Identify and give functions for each of the following: - kidney - ureter - urethra - urinary bladder - renal cortex - renal medulla - renal pelvis

Kidneys perform a number of homeostatic functions:

- Maintain volume of extracellular fluid

- Maintain ionic balance in extracellular fluid

- Maintain pH and osmotic concentration of the extracellular fluid.

- Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.

The urinary system (see left) is made-up of the kidneys, ureters, bladder, and urethra. The nephron, an evolutionary modification of the nephridium, is the kidney's functional unit. Waste is filtered from the blood and collected as urine in each kidney (W). Urine leaves the kidneys by ureters (X), and collects in the urinary bladder (Y). The bladder can distend to store urine (up to 600mL) that eventually leaves the body through the urethra (Z). (an important distiction to make here - and perhaps on the final exam - is that it would be wrong to say that the kidneys secrete urine. Only glands do that. The kidney produces urine by filtering blood and collecting the components that we call urine)


The renal cortex, renal medulla and renal pelvis are all regions of the kidney (see left) visible when it is sliced lengthwise (see also Fig. 15.6 b. Page 265). The renal cortex (W) is the outer layer it houses the Bowman's capsule and the convoluted tubules. The renal medulla (X) is the inner layer of tissue into which the loop of Henle dips. The renal pelvis (Y) is an inner space continuous with the ureter (Z) that collects the urine from the collecting ducts of several nephrons.




Identify and give functions for each of the following: - nephron - glomerulus - Bowman's capsule - afferent and efferent arterioles - peritubular capillary network - proximal and distal convoluted tubules - collecting duct - loop of Henle

The nephron (shown above) consists of a cup-shaped capsule containing capillaries and the glomerulus, and a long renal tube. (Turn to Fig. 15.8 Page 267 before continuing) Blood flows into the kidney through the renal artery. The afferent arteriole brings blood to the glomerulus. The efferent areteriole then takes this blood from the glomerulus and branches into the peritubular capillary network. Arterial pressure causes water and solutes from the blood to filter out of the capillaries of the glomerulus into the Bowman's capsule. From there the fluid flows through the proximal convoluted tubules, which include the loop of Henle, and then into the distal convoluted tubules. The distal tubule empties into a collecting duct. Fluids and solutes are returned to the capillaries that surround the nephron tubule (the peritubular capillary network).

Urine Formation - (Turn to figure 15.9 Page 268 before continuing)

There are three steps in urine formation:

1.pressure filtration, 2. selective reabsorbtion, and 3. tubular excretion.

1. Pressure filtration - Whole blood enters the glomerulus where pressure causes small molecules to move from the glomerulus to the inside of the Bowman's capsule across the thin walls of each. The filterable components (water, glucose, amino acids, urea, uric acid, creatinine) form the glomerular filtrate (which contains small dissolved molecules in approximately the same concentration as plasma). The non-filterable components (proteins, blood cells and platelets) stay within the glomerulus.

2. Selective reabsorbtion (passive and active) occurs as the filtrate moves along the proximal convuluted tubule. Nearly all of the water, glucose, amino acids, salts are returned to the blood in the peritubular capillaries by diffusion and active transport (see table 15.3). The cells lining the proximal convuluted tubule are adapted for active reabsorption with microvilli for surface area and numerous mitochondria for energy.

3. Tubular excretion is the active transport of hydrogen and amonium ions, uric acid, creatine, and drugs like penicillin from blood in the peritubular capillaries into the distal convuluted tubule. The cells lining the distal convuluted tubule are adapted for active reabsorption with numerous mitochondria for energy. They do not, however, have microvilli.

