Excretory System of Fish What is Excretion? Excretion is a process by which metabolic waste is climinated from an organism. Excretion is an essential process in all forms of life, for example in fish urine is expelled through the urethra, which is a part of excretory system. What is Excretory system of Fish? The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an organisms. So, as to help maintain internal chemical homeostasis and prevent damage to the body. How do fish excrete waste? Fish excrete both solid and liquid waste to remove toxic substances like ammonia from there body. The kidney is the fish primary excretory organ. It remove waste product from the body and also help maintain the correct balance of salt and water in fish. Organ related with excretion Kidney Ureter Urinary bladder Kidney Blood waste are filtered by the kidney. Salt water fish tend to lose water because of osmosis. There kidney return water to the body. A dissection of fish Kidney Microscopic view of Kidney Nephron Histologically Each kidney contains over approximately 1 million tiny structure called nephrons. Nephron is the structural and functional unite of the kidney. There are two types of Nephron. 1. Cortical nephron: 80% of the nephrons are short and located within the cortex. 2. Juxtamedulary nephron: 20% of nephron have long loops of Henle that extend into the medulla. Structure of NEPHRON Each nephron has two major portion . 1. A renal corpuscle (Malpighian body). 2. A renal tubule. Renal corpuscle A renal corpuscle consist of a Glomerulus surrounded by Bowman’s capsule. Glomerulus: The glomerulus is a capillary network that arises from afferent arteriole & empties into an efferent arteriole. The diameter of the efferent arteriole is smaller than that of the afferent arteriole which helps maintain a fairly high blood pressure in the glomerulus. Bowman’s capsule: It’s double walled cup like structure and it encloses the glomerulus. The wall of glomerulus and bowman’s capsule consist of a single layer of flattened epithelial cell. RENAL TUBULE The renal tubule continues from Bowman’s capsule and consist of--- ➢ Proximal convoluted tubule ➢ Loop of Henle ➢ Distal convoluted tubule Proximal convoluted tubule It is a proximal part of renal tubules next to Bowman’s capsule. Its lined with microvilli. Maximum reabsorption of water ,glucose ,amino acids and electrolytes takes place here. Loop of Henle It is U shaped middle portion of renal tubules. It is composed of ascending and descending loop. Ascending loop is thick walled and impermeable to water while descending loop is thin walled and permeable to water. Counter current mechanism is crucial role of loop of Henle. Distal convoluted tubules (DCT) It is the distal part of renal tubules that leads to collecting ducts. It is similar in structure and function with PCT. Collecting tubules It is not a part of nephron rather it is a part of kidney. The distal convoluted tubules from several nephrons empty into a collecting tubule. Several collecting tubules then unite to form a papillary duct that empties urine into a minor calyx and then into major calyx and finally into renal pelvis. Ultra Filtration Blood flows inside glomerulus under pressure due to narrowness of efferent arteriole. As a result it undergoes pressure filtration or ultrafiltration. All small volume solutes (e.g., urea, uric acid, amino acids, hormones, glucose, ions, vitamins) and water are filtered out and enter the Bowman's' capsule. The product is called nephric or glomerular filtrate 1ts volume is 125 ml/min 0 80 liters/day) Reabsorption Nephric filtrate is also called primary urine. It passes into proximal convoluted tubule. The same is surrounded by peritubular capillaries. The latter reabsorb all the useful components of nephric filtrate, e.g, glucose, amino acids, vitamins C, calcium, potassium, sodium, chloride, bicarbonate and water (75ato). Selective absorption also occurs in the region of distal convoluted tubule. Secretion (Augmentation). It occurs mostly in the distal convoluted tubule which Is also surrounded by peritubular capillaries. Smaller amount of tubular secretion also takes place in the area of proximal convoluted tubule. Tubular secretion is active secretion of waste products by the blood capillaries. It cause removal of all the waste products from blood, viz. urea, uric acid, creatinine. Extra salts, K and H are also secreted into urinary tubule to maintain a proper concentration and pH of the urine ❑Ureter The mesonephric ducts or ureters lie closed together in the median line. Anteriorly, they are separately, posteriorly the two mesonephric ducts open separately into urinary bladder. The function of the ureter is to conduct urine up to urinary bladder. Histologically, it is made up externally by tunica adventitia, the middle layer contains lamina propria and smooth muscules and outer most layer is columnar epithelial cells. ❑Urinary bladder A sac-like enlargement is clearly visible in the posterior region of the ureter. This is known as urinary bladder. But is not homologous to that of higher vertebrates. The urinary bladder usually opens to the exterior by a common urinogenital aperture in the male fish but in a female fish it known as mystus. ❑Excretory products The excretory products are, a) Ammonia b) Urea c) Uric acid d) Creatine e) Trimethylamine oxide f) Amino acide ❑Corpuscles of stannius The corpuscles of stannius is a small ductless gland (endocrine gland) situated partly or completely embedded in the kidney on its dorsal, dorsolateral and ventrolateral side. Corpuscles of stannius work in conjuncture with pituitary gland, which produces a distinct hypercalcemic effect, to maintain a relatively constant level of calcium fundulus heteroctilus. Innervati on The kidneys are richly innervated by autonomic nervous system. The nerve generally penetrate in the kidney through Blood vessels. They divide and re-divide to from plexuses. Scatterd ganglion cells are also present in the trunk kidneys.Both cholinergic and adrenergic nerves are present. Hormones and Enzymes Renin is the hormone secreated from juxtaglomerular cells.The hormone is active in increasing blood pressure and controls the glomerular filtrate. The cholinergic nerve endings secreat an enzymes acetylcholinesterase(AChE). The lower km in head kidneys indicates higher enzyme activity. Blood supply of Kidney The kidney of fish receives blood supply by the renal artery and renal portal vein. The renal artery supplies blood to glomeruli, where high blood pressure helps to separate glomerular filtrate. The renal portal veins are connected to capillary network around the kidney. Arterial blood is supplied to the kidney(glomerulus) by renal arteries arising directly from the aorta or from segmental vessels. The efferent arterioles then supply blood to peritubular capillaries. In marine and euryhaline species the peritubular capillaries also receive blood from caudal or segmental vessels,draining the fail region and constituting a renal portal system.This portal blood is thus venous. The End.