24.1 Respiratory System

• The role of the respiratory system in gas exchange:

  • Cells are bathed in interstitial fluid.

  • Cells exchange oxygen, nutrients, and waste with interstitial fluid.

  • Interstitial fluid exchanges compound with blood.

  • Blood is refreshed through exchanges with the external environment.

  • The respiratory system exchanges gases in lungs.

  • Blood gives up CO2 and picks up O2 in lungs.

  • Respiration contributes to homeostasis.

  • Respiration requires breathing and external and internal exchange of gases.

  • Oxygen enters mitochondria for cellular respiration.

  • ATP is produced with oxygen delivery.

  • CO2 is a waste product of cellular respiration.

The Human Respiratory Tract

• Includes structures that conduct air to and from the lungs.

• Lungs are the major organ of gas exchange.

• Air is filtered, warmed, and humidified as it moves through the respiratory tract.

• Nose hairs and cilia filter air in the nose and respiratory passages.

• Cilia beat upward, carrying mucus, dust, and particles to the throat to be swallowed or spit out.

• Smoking destroys cilia, leading to lung disorders.

• Air cools and loses moisture as it moves out of the respiratory tract.

• Moisture deposits on the lining of the tract and may cause dripping from the nose.

• Air still retains enough moisture to form a small cloud on a cold day when breathed out.

Upper Respiratory Tract

• The upper respiratory tract consists of the nose, pharynx, and larynx.

• The nose is the only external portion of the respiratory system.

• The nose contains two nasal cavities, which are separated by a septum.

• The nasal cavities are connected to the sinuses, which are air-filled spaces.

• The nasal cavities and sinuses help to warm, moisten, and filter the air that we breathe.

• The pharynx is a funnel-shaped passageway that connects the nasal cavity and mouth to the larynx.

• The tonsils are a ring of lymphatic tissue at the junction of the mouth and pharynx.

• Tonsillitis is an inflammation of the tonsils.

• The epiglottis is a flap of cartilage that covers the glottis, the opening into the larynx, during swallowing.

• The larynx contains the vocal cords, which vibrate to produce sound.

• Laryngitis is an infection of the larynx that causes hoarseness.

Lower Respiratory Tract

• The lower respiratory tract contains:

  • The respiratory tree.

  • Trachea (windpipe.)

  • Bronchi.

  • Bronchioles.

• The trachea:

  • Connects the larynx to the bronchi.

  • Is held open by C-shaped, cartilaginous rings.

  • Allows food to pass down through the esophagus without damaging it.

  • Divides into two primary bronchi that enter the right and left lungs.

• Bronchi:

  • Continue to branch until there are many smaller passages called bronchioles.

  • Resemble the trachea in structure.

  • Walls become thinner, rings of cartilage disappear, and the amount of smooth muscle increases as passages divide and subdivide.

• Bronchioles:

  • Each terminates in an elongated space enclosed by alveoli (little air pockets that make up the lungs.)

• Bronchitis:

  • Infection of the bronchi.

  • Nonproductive cough becomes a deep cough that produces mucus and perhaps pus.

  • Smokers with a deep cough have bronchitis and irritated respiratory tract.

  • Progression can reverse if smoking stops.

  • Chronic bronchitis is the second step toward emphysema and lung cancer caused by smoking cigarettes.

  • Lung cancer often begins in the bronchi and spreads to the lungs.

• Respiration in other animals:

  • Insects have many tracheae, little air tubes supported by rings of chitin that branch into every part of the body.

  • Tracheal system begins at spiracles, openings that perforate the insect’s body wall, and ends in very fine, fluid-filled tubules.

  • Ventilation is assisted by the presence of air sacs that draw in the air.

  • No cell is very far from the site of gas exchange, so the bloodstream does not transport oxygen, and no oxygen-carrying pigment is required.

Breathing

• The diaphragm is a muscular, membranous partition in humans.

• It divides the upper thoracic cavity from the lower abdominal cavity.

• Muscle contractions lower the diaphragm and raise the ribs during breathing.

• This increases the volume of the thoracic cavity and lungs.