NOTE: (Go to fig. 15.11 Page 270 Before continuing) The reabsorption of water occurs along the length of the nephron. Notably at the loop of Henle and collecting duct, water returns by osmosis (passive) following active reabsorption of salt (sodium ions and chloride ions) and in response to the nonfilterable proteins that remained in the blood of the peritubular capillaries when it left the glomerulus. At this point urine is isotonic to blood. The creation of hypertonic urine relies on the fact that renal medula is increasingly hypertonic as you move inwards. This situation is a result of Salt (Na+Cl-) diffusing and being actively transported out of the ascending limb of the loop of Henle (which is impermeable to water ions) into the renal medulla. In addition, urea is believed to leak from the lower portion of the collecting duct contributing to the high solute concentration (ie hypertonic environment) in the inner renal medulla. As the collecting duct passes through the renal medulla, which is increasingly hypertonic, water diffuses out of the collecting duct into the renal medulla, and the urine within the collecting duct becomes increasingly hypertonic to blood plasma.

Contrast the blood in the renal artery and the renal vein with respect to urea and glucose content

The blood in the renal vein will contain less urea than the renal artery because having passed through the kidney urea will have left the blood through pressure filtration at the glomerulus with only a small amount being passively reabsorbed at the proximal convuluted tubule.

Glucose content would be the same in both the renal artery and renal vein because although it too would have left the blood through pressure filtration at the glomerulus, almost all of the glucose is actively reabsorbed by the cells lining the proximal convuluted tubule. Reabsorption by active transport is termed selective reabsorption because only molecules recognized by carrier molecules are actively reabsorbed.

Note: The composition of urine is described in table 15.2. Urine contains all of the molecules that were filtered but not reabsorbed in the proximal convoluted tubule, as well as those molecules that underwent tubular excretion at the distal convoluted tubule.







Summary of Functions


• Helps maintain pH.

• Excretes nitrogenous wastes, excess salts and H 2 O or produces and excretes urine.

• Helps maintain water balance.

• Removes histamines, penicillin, etc.

• Helps maintain nutrient and mineral balance.

• Purifies blood.

• Regulates blood volume.

The nephron has three functions:

Glomerular filtration of water and solutes from the blood.

Tubular reabsorption of water and conserved molecules back into the blood.

Tubular secretion of ions and other waste products from surrounding capillaries into the distal tubule.

afferent arteriole • Brings blood to the glomerulus.

efferent arteriole • Brings blood from the glomerulus to the peritubular capillaries.

glomerulus: • To mechanically filter the blood

Bowman's capsule: • To receive the glomerular filtrate

proximal tubule

• Reabsorbs water.

• Selectively reabsorbs nutrients such as glucose and amino acids.

• Selective reabsorption of salts.

• Selective reabsorption of amino acids.

• Selective reabsorption of glucose.

• Active transmission of nutrients.

• Moves filtrate to the loop of Henle.

loop of Henle

• Osmoregulation.

• Maintains salt and water balance.

distal convoluted tubule

• Reabsorbs water.

• Regulates blood pH.

• Carries out selective reabsorption of K, H, NaCl and HCO.

collecting duct:

• Reabsorbs water.

• Carries urine to the renal pelvis.

• Regulation of pH.

• Regulates blood volume.

peritubular capillaries:

• Carries reabsorbed substances from the kidney nephrons to the general circulation.

Identify the source glands for ADH and aldosterone and explain how these hormones are regulated

The hypothalmus controls the pituitary gland and thus body homeostasis. It has centres for hunger, satiety, sleep, thirst, body temperature, and blood pressure. The pituitary gland is divided into two portions, the anterior pituitary and the posterior pituitary. Neurosecretory cells in the hypothalmus respond to neurotransmitter substances and produce ADH that is stored in and released from the posterior pituitary when the hypothalmus senses high osmotic pressure (low water in blood). Aldosterone is secreted by the adrenal cortex in response to low sodium levels (decreased blood volume).

(These hormones are regulated through negative feedback control. In this type of system the response dampens or even cancels the stimulus that brought about the response. This type of homeostatic regulation results in fluctuation above and below a mean. Negative feedback control is a self-regulatory mechanism.)

