24.1 Introduction to the Urinary System

The urinary system is collectively composed of

• the kidneys

• ureters

• urinary bladder

• urethra.

One of the primary functions of the kidneys is

to filter blood and convert the filtrate into urine.

Waste is then transported by the ___ from the kidneys to ___

ureters; urinary bladder.

The urinary bladder is

• an expandable, muscular sac that stores as much as 1 liter or urine until it is eliminated from the body through the urethra.

Urinary tract

The

• ureters

• urinary bladder

• urethra

are collectively known as the urinary tract because they transport the urine from the kidneys to the outside of the body.

The several essential physiologic processes that occur in the kidney when the filtrate is being converted to urine:

• Elimination of metabolic wastes. The kidneys remove waste that is present in the filtrate (e.g., urea, uric acids) to prevent these substances from reaching toxic levels within the blood.

• Regulation of ion levels. The kidneys help control the blood's ion balance, such as sodium ions (Nat), potassium ions (Kt), calcium ions (Calt), and phosphate ions (PO 43) by eliminating varying amounts of these substances in the urine, depending upon dietary intake.

• Regulation of acid-base balance. The kidneys aid in maintaining acid-base balance by altering blood levels of both hydrogen ions (Ht) and bicarbonate ions (HCO3 ) by eliminating varying amounts of these ions in the urine.

• Regulation of blood pressure. The kidneys help regulate blood pressure by excreting fluid in the urine. Regulating fluid lost in the urine helps to regulate blood volume. The kidneys also release the enzyme renin, which is required for production of angiotensin II, a hormone that increases blood pressure (see section 20.6b and L table R.7). Arguably, blood pressure regulation is one of the kidney's most important functions and is described in detail in LE section 25.4.

• Elimination of biologically active molecules. Small, biologically active molecules (e.g., hormones, drugs) are filtered from the blood, but are not reclaimed, and then become part of urine.

The other functions of the kidneys:

• Formation of calcitriol. The kidneys synthesize the final enzyme in calcitriol hormone formation. Calcitriol, which is the active form of vitamin D3, increases the absorption of calcium from the small intestine to increase blood calcium concentration (see L section 7.6).

• Production and release of erythropoietin. As the kidneys are filtering the blood, they are also indirectly measuring the oxygen level of blood. In response to low blood oxygen levels, cells within the kidney secrete erythropoietin (EPO) hormone. EPO stimulates red bone marrow to increase its rate of erythrocyte formation (see L' section 18.3b and L table R.6).

• Potential to engage in gluconeogenesis. During prolonged fasting or starvation, the kidneys may engage in gluconeogenesis to produce glucose from noncarbohydrate sources; this process helps to maintain healthy blood glucose levels during periods of extreme nutrient deprivation