• The increased volume creates a negative pressure in the thoracic cavity and lungs.

• Air flows into the lungs due to this negative pressure, which is called inspiration.

• Air does not force the lungs open, they have already opened up.

• When the rib and diaphragm muscles relax, the lungs recoil and air moves out.

• This is due to increased pressure in the lungs, which is called expiration.

Breathing In Other Animals

• Mammals and most reptiles have the same route for air in and out of their lungs.

• Humans have residual air left in their lungs due to this.

• Birds use a one-way ventilation mechanism.

• Incoming air goes through the trachea and into abdominal air sacs.

• Air then passes through the lungs into thoracic air sacs.

• Fresh air never mixes with used air in the lungs of birds.

• This greatly improves gas exchange efficiency.

Control of Breathing

• Increased concentrations of H+ and CO2 in the blood increase breathing rate in humans.

• Aortic bodies and carotid bodies are chemoreceptors that monitor the chemical content of the blood.

• Aortic bodies and carotid bodies are sensitive to changes in H+ and CO2 concentrations but minimally sensitive to lower O2 concentrations.

• The need to breathe comes from a buildup of CO2 in the bloodstream, not necessarily from a lack of O2.

• Information from the chemoreceptors goes to the breathing center in the brain, which increases breathing rate when H+ and CO2 concentrations rise.

• The breathing center is also directly sensitive to the chemical content of the blood, including its O2 content.

Lungs and External Exchange of Gases

• Mammals and most reptiles have the same route for air in and out, leaving residual air in the lungs.

• Birds use a one-way ventilation mechanism, which greatly improves gas exchange efficiency.

• Frogs and salamanders use their moist skin for gas exchange in addition to their lungs.

• Human lungs have a total surface area at least 50 times the skin’s surface area due to the presence of alveoli.

• Alveoli are lined with surfactant, which prevents the sides of the alveoli from sticking together during exhalation.

• The respiratory membrane is formed by the alveolar epithelium and the capillary epithelium.

• Emphysema is a serious lung condition that reduces the surface area for gas exchange, leading to low energy levels.

Gills of Fish

• Fish and other aquatic animals use gills as their respiratory organ.

• Water is drawn into the mouth and out from the pharynx across the gills.

• The flow of blood in gill capillaries is opposite the flow of water across the gills.

• Blood is always exposed to water having a higher oxygen content.

• About 80-90% of the dissolved oxygen in the water is absorbed.

Transport and Internal Exchange of Gases

• Hemoglobin molecules carry oxygen to the body's tissues.

• Without hemoglobin, it would take 3 years for an oxygen molecule to move from lungs to toes by simple diffusion.

• Each hemoglobin molecule contains 4 polypeptide chains: 2 α and 2 β chains.

• Each polypeptide is folded around an iron-containing group called heme.

• The iron in heme bonds with oxygen and carries it to the tissues.

• Each red blood cell contains about 250 million hemoglobin molecules.

• Each red blood cell can carry at least 1 billion molecules of oxygen.

• Hemoglobin gives up oxygen in the tissues during an internal exchange.

• Interstitial fluid always has a lower oxygen concentration than blood.

• Cells take up and utilize oxygen during cellular respiration, causing the difference in oxygen concentration.

• Hemoglobin also gives up oxygen due to warmer temperatures and lower pH in the tissues.

• The warmer temperature and lower pH are caused by cellular respiration.

• Cells give off heat and carbon dioxide as by-products during cellular respiration.

  

• Carbon dioxide enters the blood during the internal exchange.

• Interstitial fluid has a higher concentration of carbon dioxide.

• Most of the carbon dioxide is transported as bicarbonate ions (HCO3-).

• Carbon dioxide combines with water to form carbonic acid.

• Carbonic acid dissociates to hydrogen ion (H+) and HCO3-.

• H+ causes pH to lower, but only slightly due to absorption by globin portions of hemoglobin.

• HCO3- is carried in plasma.

• In the lungs, carbon dioxide diffuses out of the blood into alveoli as blood enters pulmonary capillaries.

