Kidney and Excretory System Flashcards
Always a favorite of MCAT test-makers, the excretory system can also be a major source of confusion. Use these cards to master filtration, reabsorption, and secretion, as well as hormone involvement and the complexity of the loop of Henle.
What are the two main structural components of the kidney?
The cortex and the medulla
The cortex, or outer layer of the kidney, contains the proximal and distal convoluted tubules of the nephrons. The medulla, or inner region, contains the loops of Henle.
List four primary functions of the kidneys.
- Regulation of blood pressure
- Regulation of blood osmolarity
- Regulation of blood pH
- Excretion of soluble wastes
What is filtration?
Filtration is the process by which fluid, molecules, and ions leave the blood. Filtration is promoted by hydrostatic pressure.
The material filtered into the glomerulus is called the “filtrate.”
In the context of the kidney, what is secretion?
Secretion is the process by which ions or molecules move from the blood into the nephron, joining the filtrate. These substances will then be excreted.
Secretion of protons and waste products occurs at the proximal and distal convoluted tubules.
What is reabsorption?
Reabsorption is the process by which water, molecules, and ions are transported from the filtrate back into the blood. As a result, these compounds are not excreted from the body.
Reabsorption occurs in all parts of the nephron.
Why are non-waste molecules filtered into the glomerulus?
Filtration is not a specific process. Many biologically useful molecules, if they are sufficiently small, can pass into the glomerulus.
Important molecules that enter the filtrate, including glucose and amino acids, are simply reabsorbed from the nephron. Note that proteins are too large to be filtered.
How do the kidneys eliminate extra ions that are not filtered directly into the glomerulus?
These ions are secreted.
Both K+ and H+ are secreted in the proximal and distal tubules. H+ secretion is a vital part of pH balance.
Name four molecules or ions that are reabsorbed in the proximal tubule.
A variety of biologically useful molecules are reabsorbed in the proximal tubule. Examples include water, amino acids, glucose, and bicarbonate (HCO3-).
What does plasma osmolarity measure?
Plasma osmolarity refers to the concentration of solute in the blood. High osmolarity signifies a large amount of solute and a relatively low amount of water.
In the blood, osmolarity is largely determined by the concentration of Na+ and Cl- ions.
In the loop of Henle, fluid flow in the two adjacent limbs is opposite; this contributes to a concentration gradient that increases from cortex to medulla. What term best relates to this phenomenon?
This describes the nephron’s countercurrent multiplier system.
Sodium pumps in the ascending limb trigger a progressive increase in the amount of solute in the medulla. The unique structure of the loop causes this effect to compound or “multiply” over time.
How would the osmolarity of human urine likely compare to that of a camel?
The osmolarity of human urine would be lower.
High osmolarity occurs when a large amount of solute is present in a certain volume of fluid. Since camels must conserve water, their urine would be very concentrated.
What is a portal system?
A portal system is a circulatory structure in which one capillary bed drains into another. The capillaries surrounding each nephron are similarly arranged.
In humans, the hypophyseal and hepatic portal systems are two other examples to know.
Name the two adjacent capillary beds that exchange material with the nephron.
The glomerulus and peritubular capillaries exchange fluid and solutes with the nephron.
The glomerulus, a dense web of capillaries, provides the filtrate that enters the nephron. Peritubular capillaries secrete and reabsorb substances to and from the tubules.
What is the function of the renal artery?
The renal artery transports blood from the aorta to the kidney.
Within the kidney, the renal artery splits into arterioles, which provide blood to the glomeruli.
What is the function of the renal vein?
The renal vein transports blood away from the kidney and back toward the heart.
Specifically, peritubular capillaries give rise to venules, which drain into the renal vein.
What physiological condition is regulated by the action of ADH and aldosterone?
Antidiuretic hormone (ADH) and aldosterone regulate blood pressure.
Important differences between these hormones exist, but both act on the nephrons of the kidney, eventually resulting in greater retention of water.
Which hormones act on the nephron in response to low blood pressure?
Aldosterone and ADH are produced when blood pressure is low. Through different mechanisms, both increase the nephron’s reabsorption of water, which raises blood volume.
Aldosterone is a steroid hormone, while ADH is a peptide.
Does aldosterone, ADH, or neither hormone act on the nephron in response to high blood pressure?
Neither aldosterone nor ADH activity is triggered by high blood pressure.
Be careful! ADH and aldosterone function to alleviate low, not high, pressure. When pressure is high, these hormones are not produced and excess water is not reabsorbed. This concept appears often on the MCAT.
Which functions of a nephron occur in the renal cortex?
Filtration, reabsorption, and secretion all take place within the renal cortex.
The cortex contains the glomerulus and the convoluted tubules of the nephron.
Which functions of a nephron occur in the renal medulla?
Reabsorption and urine concentration take place within the renal medulla.
The renal medulla contains the loop of Henle and the collecting duct. The loop of Henle reabsorbs both water and sodium. It also maintains the concentration gradient that allows urine to be concentrated in the collecting duct.
What is the function of the afferent arterioles?
The afferent arterioles supply blood to the glomerulus.
Remember that afferent vessels bring blood toward an organ, while efferent vessels carry blood away.
Define:
glomerulus
The glomerulus is a ball of capillaries through which fluid, molecules, and ions filter into the nephron at Bowman’s capsule.
What is the function of the efferent arterioles?
The efferent arterioles carry blood away from the glomerulus.
Through vasodilation and vasoconstriction, the efferent arterioles function in the maintenance of glomerular filtration rate.