The urinary organs extend from the abdomen through the pelvis. Since they are closely related to the genital organs, both groups of organs are often referred to collectively as urogenital organs. For didactic reasons, both systems will be discussed separately in the following chapters.
The urinary organs help regulate the level of water and minerals in the body, and thus help regulate osmotic pressure. They excrete end products of body metabolism and harmful substances into a watery fluid, the urine (the excreted metabolites are dissolved in the water component of urine). By regulating the amount of water in the body, the kidneys also influence blood pressure. Through excreting or retaining sodium, potassium, calcium and chloride ions, they are involved in regulating the level of these important electrolytes in blood. In addition, the blood’s acid-base balance is influenced by the excretion or retention of hydrogen ions. Many pharmaceutical substances are excreted through the kidneys. The kidneys also influence blood pressure by producing the enzyme renin, and the formation of red blood cells by producing the hormone erythropoietin. Lastly, they also play an important role in vitamin D metabolism.
A Overview of the urinary organs
Male urinary organs, anterior view. The urinary system consists of the following organs:
• the paired kidneys, which continuously produce urine;
• the paired ureters, which transport urine from the kidneys to the urinary bladder;
• the unpaired urinary bladder, which temporarily stores and discharges urine in a controlled manner; and
• the unpaired urethra. In women it is called the female urethra and it is solely a urinary organ, whereas in men it is called the male urethra and it is also a genital organ. In the urinary system, the urethra is involved in discharging urine from the urinary bladder to the outside of the body. In men it is also serves as a passageway for sperm.
B Basics of urine production
The nephron, illustrated above, is the smallest functional unit of the kidney (see p. 54).
In the glomeruli, richly branched capillary loops, which are supplied by branches of the renal arteries, drain an ultrafiltrate of blood called primary urine into a system of tubules. Adults produce approximately 170 liters of primary urine in 24 hours. However, in the tubular system, the primary urine is concentrated to 1% of its volume (by reabsorption of electrolytes and water back into the blood), and based on its composition, further modified with electrolytes and hydrogen ions. The volume of final urine formed in 24 hours is 1–2 liters. The final urine drains through collecting ducts into the renal pelvis and then it is carried by the ureters to the urinary bladder.
C Location of the kidneys and ureters
Horizontal section of the body at the level of the first lumbar vertebra, viewed from above. Both kidneys are embedded in a fatty, connective tissue capsule. They are located in the retroperitoneal space with one on either side of the vertebral column. The retroperitoneal space also contains the ureters (not visible in this section), which extend downward to the lesser pelvis to reach the urinary bladder. The hilum of each kidney faces medially and anteriorly (red axes).
D Location of the urinary bladder and urethra
Midsagittal section of a female (a) and male (b) pelvis, each viewed from the left side.
In both sexes, the urinary bladder is located in the lesser pelvis behind the pubic symphysis. In females it is situated in front of both the vagina and the uterus, and in males it is in front of the rectum. Depending on its degree of distension, the urinary bladder is flattened or spherical. The female urethra is straight and short, while the male urethra traverses the penis and bends multiple times along its course.
A Overview of the embryonic development of the urinary organs
The embryonic development of the urinary organs is complex and overlaps with the development of the genital and digestive organs:
• overlap with the genital system: the development of some parts of the male reproductive system (see p. 62) is closely related to the development of the mesonephric ducts, ureters, and urogenital sinus.
• overlap with the digestive system: the anal canal is derived from the cloaca.
The development of the urinary system can be divided into the development of the paired kidneys and ureters, and development of the unpaired urinary bladder and urethra. The kidneys and ureters arise from the intermediate mesoderm. The urinary bladder and urethra develop from the urogenital sinus, which formed from the ventral portion of the cloaca in the region of the future pelvic floor (see p. 47). The urogenital sinus is derived from endoderm. Thus, the urinary organs are derived from two germ layers. Over the course of development the two sets of urinary organs will connect with each other.