Basic Principles of Excretion

The excretory systems remove metabolic wastes and retain proper amounts of water, salts, and nutrients. Basically any exchange surface, including the body surface, is competent for excretion, provided it has sufficient permeability for the solutes to be excreted and a source of energy to mobilize them. Since, as mentioned above, excretion is strongly linked to osmotic and water balance, specialized compartments have been evolved. Here, excretion and osmoregulation are represented in different portions of the transporting epithelia. Typically, the excretory systems in vertebrates include the kidneys, liver, lungs, and skin and despite the great variety of architectural complexity of excretory organs, some basic structural plans and principles govern their physiology.

Ultrafiltration

This mechanism is typical of glomerular kidneys. These excretory organs result from the combination of a very large number of basic tubular structures called nephrons where water and the solutes, removed from the internal fluids, are collected to form an ultrafiltrate, a pre-urine. This occurs in a specialized region called glomerulus of Malpighi where tight connections between the endothelial cells and specialized cells of the tubulus (podocytes) optimize the exchange surface. The nephrons also involve specialized areas where the composition of the pre-urine is changed by selectively removing or adding some solutes and where the urine can be made hyper- or hyposmotic with respect to the body fluids by water regulation. This occurs in positions downstream with respect to the glomerolus. In most taxa, the formation of a pre-urine occurs through a mechanism called ultrafiltration (the pre-urine is usually defined as ultrafiltrate). In this process, a different hydrostatic and/or osmotic pressure exists between two fluids separated by a membrane or an epithelium capable of selecting the solutes that are differently capable of permeating the interface on the basis of their hydrated size. Membrane pores (provided by specific proteins called 'porines') or pores formed at the level of intercellular junctions provide preferential pathways of permeation to hydrosoluble molecules. The ultrafiltration process can be basically described as driven by a difference of hydrostatic pressure (APhydr — Pc - Pi where c and i stand for capillary and interstitium) between the two liquids -separated by the ultrafiltrating epithelium. When this interface separates a body fluid such as blood, hemolymph, or a coelomic fluid from the ultrafiltrate, the composition of the two liquid media is different for the presence of nondiffusible solutes (proteins) in the former that are almost absent in the latter. This differing fluid composition generates a difference of osmotic pressure (defined as colloidosmotic or oncotic pressure, A^onc — - that algebraically adds to the hydrostatic pressure in defining the value of the effective hydrostatic pressures (Peff):

In order to define the value of the filtration pressure (Pfilur) responsible for the water flow across the capil-lary/glomerular interfaces, the excretory tubular system exerting a back pressure (P'hydr) in the opposite direction with respect to Peff has to be taken into account; hence,

Figures 1a and 1b show a scheme of the ultrafiltration mechanism in the case of a glomerular nephron. Such filtration pressure defines the transmembrane volumetric flux per unit of membrane surface as

where Lp represents the filtration coefficient, also called hydraulic permeability or hydraulic conductivity.

Active Transport

Another very important mechanism involved in excretory processes is 'active transport'. This process is of fundamental importance in generating the ultrafiltrate in some organisms that do not perform tubular ultrafiltration and in regulating the composition of ultrafiltrate by driving the reabsorption of solutes.

In some taxa, the pre-urine is produced by active secretion that involves active (i.e., ATP-driven) transport of electrolytes to form an osmotic gradient competent to generate water flows. Typical examples of such systems are the malpighian tubuli in insects and the aglomerular kidneys of fish.

The primary active transport is catalyzed by the Na+/ K+ ATPase (also referred to as Na+/K+ pump), and the H+ ATPase (H+ pump), two plasma membrane proteins that generate either Na+ and K+ or H+ electrochemical gradients across the membrane as reported in Figures 2 a and 2b. Ions and organic molecules can move in the direction of decreasing electrochemical potentials and generate fluxes that are catalyzed by membrane-bound translocating proteins. Such movements depend indirectly on the ATPase activity of the pump. Since they are generally linked to the Na+ fluxes that dissipate such electrochemical gradient, they are referred to as secondary active transports and are permitted by membrane-bound protein carriers such as the glucose/Na+, amino acids/Na+ and the Na+/2Cl~/K+ simporter, the

Capillary endothelial ce

APhydr

Capillary endothelial ce

APhydr

Podocyte ood inle i.

Podocyte

(b) Blood inlet c

ood inle i.

hydr Ultrafiltrate rr

Blood outlet

Ultrafiltration

Ultrafiltration

Figure 1 Schematic representation of the forces that govern the ultrafiltration at the level of the glomerulus of a nephron, the basic functional unit of glomerular kidneys. The effective hydrostatic pressure (Peff) results from the combination of two opposite pressure differences: the hydrostatic pressure and oncotic pressure between the blood inside the glomerular capillaries and the liquid in the glomerulus (a). (b) Describes how the filtration pressure generates the ultrafiltrate. (c) Schematic representation of a glomerular nephron, with indication of the successive specialized sections where the ultrafiltrate, generated at the level of glomerulus, is converted to urine. The relative dimensions and lengths of the sections are arbitrary. G, glomerulus; PCT, proximal convoluted tubulus; LH, loop of Henle; DCT, distal convoluted tubulus; CD, collecting duct. The morphology of the different sections is also arbitrary.

Figure 2 Schematic representation of the vectorial transport of Na+ and K+ catalyzed by the membrane-bound ATP-driven activity of Na+/K+-ATPase (a) and H+-ATPase (b). The direction where the electrochemical potential of the transported solutes increases and the direction of their passive flow are indicated.

Figure 2 Schematic representation of the vectorial transport of Na+ and K+ catalyzed by the membrane-bound ATP-driven activity of Na+/K+-ATPase (a) and H+-ATPase (b). The direction where the electrochemical potential of the transported solutes increases and the direction of their passive flow are indicated.

Na+/H+ antiporter. Cells involved in transport of solutes are functionally polarized since they express pumps or passive transporters on different sides ofthe plasma membrane, either apical or basolateral.

(1) ultrafiltrate water and body fluids; (2) remove and concentrate waste products from body fluids; and (3) return the substances necessary for homeostasis to body fluids.

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