Renal System Part 1

Urinary System

Organs of the renal system
Macroscopically the urinary system consists of the right and the left kidney, ureters, bladder and urethra. Each kidney is surrounded by a protective fibrous capsule called the renal capsule. The outer layer of the kidneys is referred to as the cortex and the inner layer as the medulla. Inside these layers is the renal pelvis. There are two ureters the right and the left ureter and these convey urine from each kidney to the urinary bladder. From the urinary bladder there is a tube called the urethra which connects the urinary bladder with the outside world. In men round about the neck of the urethra, where it leaves the bladder, is the prostate gland which adds most of the fluid component to seminal fluid.
The arterial supply to the kidneys is via the right and left renal arteries and the venous drainage is via the right and left renal veins. The renal arteries directly leave the aorta and the renal veins drain directly into the inferior vena cava.

Macroscopic structures in relation to function
Macroscopic means large and the function of the kidneys is to filter the blood to clean it and to get rid of the impurities produced by metabolic processes. They also excrete any excess water from the blood. This water and the waste products are then excreted from the body as urine so the kidneys produce urine. Toxins which are excreted include urea and other waste products of nitrogen metabolism. The kidneys also have homeostatic functions such as maintenance of acid/base balance by selective excretion of acid or alkaline ions. The kidney deposits urine in the renal pelvis and from here the urine passes into the ureters. The ureters transport urine by the process of peristalsis to convey it down into the bladder. Between the bladder and ureter there is a valve which prevents a back flow of urine from the bladder back into the ureter, this is vital to prevent infection ascending in the urinary tract. Urine collects in the bladder progressively stretches the wall the bladder. When about 200mls of urine is present in the bladder, stretch receptors in the bladder wall are stimulated which gives the individual the desire to micturate. When this happens the sphincter between the bladder and the urethra relaxes and the muscle fibres in the wall of the bladder contract, this results in urine being expelled from the body via the urethra. In women the urethra is fairly short about 2 cm long and in men the urethra is much longer at about 18 cm.

Other renal functions
The kidneys are also involved in the homeostasis of blood pressure. When renal perfusion is low the kidneys release a hormone called renin which circulates in the blood where it converts angiotensinogen into angiotensin, this angiotensin is a very powerful vasoconstrictor, vasoconstriction will raise blood pressure. As well as producing renin the kidney also produces another hormone called erythropoietin which stimulates the red bone marrow to produce red blood cells. Erythropoietin is released in response to oxygen lack, so when there are low levels of oxygen in the blood the body will produce more red blood cells, the result of this is that more oxygen can be carried in a given volume of blood. The kidneys also convert vitamin D into its active form, without the kidney converting vitamin D into its active form the body is unable to use this vitamin.