The Urinary System

The urinary system, is a group of organs in the body that filters out excess fluid and other substances from the bloodstream. The purpose of the renal / urinary system is to eliminate wastes from the body,  regulate blood volume and pressure, control levels of electrolytes and metabolites, and  regulate blood pH.

Urinary System Functions

Figure 1.: Components of the Urinary system: Here are the major organs of the renal system.

The  renal system has many functions. Many of these functions are interrelated with the physiological mechanisms in the cardiovascular and respiratory systems.

1. Excretion : Removal of metabolic waste products from the body (mainly urea and uric acid). Metabolic wastes and excess ions are filtered out of the blood, along with water, and leave the body in the form of urine.
2. Maintains water balance: adjusts blood volume and blood pressure. The renal system alters water retention and thirst to slowly change blood volume and keep blood pressure in a normal range.(Blood pressure homeostasis)
3. Assist in maintaining electrolyte / salt balance (e.g., sodium, potassium, and calcium). Maintain salt balance in order to control blood volume
4. Assist in maintaining acid base / pH balance of blood (7.35-7.45)  by controlling the loss of hydrogen ions and bicarbonate ions in urine.
5. Secretion of hormones. Example :  Erythropoietin / EPO.

Organs of the Renal / Urinary System

The renal / urinary system organs include the kidneys, ureters, bladder, and urethra

• Kidneys: The kidneys are a pair of bean-shaped organs  which filter blood and produce urine.. They remove wastes, control the body’s fluid balance, and keep the right levels of electrolytes. Kidneys are the most complex and critical part of the urinary system.
• Ureter: Urine passes from the renal tube through tubes called ureters and into the bladder.
• Bladder: The bladder is flexible and is used as storage until the urine is allowed to pass through the urethra and out of the body.
• Urethra: Duct that transmits urine from the bladder to the exterior of the body during urination. The female and male urinary system are very similar, differing only in the length of the urethra.

Kidneys – External Anatomy

Figure 2. The kidneys are slightly protected by the ribs and are surrounded by fat for protection (not shown).

The kidneys lie on either side of the spine in the retroperitoneal space between the parietal peritoneum and the posterior abdominal wall, well protected by muscle, fat, and ribs. They are roughly the size of your fist, and the male kidney is typically a bit larger than the female kidney. The kidneys are well vascularized, receiving about 25 percent of the cardiac output at rest.

The left kidney is located at about the T12 to L3 vertebrae, whereas the right is lower due to slight displacement by the liver. Upper portions of the kidneys are somewhat protected by the eleventh and twelfth ribs. Each kidney weighs about 125–175 g in males and 115–155 g in females. They are about 11–14 cm in length, 6 cm wide, and 4 cm thick, and are directly covered by a fibrous capsule composed of dense, irregular connective tissue that helps to hold their shape and protect them. This capsule is covered by a shock-absorbing layer of adipose tissue called the renal fat pad, which in turn is encompassed by a tough renal fascia. The fascia and, to a lesser extent, the overlying peritoneum serve to firmly anchor the kidneys to the posterior abdominal wall in a retroperitoneal position.

On the superior aspect (top) of each kidney is the adrenal gland. The adrenal cortex directly influences renal function through the production of the hormone aldosterone to stimulate sodium reabsorption.

Kidney – Internal Anatomy

Externally, the kidneys are surrounded by three layers, illustrated in (Figure 3).  The kidney has three regions: Outer renal  cortex, Inner renal medulla and Renal pelvis.

Figure 3. The internal structure of the kidney is shown. (credit: modification of work by NCI)

• Renal Cortex: In a dissected kidney, it is easy to identify the cortex; it appears lighter in color compared to the rest of the kidney. The renal cortex is granular due to the presence of nephrons—the functional unit of the kidney.  Some nephrons have a short loop of Henle that does not dip beyond the cortex. These nephrons are called cortical nephrons. About 15 percent of nephrons have long loops of Henle that extend deep into the medulla and are called juxtamedullary nephrons.
• Renal Medulla: The medulla consists of multiple pyramidal tissue masses, called the renal pyramids. In between the pyramids are spaces called renal columns through which the blood vessels pass. The tips of the pyramids, called renal papillae, point toward the renal pelvis. There are, on average, eight renal pyramids in each kidney.
• Renal Pelvis: The renal pelvis leads to the ureter on the outside of the kidney. On the inside of the kidney, the renal pelvis branches out into two or three extensions called the major calyces, which further branch into the minor calyces. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.
• The renal hilum is the entry and exit site for structures servicing the kidneys: vessels, nerves, lymphatics, and ureters. The medial-facing hila are tucked into the sweeping convex outline of the cortex. Emerging from the hilum is the renal pelvis, which is formed from the major and minor calyxes in the kidney. The smooth muscle in the renal pelvis funnels urine via peristalsis into the ureter. The renal arteries form directly from the descending aorta, whereas the renal veins return cleansed blood directly to the inferior vena cava.

Blood flow through the kidney / Renal circulation.:

Blood flows to the kidneys through the right and left renal arteries. The renal arteries originate from the abdominal aorta and enter the renal hila to supply the kidneys. Inside each kidney these branch into smaller arterioles and penetrate deep into the renal medulla and renal cortex.  Blood coming into the kidney is rich in oxygen and nutrients and waste materials. Eventually, the smaller arterioles divide and become renal capillaries where blood gets filtered, and metabolic wastes are removed to form urine. Renal capillaries are located in and around the nephrons. These renal capillaries fuse and form renal veins and leave the kidney. Blood leaving the kidney via renal veins will have less waste materials as most of the wastes materials are filtered out of the blood in renal capillaries. They also have less oxygen and nutrients as these materials are used up by the kidney cells.