13.2 - The Urinary System

Structure and Functions of the Kidney

The urinary system consists of the kidneys, ureters, bladder and urethra.
It excretes nitrogenous compound wastes such as urea.
It regulates body-fluid volume, blood osmotic pressure, ion concentration in body fluids, electrolyte content and blood pH.
The kidneys consist of the cortex and medulla; urine formed in the kidneys flows into the pelvis.

Excretion: Excretes toxic nitrogenous wastes such as urea, uric acid, ammonia and creatinine.
Osmoregulation: Controls the total volume of water and ion concentration in body fluids, blood osmotic pressure, electrolyte content, and the pH of blood and body fluids.

Each kidney contains millions of functional units called nephrons.
Each nephron consists of the Bowman’s capsule, glomerulus and renal tubule.
The cup-shaped Bowman’s capsule contains a cluster of blood capillaries called the glomerulus.
The glomerulus is formed from the afferent arteriole branching from the renal artery; the capillaries merge to form the efferent arteriole.
The renal tubule consists of the proximal convoluted tubule, loop of Henle and distal convoluted tubule.
The loop of Henle is a long U-shaped tubule extending into the renal medulla.
Distal convoluted tubules from several nephrons join into a collecting duct.
Urine flows from the collecting duct into the ureter.

Urine formation involves ultrafiltration, reabsorption and secretion.

Blood enters the glomerulus under high hydrostatic pressure because the afferent arteriole has a larger diameter than the efferent arteriole.
Fluid seeps through glomerular capillary walls into the cavity of the Bowman’s capsule.
This fluid is the glomerular filtrate.
Glomerular filtrate has the same composition as blood plasma but contains no red blood cells, platelets or plasma proteins.
Red blood cells and plasma proteins remain in the blood because they are too large to seep out of the glomerulus.

Proximal convoluted tubule:
- Sodium ions ( $N a^{+}$ ) are actively pumped into the blood capillary network; chloride ions ( $C l^{-}$ ) are passively absorbed.
- All glucose and amino acids are reabsorbed through active transport.
- Solute reabsorption increases solute concentration in the blood capillaries; water enters them by osmosis.
Loop of Henle:
- Water is reabsorbed by osmosis.
- Sodium ions are reabsorbed through active transport.
Distal convoluted tubule:
- More water, sodium and chloride ions are reabsorbed.
- The amount of water and salts reabsorbed depends on their content in the blood.

Secretion moves waste materials in the blood that were not filtered earlier into the renal tubule; it is the opposite of reabsorption.
It occurs along the renal tubule and collecting duct but is most active at the distal convoluted tubule.
It occurs through simple diffusion and active transport.
Secreted substances include hydrogen ions ( $H^{+}$ ), potassium ions ( $K^{+}$ ), ammonium ions ( $N H_{4}^{+}$ ), urea, creatinine, toxic substances and some drugs.
Secretion removes toxic wastes and regulates ion levels in the blood.

Most water has been reabsorbed and only a small amount of salt remains when renal fluid reaches the collecting duct.
The remaining renal fluid is urine; a small amount of urea diffuses into the surrounding fluid and blood capillaries.
Urine normally contains water, urea, sodium chloride, uric acid and creatinine.
Urine flows through the collecting duct, ureter, bladder and urethra before being excreted.

Osmoregulation: Regulation of water and salts in the body to maintain blood osmotic pressure within a normal range.
The kidney achieves osmoregulation by regulating urine volume.

Blood osmotic pressure falls below the normal range.
Osmoreceptors in the hypothalamus and the pituitary gland are less stimulated.
The pituitary gland secretes less antidiuretic hormone (ADH).
Low ADH concentration makes the distal convoluted tubule and collecting duct walls less permeable to water.
Less water is reabsorbed from renal fluid into the blood capillaries.
A high volume of less concentrated urine is produced.
Blood osmotic pressure returns to normal.

Blood osmotic pressure rises above the normal range.
Osmoreceptors in the hypothalamus and the pituitary gland are stimulated.
The pituitary gland secretes more ADH.
High ADH concentration makes the distal convoluted tubule and collecting duct walls more permeable to water.
More water is reabsorbed from renal fluid into the blood capillaries.
A low volume of very concentrated urine is produced.
Blood osmotic pressure returns to normal.

Problem statement: What is the effect of consuming different volumes of water on the volume of urine produced?
Hypothesis: The higher the volume of water consumed, the higher the volume of urine produced.
Manipulated variable: Volume of water consumed.
Responding variable: Volume of urine collected.
Constant variables: Type of drink, pupil’s age and urine-collection time interval.
Water volumes: 250 ml, 500 ml, 750 ml and 1000 ml for four groups of pupils of approximately the same weight.
Pupils urinate before the experiment, remain at rest, and do not carry out vigorous activities.
Collect and measure urine after 20, 40 and 60 minutes; calculate the total volume produced by each group.
Conclusion: A higher volume of water consumed produces a higher volume of urine.