Excretory System

Question 1

Anatomy

Answer 1

• Kidneys, Ureters, Bladder, Urethra.
• Nephron is functional unit of kidney.
• Nephron empties into Renal Pelvis, which narrows to form Ureter.
• Urine carried through ureters to Bladder and then through Urethra to exit the body.
• Kidney divided into Cortex and Medulla. Nephrons in cortex, which loop of Henle extending into medulla. Renal pelvis found in medulla.
• Renal Artery, Renal Vein, and Ureter enter and exit through Renal Hilum.
• Kindney has Portal System (two capillary beds in series through which blood must travel before returning to heart).
• Renal artery branches out into Afferent Arteriole, which forms highly convoluted capillary bunch called Glomerulus. Bowman’s Capsule surrounds glomerulus, forming Renal Corpuscle. Blood passes through glomerulus into Efferent Arteriole, which forms second capillary bed called Vasa Recta surrounding loop of Henle. Vasa recta joins with Renal Vein.
• Nephron Anatomy: Renal Corpuscle (Bowman’s Capsule + Glomerulus) leads to Proximal Convoluted Tubule, Descending and Ascending limbs of Loop of Henle, Distal Convoluted Tubule, and Collecting Duct.

Question 2

Osmoregulation

Answer 2

• Kidney’s primary function is to regulate blood volume and osmolarity. If blood volume is low and blood osmolarity is high, kidneys will retain water, resulting in low volume, highly concentrated urine. If blood volume is high and blood osmolarity is low, excess water will be excreted while solutes will be reabsorbed in higher concentrations. Kidney function divided into three processes.

• Filtration: Blood is filtered from glomerulus into Bowman’s capsule and the collected fluid is known as Filtrate. The movement of filtrate is governed by Starling Forces (pressure differentials in both hydrostatic and oncotic pressures that cause the net movement of fluid from glomerulus to Bowman’s capsule). Hydrostatic pressure in the glomerulus is significantly higher than in Bowman’s capsule, causing fluid to move into the nephron. The osmolarity of blood is higher than that of Bowman’s capsule, resulting in oncotic pressure (osmotic pressure attributable to dissolved proteins specifically) opposing the movement of fluid into the nephron. Hydrostatic pressure is much higher than oncotic pressure, so net flow is from blood into nephron. Small dissolved molecules (glucose) will flow out in filtrate but blood proteins (albumin) and cells will not.
• Secretion: Nephrons secrete salts (K+), acids (H+), bases (ammonia NH₃) and Urea directly into tubule (anywhere besides Bowman’s capsule) by either active or passive transport, depends on needs of the body at that time; mechanism for excreting wastes too large to be filtered through glomerular pores.
• Reabsorption: Some filtered and secreted compounds are reabsorbed. Glucose, amino acids, vitamins almost always reabsorbed. Water is reabsorbed in large quantities and ions are reabsorbed under ADH and aldosterone control. Anything in filtrate not reabsorbed is excreted in urine.

• Major waste products excreted in urine are H+, K+, NH₃, and Urea. These wastes are drawn into filtrate via active transport.

Question 3

Segments of Nephron

Answer 3

• PCT: Amino acids, glucose, water-soluble vitamins, and the majority of salts are reabsorbed via active transport along with water from filtrate to interstitium and then to vasa recta. Also site of waste product secretion via active transport. Filtrate remains isotonic to interstitium.
• Loop of Henle: Descending limb is permeable only to water, and as osmolarity increases deeper into the medulla, more and more water is drawn out from filtrate into interstitium and then to vasa recta; descending limb maximizes water reabsorption by taking advantage of increasing medullary osmolarity. Ascending limb is permeable only to salts, and as osmolarity decreases closer to cortex, more and more salts are drawn out from filtrate into interstitium and then to vasa recta; ascending limb maximizes salt reabsorption by taking advantage of decreasing medullary osmolarity. As filtrate ascends the ascending limb, it encounters thick Diluting Segment composed of large mitochondria-rich cells that allow reabsorption of sodium and chloride by active transport. Diluting Segment is the only part of the nephron that allows for the production of urine that is more dilute than blood to excrete excess water (during times of overhydration). Vasa recta and loop of Henle create Countercurrent Multiplier System, in which the flow of filtrate through the loop of Henle is in the opposite direction from the flow of blood through the vasa recta, to constantly expose filtrate to hypertonic blood to maximize water reabsorption.
• DCT: Responds to Aldosterone to promote sodium reabsorption (along with water, which follows osmotically active sodium). Increases urine concentration and decreases urine volume. Also site of waste product secretion via active transport.
• Collecting Duct: Responds to Aldosterone and ADH to promote water reabsorption (although CD almost always reabsorbs water). Further increases urine concentration and decreases urine volume. When very well hydrated, CD will be fairly impermeable to salt and water; when dehydrated, ADH and aldosterone act to increase reabsorption of water. Filtrate leaves the tubule and collects in the renal pelvis to be carried through the ureters to the bladder.

• Solutes and water that enter the Interstitium (connective tissue surrounding the nephron) are picked up by Vasa Recta to be returned to circulation.

Question 4

Other Functions of Excretory System

Answer 4

• Blood Pressure: Decreased blood pressure stimulates the release of renin from juxtaglomerular cells in the kidney. Renin then cleaves angiotensinogen (liver protein) to form angiotensin I. ACE in lungs metabolizes angiotensin I to angiotensin II, which promotes the release of aldosterone from adrenal cortex. Aldosterone increases reabsorption of sodium along with water in DCT and CD (without change in blood osmolarity). This reabsorption of isotonic fluid increases blood volume and therefore blood pressure. 
• ADH is synthesized by the hypothalamus and released by the posterior pituitary in response to high blood osmolarity to lower blood osmolarity by increasing permeability of the collecting duct and promoting water retention, which increases blood pressure.
• Cardiovascular system also regulates blood pressure by vasoconstricting or vasodilating. Constriction of the afferent arteriole leads to a lower pressure of blood reaching the glomeruli, which are adjacent to the juxtaglomerular cells. Therefore, this vasoconstriction will secondarily lead to renin release to raise blood pressure.
• Acid-Base Balance: When blood pH is too low, the kidneys excrete more hydrogen ions and increase reabsorption of bicarbonate, resulting in a higher pH. When blood pH is too high, the kidneys can excrete more bicarbonate an increase the reabsorption of hydrogen ions. This is slower than the respiratory response, but it is a highly effective way for the body to maintain acid-base balance.
• The respiratory system can react to derangements of pH quickly. If the blood pH is too low, carbon dioxide can be blown off by increasing respiratory rate, which favors the conversion of hydrogen ions and bicarbonate to water and carbon dioxide, thereby increasing pH. If the blood pH is too high, then decreasing the respiratory rate causes the opposite effects.