1600s, physicians examined urine to diagnose conditions like edema (dropsy).
• The flask holding the urine, called a matula, was the symbol for the medical profession from the 1200s to the 1600s.
• This ancient practice of urine inspection has been replaced, improved, and quantified by the modern science of urology.

Dehydration and Urine Colour

• Athletes whose sweat loss exceeds fluid intake may become dehydrated during activity.
• Even slight dehydration (a 1–2 percent loss in body weight) has a negative effect.
• A doctor of sports medicine, L.E. Armstrong, devised a standardized reference chart for urine colour for athletes to determine their hydration. People who are well-hydrated produce urine that is “very pale yellow”, “pale yellow”, or “straw coloured.”

Overview of the Excretory System

• The basic function of the excretory system is to regulate the volume and composition of body fluids by removing wastes and returning needed substances to the body for reuse.

The Problem of Wastes

• A waste is any substance that is produced by the body and is present in excess of the body’s needs.
• Examples of wastes: carbon dioxide, water, ions of sodium (Na+), chloride (Cl-), and hydrogen (H+), and compounds from the breakdown of proteins and nucleic acids.
• Nitrogenous wastes like ammonia, urea, and uric acid pose a greater immediate threat than others.
• Ammonia is highly toxic but is quickly converted to the less toxic compound, urea, in the liver.
• Urea makes up the majority of nitrogenous waste in the body, and about half of it is eliminated in urine.
• Uric acid is present in much lower concentrations and also passes out of the body in urine.

The Solution to Wastes: Excretion

• Excretion is the process of separating wastes from the body fluids and eliminating them.
  • 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 pigments and other cellular chemicals.
• Most metabolic wastes are dissolved or suspended in solution and are excreted by the excretory (also called the urinary) system.

The Organs of the Excretory System

• Humans have two fist-sized kidneys, located in the area of the lower back on each side of the spine.
• A large cushion of fat usually surrounds the kidneys, offering some protection.
• Humans can function with only one kidney.
• The kidneys release urine into two muscular, 28-cm-long tubes called 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.
• Urine exits the bladder and the body through a tube called the urethra.
  • In males, the urethra is approximately 20 cm long and merges with the vas 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 Body’s Blood-Cleansers

• Within each kidney, the mouth of its ureter flares open to form a funnel-like structure called the renal pelvis.
  • The renal pelvis has cup-like extensions that receive urine from the renal tissue.
• Kidney tissue is divided into two sections: an outer renal cortex and an inner renal medulla.
• Embedded within the renal cortex and extending into the renal medulla are over one million microscopic structures called nephrons.
• Closely associated with these nephrons is a network of blood vessels.
• The nephrons are responsible for filtering various substances from blood, transforming it into urine.
• Each nephron is organized into three main regions: a filter, a tube, and a duct.
  • A Filter: The filtration structure at the top of each nephron is a cap-like formation called the Bowman’s capsule. Within each capsule, a renal artery enters and splits into a fine network of capillaries called a glomerulus.
    • The walls of the glomerulus act as a filtration device; they are 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.
  • A Tubule: The Bowman’s capsule 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 three sections: the proximal tubule, the loop of Henle, and the distal tubule.
    • Like the small intestine, this tubule absorbs substances that are useful to the body, such as glucose and a variety of ions, from the filtrate passing through it.
    • Unlike the small intestine, this tubule also secretes substances into the tissues surrounding it.
  • A Duct: The tubule empties into a larger pipe-like channel called a collecting duct.
    • The collecting duct functions as a water-conservation device, reclaiming water from the filtrate passing through it so that very little precious water is lost from the body.
    • The filtrate that remains in the collecting duct is a suspension of water and various solutes and particles and is now called urine.
    • The solutes and water reclaimed during reabsorption are returned to the body via the renal veins.

9.2 Urine Formation in the Nephron

• Four processes are crucial to the formation of urine:
  • Glomerular filtration: moves water and solutes, except proteins, from blood plasma into the nephron.
  • 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.
• Two factors contribute to this filtration:
  • The permeability of the capillaries of the glomerulus.
  • Blood pressure within the glomerulus is about four times greater than it is in capillaries elsewhere in the body.
• Each day, 1600 L to 2000 L of blood passes through your kidneys, producing about 180 L of glomerular filtrate.
• This filtrate is chemically very similar to blood plasma.

Tubular Reabsorption: Recovery of Substances in the Proximal Tubule

• About 65 percent 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 involves both active and passive transport mechanisms.
• The cells of the proximal tubule are richly endowed with mitochondria, which use the energy-releasing power of ATP to drive the active transport of sodium ions (Na+), glucose, and other solutes back into the blood.

Focussing on the Loop of Henle in the Proximal Tubule

• The function of the loop of Henle is 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 permeability of the descending limb, water diffuses from the filtrate to the capillaries by osmosis.
• 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.
• 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 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.
• Potassium ions (K^+), and hydrogen ions (H^+) are actively secreted into the distal tubule from the bloodstream in the capillaries to maintain the pH of the blood.
• Substances that are not normally part of the body, such as penicillin and other drugs, are secreted into the distal tubule.

