Urinary system

The same organs are involved in urine production in males and females. The urinary system consists of the kidneys, each with a ureter leading to the bladder. The bladder drains into the urethra, which carries the urine to its evacuation point.
These organs and ducts are located in the abdomen; the kidneys are located under the lumbar arch, near the first lumbar vertebrae; the left kidney is slightly more towards the rear than the right one; the two ureters run into the dorsal side of the bladder, which is just in front of the pelvis.
The urethra follows a different route depending on the sex of the dog. In bitches, it is shorter and generally wider, leading to the vestibule through a small papilla. In males, it is longer, narrower and split into three sections: prostatic, membranous and penile.

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Structure of the kidneys

The kidneys are composed of an external cortex, an internal medulla and a renal pelvis, the broadened top part of the ureter.

The nephrons are the functional parts of the kidneys. They are relatively long tubules which run into collecting ducts. They are split into various parts: a glomerulus, a cluster of capillaries, with a proximal tubule consisting of a convoluted portion and a straight portion, an intermediate tubule and a distal tubule (which also has a convoluted portion and a straight portion). Each tubule runs into a collecting duct via a short connecting tubule. In simplified terms, the glomeruli and the convoluted portions of the tubules could be said to be grouped in the renal cortex, whereas the straight portions of the tubule, forming the Loop of Henle, forms the medulla.

© Diffomédia/Royal Canin

Production of urine

Urine is produced in the kidney nephrons in various stages to enable the elimination of part of the body’s waste, among other functions. Other kidney functions include regulation of ions, acids and bases.

The first stage in the production of urine is blood filtration, the purpose of which is to produce “primary” urine. To do so, the blood passes through fenestrated capillaries (small arteries with perforated walls) in the renal tubules. Substances that are sufficiently small can pass through these capillary walls because of the large difference in pressure and be collected in the renal tubules.

The resulting filtrate solution is “primary” urine, because its composition is modified before it is evacuated. At this point, it is very similar to plasma.

Filtration is followed by absorption in the convoluted portion of the tubules, especially the proximal tubule. This returns substances and ions the body needs to the bloodstream. These transports are often combined with the absorption of water, which is a passive process, while absorption without water requires the expenditure of energy by cells.

Chloride, sodium and potassium are the main ions absorbed. All glucose and proteins, as well as some amino acids and organic acids are absorbed by the convoluted portions of the tubules.

Some substances enter the urine as a result of secretion, which also occurs in the convoluted portion of the proximal tubule. This mechanism involves substances present in the blood (such as contrast media used in medical examinations or medications such as penicillin) and substances created in the tubule’s epithelium (ammonia for example). Both active and passive mechanisms are involved, as well as exchanges.

Urine attains its final form in the last part of the nephron, the collecting duct. Regulatory mechanisms are triggered to determine the concentration of the urine and increase its acidity.

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Urine

Daily evacuated volume: 25-40 ml/per kg of body weight
pH: 5-7.
Urea: 300-800 mg/kg of body weight per day

Urine storage and evacuation

The urine, in its final form, flows from the collecting ducts into the renal pelvis, a small pouch which empties into the urethra. It is conveyed to the bladder, a watertight organ capable of swelling dramatically, where it is stored between urinations. A sphincter between the bladder and the urethra prevents incontinence. Urination occurs when the bladder accumulates a given volume of urine. The bladder, which is composed of many smooth muscle fibres, contracts and the urethral sphincter relaxes. The urine is then evacuated under pressure.

This process is regulated by the nervous system. The brain has voluntary and conscious control of urination. Nerves originating in the lumbar, sacral and pelvic regions cause contraction and relaxation of the bladder.

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Pourquoi l’urine de mon chien brûle-t-elle le gazon ?

CALLALLOO Canis_Fotolia

One of the major functions of the kidneys is to excrete nitrogen waste products derived from the breakdown of proteins. Consequently, when dogs urinate on the lawn, grass may be damaged as a result of exposure to excessive amounts of nitrogen contained in the urine. This damage is usually recognized as dead brown patches of grass, created because of dehydration caused by the nitrogen. Female dogs are more likely to contribute to “lawn burn” because their urination habits most often involve squatting and releasing larger volumes of urine than the typical marking behavior of the male dog.

Bowles

Mary Bowles, DVM, DACVIM
Associate Professor
Center for Veterinary Health Sciences
Oklahoma State University
Stillwater, Oklahoma 74078
(USA)

Regulation

The urinary function is primarily regulated through control of the kidneys which happens in various ways. External factors, especially blood circulation, can play a part; indeed the amount of urine produced by the kidneys is heavily dependent on the amount of blood filtered. When the blood volume decreases, less urine will be evacuated; when it increases, more urine will be evacuated.

The nervous system also plays a role, influencing both kidney function and bladder function (urination). The kidneys are stimulated by numerous nerves, which have an impact on the blood vessels in the kidneys. They can rapidly decrease the rate of renal perfusion, resulting in a decreased volume of evacuated urine.

Hormones have the greatest regulatory effect. Many hormones are involved in controlling the evacuation of water and ions, but most of them only come into play in pathological situations. The most important of these hormones is vasopressin, also called antidiuretic hormone (ADH). Vasopressin is secreted by the pituitary gland, which is located at the base of the brain. It acts on the end of the convoluted portion of the distal tubule and on the nephron’s collecting tubule. Secretion is triggered by a rise in osmotic pressure in the blood (due to the fall in the amount of water in the blood compared with other substances) or a fall in arterial pressure. Other stimuli that can also trigger secretion of vasopressin are stress, a drop in ambient temperature and physical exercise.

Vasopressin is trapped by receptors on the surface of the cells of the collecting duct, resulting in the increased absorption of water in the nephron. This enables the animal to conserve some of the water in its cells.

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