• Hemoglobin gives up H+ it has been carrying during this reaction.

• Carbon dioxide diffuses out of the blood into the alveoli of the lungs.

• If the blood level of H+ rises, the breathing center in the brain increases the breathing rate.

• As more CO2 leaves the blood, the pH of the blood is corrected.

24.2 Urinary System

• The urinary system in vertebrate animals, including humans, is important for homeostasis. The kidneys are the primary organs responsible for the following functions:

  • Excretion of nitrogenous wastes, such as urea and uric acid.

  • Maintenance of the water-salt balance of the blood.

  • Maintenance of the acid-base balance of the blood.

• The kidneys in humans are bean-shaped, reddish-brown organs.

• They are about the size of a fist and located on each side of the vertebral column, just below the diaphragm.

• The kidneys are partially protected by the lower rib cage.

• Urine made by the kidneys is conducted from the body by the other organs in the urinary system.

• Each kidney is connected to a ureter, a tube that takes urine from the kidney to the urinary bladder.

• Urine is stored in the urinary bladder until it is voided from the body through the single urethra.

• In males, the urethra passes through the penis; in females, it opens in front of the opening of the vagina.

• In females, there is no connection between the genital (reproductive) and urinary systems.

• In males, there is a connection between the genital and urinary systems, since the urethra also carries sperm during ejaculation.

• In amphibians, birds, reptiles, and some fishes, the bladder empties into the cloaca, a common chamber and outlet for the digestive, urinary, and genital tracts.

Human Kidney

• When a kidney is sectioned longitudinally, three major parts can be distinguished:

  • Renal cortex: the outer region of the kidney with a granular appearance.

  • Renal medulla: consists of cone-shaped renal pyramids, located inside the renal cortex.

  • Renal pelvis: the innermost part of the kidney, which is a hollow chamber where urine collects.

• Urine is carried from the renal pelvis to the bladder by a ureter.

• The cortex and medulla of each kidney are composed of about 1 million tiny tubules called nephrons.

• The nephrons of a kidney produce urine.

Nephrons

• Each nephron is composed of several parts.

• The glomerular capsule is formed by the closed end of a nephron.

• Filtration of the blood occurs at the glomerular capsule.

• The inner layer of the capsule is made up of specialized cells that allow easy passage of molecules.

• The proximal convoluted tubule is a portion of the nephron that follows the capsule.

• The proximal convoluted tubule is lined by cells with many mitochondria and tightly packed microvilli.

• The nephron loop comes after the proximal convoluted tubule and has a descending and ascending limb.

• The distal convoluted tubule follows the nephron loop.

• Several distal tubules enter one collecting duct.

• A collecting duct delivers urine to the renal pelvis.

• The nephron loop and the collecting duct give the pyramids of the renal medulla their striped appearance.

• Each nephron has its own blood supply.

• Various exchanges occur between parts of the nephron and a blood capillary as urine forms.

Urine Formation

• Urine formation requires three steps: filtration, reabsorption, and secretion.

• Filtration is the movement of small molecules from a blood capillary to the inside of the glomerular capsule as a result of blood pressure.

• Small molecules such as water, nutrients, salts, and urea move to the inside of the capsule.

• Plasma proteins and blood cells are too large to be part of this filtrate, so they remain in the blood.

• If the composition of the filtrate were not altered in other parts of the nephron, death from loss of nutrients (starvation) and loss of water (dehydration) would quickly follow.

• The next step, reabsorption, helps prevent this.

• Reabsorption of Solutes:

  • Water and other substances move from the proximal convoluted tubule into the blood.

  • Nutrients such as glucose and amino acids also return to the blood.

  • Some molecules, such as glucose, are both passively and actively reabsorbed.

  • Cells of the proximal convoluted tubule have numerous microvilli and mitochondria.

  • Glucose in urine is a sign of diabetes mellitus.

  • Sodium ions (Na+) are actively pumped into the peritubular capillary, and then chloride ions (Cl–) follow passively.

  • Water moves by osmosis from the tubule into the blood.

  • About 60-70% of salt and water are reabsorbed at the proximal convoluted tubule.