ADH increases the amount of water in the blood cancelling the stimulus to the hypothalmus shutting off the secretion of ADH (negative feedback).

Aldosterone responds to a decrease in blood volume, this hormone functions to return blood volume to normal levels by causing sodium and potassium ions to be reabsorbed by active transport at the distal tubule or collecting duct. This increases the Na+ concentration of the blood (therefore, more fluid returns from the tissues by osmosis and the blood volume increases) shutting off the signal to the adrenal cortex to secrete aldosterone.

Relate ADH, aldosterone, and the nephron to the regulation of water and sodium levels in the blood

The kidneys and the hypothalamus work together to regulate blood volume through negative feedback. (Note: diuresis means increased amount of urine)

• Hypothalamus senses high osmotic pressure (a decrease in blood volume).

• ADH (antidiuretic hormone) is released (fxn is to return blood volume to normal levels)

• Kidneys increase their retention of H 2 O.

• Distal tubule becomes more permeable to water.

• Collecting duct becomes more permeable to water.

• An increased amount of fluid/water/solution/extra cellular fluid is reabsorbed by the peritubular capillary network.

• Results in increased blood volume.

• Results in decreased osmotic pressure.

• Negative feedback occurs in the posterior pituitary which stops ADH secretion.

• Negative feedback occurs in the hypothalamus which stops ADH secretion.

(When too much fluid is present in the blood sensors in the heart signal the hypothalmus to cause a reduction of the amount of ADH in the blood.)

Aldosterone responds to a decrease in blood volume (renin stimulates its release). Aldosterone is a hormone functions to return blood volume to normal levels.

• Sodium and potassium ions are reabsorbed by active transport at the distal tubule or

collecting duct.

• This increases the solute concentration of the blood; therefore, more fluid returns from

the tissues and the blood volume increases.

A Case Study of the Urinary System

Suppose Will Brown's plasma was analyzed before and after a ten kilometre cross-country run. During the run, Will became dehydrated. His resulting lowered blood

volume would be detected by his body and one of three homeostatic mechanisms would return it to normal.

Path One:

• Stretch receptors in the walls of the arteries detect that the plasma lacks sufficient water.

• ADH (antidiuretic hormone) produced by hypothalamic neurons is transported to the posterior pituitary where it is released into the blood.

• ADH increases the permeability of the distal convoluted tubule and the collecting duct in the nephron.

• This increased permeability causes more water to be reabsorbed.

• Increased water in the plasma will return ion concentrations to normal levels.

• As the blood becomes more dilute, ADH ceases to be produced and released.

• This mechanism is an example of negative feedback.


Path Two:

• Osmoreceptors in the hypothalamus detect that the plasma lacks sufficient water.

• The hypothalamus causes sensations of thirst.

• The student drinks.

• Water is absorbed from the stomach, small intestine and large intestine into the blood.

• Blood volume is increased.


Path Three:

• Kidney releases an enzyme when blood volume and Na+ is low which

eventually targets adrenal gland.

• Adrenal gland releases aldosterone.

• Aldosterone targets kidney tubules.

• This increases Na + recovery in the nephrons.

• This results in increased water recovery and subsequent increase in

blood volume.

[ The Following Is For Your Interest Only]

Nitrogen wastes are a by product of protein metabolism. Amino groups are removed from amino acids prior to energy conversion. The NH2 (amino group) combines with a hydrogen ion (proton) to form ammonia (NH3). Ammonia is very toxic and usually is excreted directly by marine animals. Terrestrial animals usually need to conserve water. Ammonia is converted to urea, a compound the body can tolerate at higher concentrations than ammonia. Birds and insects secrete uric acid that they make through large energy expenditure but little water loss. Amphibians and mammals secrete urea that they form in their liver. Amino groups are turned into ammonia, which in turn is converted to urea, dumped into the blood and concentrated by the kidneys.]