Reabsorption from the Collecting Duct

• The filtrate entering the collecting duct still contains a lot of water.
• The concentration of ions along its length increases.
• This causes the passive reabsorption of water from the filtrate in the collecting duct by osmosis.
• The permeability to water in the distal tubule and the collecting duct is increased if blood plasma is too concentrated.
• 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 four times as concentrated by the time it exits the duct.
• This filtrate—which is approximately 1 percent of the original filtrate volume—is now called urine.

9.3 Excretory System

• The amount of water reabsorbed from the filtrate influences two important characteristics of blood: its volume and the concentration of plasma solutes.
• The greater the concentration gradient, the greater the osmotic pressure becomes.
• Osmotic pressure affects many cellular activities, especially the exchange of materials between cells and blood.
• Blood volume influences blood pressure and, thus, affects the health of the cardiovascular system.
• If we drink too much, the kidneys allow more water to pass into the urine. If water is scarce, the kidneys conserve water by producing concentrated urine.

Regulating Reabsorption of Water

• Osmoreceptors are cells that are sensitive to osmotic pressure and are mostly located in the hypothalamus.
• When blood plasma becomes too concentrated, osmotic pressure increases.
• Osmoreceptors in the hypothalamus send impulses to the adjacent pituitary gland in the brain that causes the release of antidiuretic hormone (ADH).
• ADH travels through the blood to the kidneys, where it increases the permeability of the distal tubule and the collecting duct, allowing more water to be reabsorbed into the blood. This dilutes the blood and lowers osmotic pressure to normal.
• Conversely, if blood plasma is too dilute, osmoreceptors stop or prevent the release of ADH.
• The ethanol in alcoholic beverages is a diuretic, so it increases the volume of urine.
• Caffeine is also a diuretic.

Reabsorption of Salts

• The kidneys regulate salt balance in the blood by controlling the excretion and reabsorption of various ions.
• Sodium ion (Na^+) is the most abundant ion in blood plasma.
• A drop in blood (Na^+) concentration is normally compensated by the kidneys under the influence of the hormone aldosterone.
• Aldosterone stimulates the distal tubules and collecting ducts to reabsorb (Na^+).
• Aldosterone also stimulates the secretion of potassium ions (K^+) into the distal tubes and collecting ducts if (K^+) concentration in the blood is too high.

Maintaining Blood pH

The pH of body fluids stays at about 7.4 via three main mechanisms:

• The acid-base buffer system buffers the blood—prevents changes in (pH) by adding or removing hydrogen ions (H^+).
• One of the key buffering reactions in the blood involves carbonic acid (H2CO3)
  and bicarbonate ions (HCO3^-); H^+ + HCO3^- \rightleftharpoons H2CO3 \rightleftharpoons H2O + CO2
• An increased breathing (respiration) rate pulls the reaction to the right to generate (CO_2).These mechanisms to control acid-base balance are aided by the more powerful actions of the kidneys.
• The kidneys excrete (H^+) and reabsorbing (HCO_3^-)
  as needed to maintain normal blood (pH).

Upsetting the Balance of the Excretory System

• The composition of urine reflects the amounts of water and solutes that the kidneys must remove from or retain in the body to maintain homeostasis.
• Note, however, that urine composition varies greatly over the course of a day due to factors such as dietary intake, physical activity, emotional stress, and fatigue.
  Disorders of the Excretory System
• One of the most common disorders of the excretory system is a urinary tract infection.
• If the bladder has a bacterial or viral infection, the disorder is called cystitis; if only the urethra is involved, the condition is called urethritis.
• Most kidney stones form due to excess calcium in the urine.

Problems with Kidney Function

Renal insufficiency is a general term used to describe the state in which the kidneys cannot maintain homeostasis due to damage to their nephrons.

• Some causes of nephron damage include:
  • Kidney infection
  • High blood pressure
  • Diabetes mellitus
  • Trauma from a blow to the lower back or constant vibration from machinery
  • Poisoning by heavy metals such as mercury and lead
  • Atherosclerosis
  • Blockage of the tubules

Hemodialysis and Peritoneal Dialysis

• Dialysis is the diffusion of dissolved substances through a semipermeable membrane.
• Substances move across a membrane from the area of greater concentration to one of lower concentration.
• There are two main types of renal (kidney) dialysis: hemodialysis and peritoneal dialysis.
  • Hemodialysis utilizes an artificial membrane in an external device— in essence, an artificial kidney—that is connected to an artery and a vein in a person’s arm.
  • Peritoneal dialysis utilizes the lining of the intestines, called the peritoneum, as the dialysis membrane. Dialysate is introduced to the abdominal cavity.

Kidney Transplants

• Dialysis enables people with kidney disease to live their lives in a relatively unchanged way. However, dialysis is not a cure, and it is not intended to be a long-term solution to the problem of kidney disease.
• Individuals with 10 percent or less kidney function will eventually have to replace their kidneys.

The Kidney-Coronary Connection

• High blood pressure is one of the main reasons that kidneys begin to fail.

Metabonomics

• Metabonomics is the study of the complete metabolic response of an organism to various environmental or genetic stimuli.
• Changes in the levels of metabolites can be measured in body fluids such as blood or urine,
• One technology used by researchers to analyze body fluids is nuclear magnetic resonance (NMR) spectroscopy.