The urinary (or excretory) system is responsible for the elimination or removal of waste products removed from the blood by the kidneys to maintain a relatively constant internal environment (i.e., homeostasis) of the body fluid. This is done by producing and excreting urine that is of the right volume and composition.
The urinary system consists of:
- Two kidneys – paired reddish-brown organs that filter excess plasma and plasma constituents from the filtrate before excreting them.
- Two ureters – muscular tubes which carry urine from the kidneys to the urinary bladder.
- A urinary bladder – a hollow muscular organ whose size and location depend on the amount of urine it contains.
- A pelvic urethra – extends from the urinary bladder across the floor of the pelvic canal to the ischial arch.
The urinary system
To carry out a variety of functions, tissue cells metabolize nutrients ingested through the digestive tract and oxygen inhaled through the lungs. Various waste products are produced during this process, including faeces, carbon dioxide, metabolic water, urea, heat, excess salt, and so forth. To preserve homeostasis, the urinary system removes these waste products. The excretory organs involved in the excretion (or removal) of waste products include the:
- Alimentary canal.
- Skin (sweat glands).
- Lungs.
- Kidneys.
The alimentary canal, skin, and lungs are in direct contact with the external environment and can, therefore, rid themselves directly of the waste products that they eliminate. The kidneys are deep inside the body and have no direct opening to the external environment. The waste products that the kidneys extract from the blood must be eliminated via the urinary system.
Waste materials and water are taken out of the blood in the kidneys to form urine, which collects in the bladder and then passes out of the body.
Structure and Function of the Kidneys
The kidneys of most animals are roughly bean-shaped, with the exceptions of the right equine kidney, which is heart-shaped, and the kidneys of the ox, which have distinctive lobes.
Right kidney of the horse (left), the ox (middle), and the sheep (right)
The kidneys are in the dorsal part of the abdominal cavity on each side of the aorta and caudal vena cava, just ventral to the first few lumbar vertebrae.
Internal anatomy of the kidney of a horse (A), bovine kidney (B), and porcine kidney (C)
The hilus is a concavity (or concave surface) on the medial aspect of each kidney where arteries and nerves enter and the ureter, veins, and lymphatic vessels exit. The wide origin of the ureter in the kidney is the renal pelvis, which receives urine from the collecting tubules of the kidney. The cavity in the kidney that contains the pelvis is the renal sinus. The bovine kidney lacks a renal pelvis; instead, the ureter arises directly from the merging of individual calyces.
The portion of the kidney immediately surrounding the renal pelvis is the renal medulla, which appears striated due to the arrangement of collecting tubules radially. In addition to collecting tubules, the medulla contains a few loops of Henle (descending and ascending loops). The medulla is surrounded peripherally by the renal cortex, which contains the renal corpuscles, the histological units of filtration. Many renal corpuscles gives the cortex a granular appearance; the cortex also contains proximal and distal convoluted tubules and other segments of loops of Henle.
The medulla and cortex are organised into lobes, which are cone-shaped collections of renal tissue. The medullary portion of each lobe forms a renal pyramid with the renal papilla pointing toward the renal pelvis. Each pyramid in the bovine kidney corresponds to one of the grossly apparent lobes of the bovine kidney. In pigs and small ruminants, adjacent cortices of individual lobes are fused, resulting in a smooth kidney surface. However, the persistence of distinct papillae protruding into the renal pelvis reveals the individual nature of the porcine lobes. Individual papillae in horses and small ruminants are fused, like the cortex. Therefore, they appear as a single longitudinal ridge, the renal crest, projecting into the renal pelvis. The renal pelvis collects urine discharged from the collecting tubules of the renal crest, which is then transported to the ureter.
Individual pyramids project into minor calyces, cuplike diverticula of the common collecting space within the renal hilus, in the ox and pig kidneys. These then drain into the major calyces. The major calyces of the porcine kidney empty into the renal pelvis, whereas the major calyces of the bovine kidney empty directly into the ureter due to the absence of a renal pelvis.
Canine kidney (A) and small ruminant kidney (B)
Functions of the Kidneys:
- The kidneys maintain the body’s balance (i.e., homeostasis) of water (H2O), various electrolytes, acids, and bases by filtering plasma and plasma components from the blood. The kidneys help to maintain the composition of plasma by filtering excess components from the blood and excreting them into the filtrate and reabsorbing lacking components from the filtrate or just simply retaining lacking components.
- The kidneys excrete metabolic waste products (urea-nitrogenous waste) in the urine, which is necessary because these substances can poison cells and slow metabolism if not removed.
- The kidneys also have a hormonal function – the adrenal cortex of each kidney produces aldosterone (a steroid hormone) that helps with the balance of sodium (Na) and potassium (K); in addition, the kidneys are influenced by the antidiuretic hormone to reabsorb water from the filtrate.
There are two adrenal glands located near the kidneys. Variables such as shape, size, and location vary among species. Each gland consists of two regions: the adrenal cortex (the outer part of each gland) and the adrenal medulla (the inner part of each gland).