Chapter 26

MAINTAINING THE INTERNAL ENVIRONMENT CHAPTER 26

HOW THE ANIMAL BODY MAINTAINS HOMEOSTASIS • Homeostasis may be defined as the dynamic constancy of the internal environment. • Conditions fluctuate continuously within narrow limits.

HOW THE ANIMAL BODY MAINTAINS HOMEOSTASIS • To maintain internal constancy, the vertebrate body uses: • Sensors that measure each condition of the internal environment. • An integrating center that contains the set point, or proper value for a particular internal condition. • Effectors, which are muscles or glands that can change the value of the condition back toward the set point. • The activity of the effectors is influenced by the effects they produce in a negative feedback loop.

HOW THE ANIMAL BODY MAINTAINS HOMEOSTASIS • Regulating body temperature • Humans, as well as other mammals and birds, are endothermic. • This means that they can maintain relatively constant body temperature. • Other vertebrates are ectothermic, meaning their body temperatures depend more or less on the environmental temperature. • But they can modify their behavior to affect body temperature.

HOW THE ANIMAL BODY MAINTAINS HOMEOSTASIS • Regulating blood glucose • Excess glucose is stored in the liver as glycogen under the influence of the hormone insulin, which is released from the pancreas. • When glucose levels are low in the blood, the pancreas releases the hormone glucagon, which stimulates the liver to convert glycogen back to glucose.

CONTROL OF BLOOD GLUCOSE LEVELS

REGULATING THE BODY’S WATER CONTENT • Animals use various mechanisms for osmoregulation, the regulation of the body’s osmotic composition. • This refers to how much water and salt the body contains. • The proper operation of many vertebrate organ systems requires that the osmotic concentration of the blood be kept within narrow bounds.

REGULATING THE BODY’S WATER CONTENT • In many animals and single-celled organisms, the removal of water and salts from the body is coupled with the removal of metabolic wastes through the excretory system.

REGULATING THE BODY’S WATER CONTENT • For example, protists, like Paramecium, employ contractile vacuoles.

Anterior contractile vacuole

Cilium Feeder canal Contractile vacuole

Excretory pore Endoplasmic reticulum

Posterior contractile vacuole

REGULATING THE BODY’S WATER CONTENT • Flatworms employ a system of excretory tubules called protonephridia to expel fluids and wastes from the body.

Excretory pores

Flame cell Cilia Collecting tubule

REGULATING THE BODY’S WATER CONTENT • Other invertebrates have a system of tubules that open both to the inside and to the outside of the body. • In annelids, these tubules are called nephridia.

Nephridium

Coelomic fluid

Bladder Capillary network

Pore for Nephrostome urine excretion

REGULATING THE BODY’S WATER CONTENT • The excretory organs in insects are called Malpighian tubules, which are extensions of the digestive tract.

Malpighian tubules Waste molecules K+ Water

Air sac Malpighian tubules

Mid gut

Mid gut Intestine

Hindgut Rectum Poison sac Rectum

Anus

Water and K+

REGULATING THE BODY’S WATER CONTENT • Kidneys are the excretory organs in vertebrates. • Kidneys create a tubular fluid by filtration. • The filtrate contains many valuable nutrients in addition to waste products. • Selective reabsorption ensures that these nutrients and water are reabsorbed into the blood, while wastes remain in the filtrate.

EVOLUTION OF THE VERTEBRATE KIDNEY • The kidney is a complex organ made up of many repeating units called nephrons. • Blood pressure forces the fluid in the blood through a capillary bed at the top of each nephron, called a glomerulus. • The glomerulus excludes blood cells, proteins, and other large molecules from the filtrate. • The remainder of the nephron tube reabsorbs anything else useful from the filtrate

BASIC ORGANIZATION OF THE VERTEBRATE NEPHRON Proximal arm

Distal arm Glomerulus

Neck

Glucose Amino acids Divalent ions

Intermediate segment

H2O

NaCl

H2O

H2O H2O

NaCl

H2O

H2O

Collecting duct

EVOLUTION OF THE VERTEBRATE KIDNEY • Only birds and mammals can reabsorb water from the glomerular filtrate to produce a urine that is hypertonic to (more concentrated than) blood.

EVOLUTION OF THE VERTEBRATE KIDNEY • Kidneys are thought to have evolved first among the freshwater fish. • The body fluids of a freshwater fish have a greater osmotic concentration than the surrounding water. So, • Water tends to enter the body from the environment. • Solutes tend to leave the body and enter the environment.

EVOLUTION OF THE VERTEBRATE KIDNEY • Freshwater fish address these problems by

NaCl

Large glomerulus

NaCl

Active tubular reabsorption of NaCl

Kidney tubule

• Not drinking water. • Excreting a large volume of dilute urine. • Reabsorbing ions (mainly NaCl) from the nephron. • Actively transporting NaCl across the gills from the surrounding water into the blood.

