Excretory System Overview

Excretory System

• The excretory system is a collection of organs responsible for disposing waste from the body and maintaining osmotic balance in blood and plasma.

Osmoregulation

• Osmoregulation is the process by which organisms control the balance of water and solutes within their bodies to maintain internal stability despite changes in external environmental conditions.

Processes of Excretion

• There are 4 major processes of excretion: filtration, reabsorption, secretion, and excretion.
• Filtration: pressure-filtering of body fluids producing a filtrate (water, salts, sugar, amino acids, N-waste).
• Reabsorption: reclaiming of valuable solutes (glucose, salts, amino acids) from the filtrate.
• Secretion: addition of larger molecules like toxins and other excess solutes from the body fluids to the filtrate.
• Excretion: the filtrate leaves the system.

Primary Excretory Organs

• The primary excretory organs in fish are the kidney, gills, and skin.

Fish Kidney Structure

• The kidney is composed of the renal cortex (outer layer), renal medulla (inner layer), and renal pelvis (central area).
• Renal cortex: responsible for filtering the blood and removing waste products.
• Renal medulla: responsible for regulating the balance of fluids and electrolytes in the body.
• Renal pelvis: receives blood from the renal arteries and returns it to the renal veins.

Types of Kidneys in Fish

• Anterior kidney (head kidney or pronephros): plays a role in excretion during early development but is later replaced by the mesonephros and metanephros.
• Posterior kidney (excretory kidney or metanephros): the permanent kidneys in vertebrates, responsible for filtering blood, removing waste products, regulating electrolyte balance, and maintaining proper fluid levels.

Freshwater vs Saltwater Fish Kidney

• Freshwater fish kidneys excrete excess water and retain electrolytes, while saltwater fish kidneys excrete excess electrolytes and retain water.

Gills in Excretion

• Gills play a role in excretion through the process of diffusion, removing metabolic waste such as carbon dioxide and ammonia.
• Gills also regulate the balance of ions and maintain proper pH levels in the fish's body.

Skin in Excretion

• Fish excrete waste through their skin via sweat glands.
• Sweat contains salt, excess oils, water, and other unnecessary substances, which are then excreted through small pores.
• Sweating also helps regulate body temperature during evaporation.

Excretory Organs in Shellfish

• Coelom: a body cavity that serves as the excretory organ in shellfish such as clams and oysters.
• Waste products are eliminated through the coelom via the process of filtration.