Chapter 44 (3)

Much of the movent of solutes between internal fluids and their external environment is handled by the __. These systems are central to __ because thye dispose of metabolic wastes and control body fluid composition.

Animals across a wide range of species produce a fluid waste called __ through four basic steps. What arethey?

In the first step, what happens?
-- In most cases, hydrostatic pressure drives a process of __. Cells, as well as proteins and other large molecuels, cannot cross the epithelial membrane and remain in the body fluid. In contrast, water and small solutes cross the membrane forming a solution called the __.

The __ is converted into a waste fluid by the specific transport of materials into or out of the filtrate. The process of selective __ recovers useful moelcuesl and water from teh __ and returns them to the body fluids. Valuable solutes are reabsorbed by active transport. Nonessential solutes and wastes are left in the filtrate or are added to it by selective __, which also occurs by active transport. The pumping of various solutes adjusts the osmotic movement of water into out of the __. In the last step- __- the processed filtrate is released from the body as urine.

The systems that perform the basic __ functions vary widely among animal groups. However, they are generally built on a complex network of tubules that provide a large surface area for the exchange of water and solutes, including nitrogenous wastes.

Flatworms, which lack a coelom or body cavity, have excretory systems called __. The __ form a network of dead-end tubules connected to external openingts. THe tubules branch throughout the body. Cellular units called __ cap the branches of each __. Formed from a tubule cell and a cap cell, each __ has a tuft of cilia projecting into the tubule.

During __, the beating of the cilia draws water and solutes from the interstitial fluid through the flame bulb, releasing filtrate into the tubule network. The processed __ then moves outward through the tubules and empties as urine into the external environment. The urine excreted by freshwater flatworms has a low solute concentration, helping to balance the osmotic uptake of water from the environment.

__ are also found in rotifers, some annelids, mollusc larvae, and lancelets. Among these animals, the funciton of the __ varies. In the freshwater flatworms, __ serve mainly in osmoregulation. Most metabolic wastes diffuse out of the animal across the body surface or are excreted in the __ and eliminated through the mouth. However, in some parasitic flatworms, which are __ to the surrounding fluids of their host organisms, the main ufnction of __ is the disposal of nitrogenous wastes. Natural selection has thus adapted __ to distinct tasks in different environments.

Most annelids have __, excretory organs that open internally to the coelom. Each segment of a worm has a pair of __, which are immersed in coelomic fluid and enveloped by a capillary network. A ciliated funnel surrounds the internal opening. As the cilia beat, fluid is drawn into a collecting tubule, which includes a storage bladder that opens to the outside.

The __ of an earthworm has both excretory and osmoregulatory functions. As urine moves along the tubule, the __ bordering the lumen reabsorbs most solutes and returms them to the blood in the capillaries.

Nitrogenous wastes remain in the tubule and are excreted to the outside. Earthworms inhabit damp soil and usually experience a net uptake of water by osmosis through their skin. Their __ balance the water influx by producing urine that is __ (hypoosmotic to body fluids).

Insects and other terrestrial arthropods have organs called __ that remove nitrogenous wastes and also function in osmoregulation. The __ extend from dead-end tips immersed in hemolymp (circulatory fluid) to openings into the digestive tract. THe __ step common to other excretory systems is absent.

Instead, teh transport epithelium that lines the tubules secretes certain solutes, including nitrogenous wastes, from teh hemolymph into the lumen of the tubule. Water follows the solutes into the tubule by osmosis, and the fluid then passses into the rectum. There, most solutes are pumped back into the hemolymph, and water reabsorption by osmosis follows. The nitrogenous wastes- mainly insoluble __- are eliminated as nearly dry matter along with the feces. Capable of conserving water very differnetly, the insect excretory system is a key adaptation contributing to these animals' tremendous success on land.

In vertebrates and some other chordates, a specialized organ called the kidney functions in both __ and __. Like the excretory organs of most animal phyla, kidneys consist of tubules. THe numerous tubules of these compact organs are arranged in a highly organized manner and closely associated with a network of capillaries.

The vertebrate __ also includes ducts and other structures that carry urine from the tubules out of the kidney and, eventually, out of the body.

True or False:
Vertebrate kidneys are typiccally nonsegmented. But hagfishes, which are invertebrate chordates, have kidneys with segmentally arranged excretory tubules; so the excretory structures of vertebrate ancestors may have been segmented.
True False

The excretory system of mammals centers on a pair of kidneys. In humans, each kidney is about 10 cm long and is suppplied with blood by a __ and drained by a __.

Blood flow through the kidneys is voluminous. The kidneys account for less than 1% of human body mass but receive roughly 25% of the blood exiting hte heart. Urine exits each kidney through a duct called the __, and oth __ drain tnto a common __.

During urination, urine is expelled fromt eh bladder through a tube called the __, which empties to the outside near hte vagina in females and through the penis in males. Urination is regulated by sphincter muscles close to the junction of the __ and the bladder.

the mammalian kidney has an outer __ and an inner __. Microscopic excretory tubules and their associated blood vessels pack both regions. Weaving back and forth across the cortex and medulla is the __, the functional unit of hte vertebrate kidney. A __ consists of a single long tubule as well as a ball of capillaries called hte __. The blidnd end of the tubule forms a cup-shaped swelling called __, which surrounds the __. Each human kidney contains about a million __, whth a total tubule length of 80 km.

Filtration occurs as blood pressure forces fluid fromt eh blood in the __ into the lumen of __/ The porous capillaries and specialized cells of the capsule are permeable to water and small solutes, but not to blood cells or large moleucles such as plasma protiens. Thus, the filtrate in __ contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules. Because filtration of small moleules is nonselective, the mixture mirrors the concentrations of tehse substances in blood plasma.

From Bowman's capsule, the filtrate passes into the __, the first of thre major regions of the __. Next is the __, a hairpin turn with a descending limb and an ascending limb.

THe __, th last region of the nephron, empties into a __, which receives processed filtrate from many nephrons. This iltrate flows from all of the collecting ducts of the kidney into the __, which is drained by the ureter.

Among the vertebrates, only mammals and some birds have loops of Henle. In the human kidney, 85% of the nephrons are __, which have short loops of Henle and are almost entirely confined to the renal cortex. THe other 15% the __, have loops that extend deeply into the renal medulla. It is the __ that enable mammals to produce urine that is hyperosmotic to body fluids, an adaptation that is extremely important for water conservation.

True or False:
The nephron and the collcting duct are lined by a transport epithelium that processes the filtrate forming the urine. One of this epithelium's most important tasks is reabsorption of solutes and water.
True False

Each nephtron is supplied with blood by an __, an offshoot of the renal artery that branches to form the capillaries of the glomerulus. THe capillaries converge as they leave the glomerulus, forming an __. Branches of this vessel form the __, which surround the proximal and distal tubles. A third set of capillaries extend downward and from the __, hairpinshaped capillaries that serve the long loop of Henle of juxtamedullay nephrons.

The direction of blood flow within the capillaries of the __ is opposite that of the filtrate in the neighboring __.
In other words, each ascending portion of the __ lies next to the descending portion of a __, and vice versa. Both the tubules and capillaries are immersed in interstitial fluid, through which various sunstances diffuse between the plasma within capillaries and the filtrate w/in the nephron tubule. Although they don't exchange materials diretly, the __ and the __ function together as part of a countercurrent system that enhances nephron efficiency.