Food, fresh water (passes over gills)

Kidney: Excretion of dilute urine

Gills: Active absorption of NaCl, water enters osmotically

Intestinal wastes

Urine

Freshwater fish

EVOLUTION OF THE VERTEBRATE KIDNEY • Marine fish probably evolved from freshwater ancestors.

Glomerulus reduced or absent Stomach: Passive reabsorption of NaCl and water

• Their bodies are hypotonic to the surrounding seawater. So, • Water tends to leave their bodies through osmosis across the gills. • They lose water in their urine. • To compensate, marine fish drink lots of seawater • They excrete isotonic urine.

MgSO4 MgSO4

Active tubular secretion of MgSO4

Food, seawater

Gills: Active secretion of NaCl, water loss

Intestinal wastes: MgSO4 voided with feces

Marine fish

Kidney: Excretion of MgSO4, urea, little water

EVOLUTION OF THE VERTEBRATE KIDNEY • Elasmobranchs solve the osmotic problem posed by their seawater environment by reabsorbing urea from the nephron tubules. • The blood is approximately isotonic to the surrounding sea. Kidney

Glomerulus Urea Urea

Kidney tubule

Cartilaginous fish

EVOLUTION OF THE VERTEBRATE KIDNEY • The amphibian kidney is like that of freshwater fish. • Amphibians produce a very dilute urine and actively transport Na+ across their skin.

• The kidneys of terrestrial reptiles reabsorb much of the salt and water in the nephron tubules. • Their urine is still hypotonic but they can absorb additional water in the cloaca

EVOLUTION OF THE VERTEBRATE KIDNEY • Because mammals and birds can produce hypertonic urine, they can excrete their waste products in a small volume of water.

• The kidneys of some mammals are even more extremely efficient at conserving water. • The kidneys of the kangaroo rat are so efficient it never has to drink water; it can obtain all the water it needs from its food and aerobic cell respiration.

EVOLUTION OF THE VERTEBRATE KIDNEY • Birds have relatively few or no nephrons Salt glands with long loops. • At most, they can only reabsorb enough water to produce a urine that is about twice the concentration of their blood • Marine birds solve the problem of water loss by drinking sea water and excreting excess salt through salt glands near the eyes.

Salt secretion

THE MAMMALIAN KIDNEY • In mammals, each kidney receives blood from a renal artery, and it is from this blood that urine is produced. • Urine drains from each kidney through a ureter. • The ureters carry urine to a urinary bladder. • Urine passes out of the body through the urethra.

THE MAMMALIAN KIDNEY • Within the kidney, the mouth of the ureter flares open to form a funnel-like renal pelvis. • The renal tissue is divided into: • An outer renal cortex • An inner renal medulla

Nephron

Renal cortex

Renal medulla

Renal artery Renal vein

Ureter

THE MAMMALIAN KIDNEY • The mammalian kidney is comprised of roughly 1 million nephrons, each of which is composed of three regions: • Filter

• The filtration device at the top of each nephron is called the Bowman’s capsule which receives filtrate from the glomerular capillaries.

• Tube

• The Bowman’s capsule is connected to a long renal tubule, which includes the Loop of Henle, that acts as a reabsorption device.

• Duct

• The renal tubule empties into a collecting duct that operates as a water conservation device.

THE MAMMALIAN KIDNEY There are five steps involved in the formation of urine in the kidney: Total solute concentration (mOsm)

•

1. Pressure filtration 2. Reabsorption of water 3. Selective reabsorption 4. Tubular secretion 5. Further reabsorption of water

Glomerulus

Proximal Distal tubule tubule

1 Na+ – Cl H2O

300

4 Nitrogenous wastes Collecting duct

Cortex

2 H2O

600

3 Na+ H2O Cl–

Outer medulla

5 H2O Loop of Henle

1200 http://youtu.be/TzwPmz5V6Xg

Bowman's capsule

Inner medulla

Urea H2O

ELIMINATING NITROGENOUS WASTES • Amino acids and nucleic acids are nitrogencontaining molecules. • When animals metabolize these substances, they produce nitrogen-containing byproducts, called nitrogenous wastes, that must be eliminated by the body.

ELIMINATING NITROGENOUS WASTES • The first step in the metabolism of amino acids and nucleic acids is the removal of the amino (—NH2) group. • This group is then combined with H+ to form ammonia (NH3). • This takes place in the liver.

ELIMINATING NITROGENOUS WASTES • Ammonia is quite toxic and is safe only in very dilute concentrations. • Fish and tadpoles, ammonia can be directly eliminated across the gills or excreted in dilute urine. • In sharks, adult amphibians, and mammals, the nitrogenous waste is eliminated as urea, which is less toxic. • Reptiles, birds, and insects excrete nitrogenous wastes in the form of uric acid, which can be excreted with very little water.

NITROGENOUS WASTES Amino acids and nucleic acids

1

Catabolism

Ammonia by-product

2

3 Converted to urea

Eliminated directly

Urea

NH2

Ammonia

O NH3

4 Converted to uric acid

Uric acid

O H N

C HN

NH2

O O

N H

N H

Most fish

Mammals, some others

Reptiles and birds