Purpose of the urinary system is to remove wastes (e.g., urea) from the body, control level of electrolyte and metabolite, and regulate blood volume and blood pressure.
Urine, made in the kidneys flows through the ureters, urinary bladder, and urethra.
< Kidney >
Fig. 6-3. Location of kidney.
The kidney is retroperitoneal organ that is located close to the back (Fig. 4-38). Precisely, the kidney is situated at the space between R11, R12 and lumbar vertebrae. The right kidney is slightly lower than the left because the right kidney is pressed down by the large liver (Fig. 4-40).
The humorous reason of the bilateral asymmetry is introduced.
Even though the kidneys are not very big organs, they receive a large amount of blood supply (about 20% of blood from the heart) via the renal arteries (Fig. 10-52). After making urine, the blood leaves the kidneys via the renal veins (Fig. 10-51).
Fig. 6-6. Glomerulus, glomerular capsule.
The glomerulus is a twisted capillary that originates from the renal artery. Blood enters the glomerulus from the afferent arteriole. Then, blood leaves the glomerulus through the efferent arteriole. The glomerulus is surrounded by a cup-like sac called the glomerular capsule that leads to the renal tubule.
The efferent arteriole proceeds to become the peritubular capillary.
Fig. 6-8. Formation of urine.
Urine is formed through three steps: filtration between the glomerulus and glomerular capsule, reabsorption and secretion between the peritubular capillary and renal tubule.
Fig. 6-9. Filtration between glomerulus, glomerular capsule.
Filtration, the first step, happens from the glomerulus to the glomerular capsule. The efferent arteriole is thinner than the afferent arteriole (Fig. 6-6); therefore, blood pressure in the glomerulus is high. As a result, blood plasma (Fig. 11-3) is filtered through the glomerular holes into the glomerular capsule. In case of hypertension, drugs to dilate the afferent arterioles or to constrict the efferent arterioles can be prescribed, so as to increase the urine formation and decrease the blood pressure.
Additionally, difference of the osmotic concentration of blood and urine effects filtration. The difference of the osmotic concentration induces solvent movement through a semipermeable cell membrane (Fig. 16-12).
From the glomerular capsule, urine flows to the renal tubule.
Amount of filtration from the glomerulus is 100 times larger than amount of optimal urine output. So reabsorption, the second step, happens from the renal tubule to the peritubular capillary to decrease urine in volume. Sodium (Na) in the renal tubule is reabsorbed into the peritubular capillary using energy. Then water is reabsorbed into the peritubular capillary where blood shows high osmotic concentration.
Even after filtration, blood in the peritubular capillary still has wastes and toxins. So secretion, the third step, happens to move them to the renal tubule using energy.
Fig. 6-13. Glomerular capsule, sequential renal tubules.
The renal tubule is divided into the proximal tubule, nephron loop, distal tubule, and collecting tubule. They alternately pass in the renal cortex and renal pyramid (renal medulla). The proximal and distal tubules (as well as the nephron loop) are surrounded by the peritubular capillaries (Fig. 6-8). Urine in the collecting tubule drips into the minor calyx (Fig. 6-14).
Fig. 6-14. Kidney, ureter.
When cutting the kidney, the renal cortex, renal medulla (renal pyramid) are discriminated. The color difference between the two results from the glomeruli, which are found only in the renal cortex (Fig. 6-13).
The urine from the renal pyramid flows along the minor calyx (Fig. 6-13), major calyx, and renal pelvis, all of which are located in the renal sinus. The renal sinus also involves the renal artery (Fig. 10-52) and renal vein (Fig. 10-51).
< Ureter, urinary bladder, urethra >
Urine descends through the ureter by peristalsis of the smooth muscle (Fig. 16-30).
The ureter has three sites where it is normally narrowed: the initial site (border between the renal pelvis and ureter), the site where the ureter crosses the common iliac artery (Fig. 10-53), and the terminal site (border between the ureter and urinary bladder) (Fig. 6-14).
Fig. 6-17. Urinary bladder.
The empty urinary bladder is a triangular pyramid-shaped structure, while the full urinary bladder stretches to become nearly spherical. Thus, a pig’s urinary bladder filled with air was used as a soccer ball in the past.
At ordinary times, we keep urine in the urinary bladder by the contraction of the internal urethral sphincter (smooth muscle) which is a part of the urinary bladder wall and the external urethral sphincter (skeletal muscle) (Fig. 7-15).
However, during urination, we relax these sphincters; simultaneously, we contract the detrusor (smooth muscle) in the urinary bladder (Fig. 7-15) and the external oblique muscle, etc. (skeletal muscle) in the abdominal wall (Fig. 3-33).
In both male and female, harmonious contraction and relaxation of the smooth and skeletal muscles are essential for restraining or causing urination.
The male urethra is composed of the prostatic urethra, MEMBRANous urethra (surrounded by the MEMBRANe-like external urethral sphincter), and SPONGy urethra (surrounded by the corpus SPONGiosum) (Fig. 7-17).
In the midsagittal plane, the spongy urethra is curved twice. Unlike the root of penis, the body of penis is free-floating (Fig. 7-20).
Female’s internal urethral sphincter is not well organized (Fig. 6-23). As a result, urinary incontinence happens more frequently in female.
Female has neither a prostatic urethra nor a spongy urethra (Fig. 6-21). The female urethra, just encircled by the external urethral sphincter, is short. Therefore, the infection of the urinary bladder through the urethra is more frequent in female than in male.
The urethral opening is close to the vaginal opening (Fig. 7-38) and anus (Fig. 4-36). This is another reason of frequent infections of the female urinary bladder.
The male urethra is the passage of both urine and semen (Fig. 7-13).
In contrast, the female urethra belongs only to the urinary system.