  • Some of the urea and other types of nitrogenous wastes excreted by humans are passively reabsorbed, but most remain in the filtrate.

• Secretion:

  • Secretion is the transport of substances into the nephron by means other than filtration.

  • Substances such as uric acid, hydrogen ions, ammonia, and penicillin are eliminated by secretion.

  • The process of secretion helps rid the body of potentially harmful compounds that were not filtered into the capsule.

• Regulation of Water-Salt Balance and pH:

  • All animals maintain water-salt balance and pH of the internal environment.

  • Humans use long nephron loops to secrete hypertonic urine.

  • Ascending limb of the nephron loop pumps salt and urea into the renal medulla, and water follows by osmosis.

  • Hormones regulate water-salt reabsorption by kidneys.

  • Coffee interferes with one hormone, causing the production of more urine.

  • Kidneys regulate pH of blood through bicarbonate buffer system and regulation of breathing rate.

  • Kidneys excrete a wide range of acidic and basic substances.

  • Kidneys reabsorb bicarbonate ions and excrete hydrogen ions to maintain normal blood pH.

  • Urine is usually acidic, indicating excess hydrogen ions are produced and excreted.

  • Ammonia and phosphate provide means of buffering hydrogen ions in urine.

Regulation of Water-Salt Balance in Other Animals:

• Insects have a unique excretory system consisting of Malpighian tubules attached to the gut.

• Uric acid is the primary nitrogenous waste product of insects.

• Uric acid diffuses from the hemolymph into the Malpighian tubules.

• Water follows a salt gradient established by active transport of potassium (K+).

• Water and useful substances are reabsorbed at the rectum.

• Uric acid leaves the body at the anus.

• Insects in dry environments reabsorb most of the water and excrete a dry, semisolid mass of uric acid.

• Insects that live in water or eat large quantities of moist food reabsorb little water.

• Most animals can regulate the blood levels of both water and salt.

• Freshwater fishes take up salt in the digestive tract and gills and produce large amounts of dilute urine.

• Saltwater fishes take in salt water by drinking, but then they pump ions out at the gills and produce only small amounts of concentrated urine.

Problems with Kidney Function

• Illnesses such as diabetes, hypertension, and inherited conditions can cause progressive renal disease and renal failure.

• Urinary tract infections, an enlarged prostate gland, pH imbalances, or excessive calcium intake can lead to kidney stones.

• Kidney stones form in the renal pelvis and can block the renal pelvis or ureter, leading to back pressure that destroys nephrons.

• The presence of albumin, white blood cells, or red blood cells in the urine is one of the first signs of nephron damage.

• If more than two-thirds of the nephrons are inoperative, urea and other waste substances accumulate in the blood.

• Retention of water and salts is of greater concern than nitrogenous waste in the blood.

• Edema, fluid accumulation in the body tissues, may occur.

• An imbalance in the ionic composition of body fluids can lead to loss of consciousness and even heart failure.

Hemodialysis and Kidney Transplants

• Patients with renal failure can undergo hemodialysis using an artificial kidney machine or continuous ambulatory peritoneal dialysis (CAPD).

• Dialysis is the diffusion of dissolved molecules through a semipermeable membrane with pore sizes that allow only small molecules to pass through.

• In an artificial kidney machine, the patient's blood is passed through a membranous tube in contact with a dialysis solution or dialysate.

• Substances more concentrated in the blood diffuse into the dialysate, and substances more concentrated in the dialysate diffuse into the blood.

• The dialysate is continuously replaced to maintain favorable concentration gradients.

• In CAPD, the peritoneum is the dialysis membrane, and a fresh amount of dialysate is introduced directly into the abdominal cavity from a bag.

• Waste and salt molecules pass from the blood vessels in the abdominal wall into the dialysate before the fluid is collected 4 or 8 hours later.

• Patients with renal failure may undergo a transplant operation, receiving a functioning kidney from a donor.

• Receiving a kidney from a close relative has the highest chance of success.

• The current 1-year survival rate for a kidney transplant is 98%, and the 5-year survival rate is more than 91%.