Biology 101 - Kidney and Excretory System Part 2

It supplies blood to the glomerulus.

Remember that afferent vessels bring blood toward an organ, while efferent vessels carry blood away.

It is a ball of capillaries through which fluid, molecules, and ions filter into the nephron at Bowman’s capsule.

It carries blood away from the glomerulus.

Through vasodilation and vasoconstriction, the efferent arterioles function in the maintenance of glomerular filtration rate.

It is the main functional unit of the kidney. It is responsible for filtration, secretion, and reabsorption.

Fluid, molecules, and ions enter Bowman's capsule from the glomerulus.

The filtrate then enters the proximal convoluted tubule and descends the loop of Henle.

After ascending the loop of Henle, the filtrate enters the distal convoluted tubule.

Finally, the filtrate enters the collecting duct.

A rounded structure at the beginning of the nephron. It collects the filtrate exiting the glomerulus.

Proximal Convoluted Tubule

| PCT

Most reabsorption and secretion occurs in the PCT.

It provides a high surface area for effective secretion and reabsorption of substances.

Vitamins, glucose, and amino acids

As biologically useful molecules, these substances are usually completely reabsorbed. Note that water and some ions are also reabsorbed in the proximal convoluted tubule.

It is a "U-shaped" structure that follows the proximal convoluted tubule. It generates a concentration gradient that assists the collecting duct in water reabsorption.

It would facilitate the production of urine with high osmolarity (more concentrated).

A longer loop can generate a larger gradient through the countercurrent multiplier system. This gradient then draws more water out of the collecting duct through osmosis.

It is the site of water reabsorption.

The descending limb is permeable to water but not to solutes. As a result, the filtrate becomes more concentrated as water is reabsorbed down the length of the limb.

Water can passively flow out of the descending limb instead of requiring energy.

A high solute concentration is present in the deeper regions of the medulla. The water permeability of the descending branch allows water to flow out, down its gradient, without ions moving inward.

It is the site of active Na+ transport.

The ascending limb is permeable to salt and not water. Sodium ions are pumped out of the limb, decreasing the osmolarity of the filtrate inside.

Water is prevented from reentering the loop of Henle.

Salt must actively be pumped out of the ascending loop to achieve the proper concentration gradient within the kidney. If water followed, this process would be disrupted.

The distal convoluted tubule is located between the ascending limb of the loop of Henle and the collecting duct.

The DCT fine-tunes the reabsorption and secretion already performed by the proximal convoluted tubule.

Specifically, the DCT is involved in K+ and H+ secretion as well as plasma calcium regulation.

Elevated plasma osmolarity and low blood volume, both markers of low blood pressure, trigger the release of aldosterone from the adrenal glands. Aldosterone signals the DCT to reabsorb more Na+. Water follows the Na+ and is thus reabsorbed.

Increased water reabsorption raises blood volume and thus blood pressure.

The PCT is lined with a brush border, or epithelium covered in microvilli. As in the small intestine, this structure maximizes surface area.

The epithelium of the DCT does not possess a brush border.

It carries the filtrate to the ureter. It mainly functions to concentrate the urine when necessary.

Urine concentration is promoted by the hormone ADH and assisted by the concentration gradient maintained by the loop of Henle.

Low blood pressure triggers the release of antidiuretic hormone (ADH) from the posterior pituitary.

ADH causes the collecting duct to become permeable to water, facilitating passive reabsorption.

Increased water reabsorption raises blood volume and thus blood pressure.

A dehydrated person's urine would have a high osmolarity. In other words, it would be concentrated.

Dehydration, or water loss, results in low blood volume. This triggers ADH and aldosterone to increase the reabsorption of water in the nephron. Urine that contains less water compared to solute will have a higher osmolarity.

ureter

A thin tube, transports urine from the kidney to the bladder.

The collecting ducts of the nephrons drain into the ureters.

urinary bladder

A sac lined with smooth muscle, stores urine. Its epithelium can become compact to accommodate a larger volume.

Note that this structure is distinct from the gall bladder, which functions in digestion.

urethra

It is the final tube through which urine is excreted.

In males, the urethra also functions in reproduction as a pathway for semen during ejaculation.

aldosterone

A mineralocorticoid, may affect this patient's kidneys. Since this hormone is released by the adrenal cortex, impairment of that structure could cause improper aldosterone release.

Aldosterone is a steroid, like the other products of the adrenal cortex. This can be remembered by recalling their broad classification: corticosteroids.

Aldosterone acts on the distal convoluted tubule in response to low blood pressure. It directly facilitates the reabsorption of sodium ions.

Indirectly, aldosterone also increases the reabsorption of water, which passively follows Na+ out of the distal convoluted tubule. This water retention raises blood volume, increasing the blood pressure.

increases

Renin, an enzyme, is secreted from the kidneys and indirectly leads to aldosterone release. This series of reactions, known as the renin-angiotensin system, is often a target of blood pressure medication.

Antidiuretic hormone, or ADH, may affect this patient's kidneys. This peptide hormone is stored in and released from the posterior pituitary.

Note that ADH is produced in the hypothalamus, not the posterior pituitary. It is transported to this gland after synthesis.

ADH acts on the collecting duct and distal tubule in response to low blood pressure. It directly facilitates the reabsorption of water, thus increasing blood volume and pressure.

Specifically, ADH increases the number of aquaporins, or water channels, in the cells that line this part of the nephron.

Urine osmolarity, or solute concentration, increases.

ADH causes the increased reabsorption of water in the distal convoluted tubule and the collecting duct. More water retention leads to concentrated urine.

antidiuretic hormone

| (ADH)

It promotes the reabsorption of Na+, which leads to water retention. ADH increases water reabsorption directly.

It is a steroid hormone, while ADH is a peptide.

It is released from the adrenal cortex, while ADH is secreted from the posterior pituitary.

The primary stimulus for aldosterone release is reduced blood flow to the kidney, while the trigger for ADH release is elevated plasma osmolarity.

The body can regulate blood pressure, osmolarity, pH, and waste products.

Blood pressure is regulated by varying the excretion of water and Na+. Osmolarity is lowered through the excretion of ions; in the case of H+ and HCO3-, this also alters pH. Excreting urea eliminates waste product.

The kidneys will secrete H+.

Normal plasma pH is around 7.4, and acidosis occurs at levels below 7.35. Thus, the pH in this example is too acidic, and protons must be secreted.

The kidneys will secrete a larger fraction of HCO3-, resulting in its elimination from the body.

Normal plasma pH is around 7.4, and alkalosis occurs at levels above 7.45. Thus, the pH in this example is too basic. Bicarbonate, or HCO3-, is an important base; its excretion will make blood more acidic.

The nitrogen from amino acids is converted to ammonia, then urea. Urea is filtered in the nephron before being concentrated and excreted as urine.

Urea, below, is a soluble, nontoxic form of ammonia. Its excretion in the urine is a simple disposal method for the products of protein catabolism.

The glomerulus may be impaired.

Ordinarily, the glomerulus cannot filter proteins and blood cells into Bowman's capsule due to their large size. Glomerular damage could impact this size-based filtration, resulting in the appearance of blood cells and proteins in the urine.

Diabetes is often characterized by the presence of glucose in the urine.

Normally, the proximal convoluted tubule reabsorbs virtually all of the glucose present in the filtrate. However, diabetes results in chronically elevated blood glucose levels. When these levels exceed the maximum amount that can be reabsorbed, glucose will be excreted.