Urinary system

Chapter 10 Urinary system

If the metabolic processes of the body are to function effectively, the chemical composition and volume of the tissue fluid must be kept constant. The most important function of the urinary system – and principally that of the kidney – is to maintain this constant internal environment, described as homeostasis.

The urinary system lies in the abdominal and pelvic cavities. It is anatomically linked with the genital or reproductive system and may be referred to as the urogenital system. Both systems share the urethra which runs through the penis of the male and joins the vagina of the female.

The parts of the urinary system are:

The functions of the urinary system are:

The kidney

There are two kidneys lying in the cranial abdominal cavity, one on each side of the midline ventral to the lumbar hypaxial muscles (Fig. 10.1). Each kidney is closely attached to the lumbar muscles by a covering of parietal peritoneum. There is no mesenteric attachment, as seen in other abdominal organs, and the kidney is described as being retroperitoneal. The right kidney lies slightly cranial to the left because the stomach has evolved to lie on the left side of the abdomen, pushing the left kidney out of position. Lying close to the cranial pole of each kidney are the ovaries of the female and the adrenal glands (Fig. 10.2).

Macroscopic structure

The kidneys of the cat and dog have a characteristic bean shape and the indented area is known as the hilus. This is the point at which blood vessels, nerves and the ureters enter and leave the kidney. The kidneys are normally a deep reddish-brown but the colour may be affected by any substance filtering through them. On a lateral radiograph of the abdomen, a normal kidney can be seen to be equivalent in size to approximately 2.5 vertebrae (Fig. 10.3). The outer surface may be surrounded by a layer of fat, which acts as an energy reserve and protects the kidney from external damage.

When examining the cut surface of a normal kidney cut longitudinally, it is possible to see four layers (Fig. 10.4). From the outside inwards these are:

Blood supply

Arterial blood is carried from the aorta in a single renal artery to each kidney (Fig. 10.4). This carries 20% of cardiac output. Within the tissue of the kidney, the renal artery divides into several interlobar arteries, which pass between the renal pyramids and into the cortex. Here capillaries supply the renal tubules and also give off numerous capillary networks known as glomeruli (sing. glomerulus) (Fig. 10.5). Each glomerulus supplies an individual nephron. The capillaries then recombine to form interlobar veins, which enter the single renal vein. This carries venous blood to the caudal vena cava.

Blood entering the kidney carries oxygen, nutrients and waste products from the tissues of the body; blood leaving the kidney carries carbon dioxide produced by the kidney tissues, but the nitrogenous waste products have been removed via the glomeruli.

Microscopic structure

The functional unit of the kidney is the nephron (Fig. 10.5). Each kidney contains about a million nephrons, which are closely packed together. They are responsible for the filtration of blood and the production of urine. Each nephron is a long tubule divided into several parts:

Glomerular capsule – a cup-shaped structure enclosing a network of blood capillaries called the glomerulus (Fig. 10.6). The capsule may also be known as Bowman’s capsule. The capsule and the glomerulus form the renal corpuscle. The basement membrane of the inner surface of the capsule, which is in close contact with the endothelium of the glomerular capillaries, is lined by podocyte cells between which are tiny pores (Fig 10.6). These pores are of such a size that they will allow the passage of fluid and small molecules, but restrict the passage of larger molecules. The outer surface of Bowman’s capsule is continuous with the epithelium of the proximal convoluted tubule. Fluid filtered by the capsule drains into the space between the two layers and continues into the next part of the nephron.

Proximal convoluted tubule – a long, twisted tube leading from the neck of the capsule and lying in the renal cortex (Fig. 10.5). The tubules are lined in simple cuboidal or columnar epithelium. The side of the epithelium directed towards the lumen of the tubule is lined by fine microvilli forming a ‘brush border’. This increases the surface area for the reabsorption of water and electrolytes.

Renal function – the formation of urine

Blood is filtered by the kidneys and the resulting filtrate undergoes a series of modifications within the renal tubules to produce urine. This urine is very different in composition and volume from the original filtrate. For every 100 L of fluid filtered from the blood only 1 L is produced as urine – 99% of the original filtrate is reabsorbed back into the blood. The changes made to the filtrate reflect the status of the extracellular fluid (ECF) and in particular that of the blood plasma.

Blood enters the kidney and is carried to the capillaries forming the glomeruli.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Urinary system

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