9.9

• The mammalian urinary system consists of the kidney, ureter, bladder, urethra, and the structures within the kidney include the renal vein, renal artery, and the ureter.
• The gross structure of a mammalian kidney consists of a fibrous capsule, cortex, and medulla.
• The cortex of a mammalian kidney consists of Bowman’s capsules, convoluted tubules, and blood vessels.
• The medulla of a mammalian kidney consists of collecting ducts, loops of Henle, and blood vessels.
• The structure of a nephron consists of a glomerulus, Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.
• The blood vessels associated with a nephron include a wide afferent arteriole from the renal artery that enters the renal capsule and forms a glomerulus, an efferent arteriole that branches to form a capillary network that surrounds tubules, and a loop of Henle that extends from the cortex into the medulla.
• The sections of a nephron include Bowman’s capsule at the start of the nephron, which is cup-shaped and surrounds the glomerulus, and the proximal convoluted tubule (PCT), which is a series of loops surrounded by capillaries and has walls made of epithelial cells with microvilli.
• The loop of Henle is a hairpin loop that extends from the cortex into the medulla.
• The distal convoluted tubule is similar to the PCT but has fewer capillaries.
• The collecting duct is a DCT from several nephrons that empty into a collecting duct, which leads into the pelvis of the kidney.
• Urea is produced by hepatocytes deaminating excess amino acids to form ammonia, and the ornithine cycle in liver cells converts ammonia to urea, which is less toxic.
• Permeability of walls is determined by the action of hormones.
• Reabsorption of water can occur through osmosis or active transport.
• Osmoregulation is controlled by negative feedback homeostatic mechanisms.
• The collecting duct plays a role in osmoregulation.
• The hypothalamus plays a role in osmoregulation by causing osmoreceptors to shrink when water is lost, triggering the hypothalamus to produce more antidiuretic hormone (ADH).
• Reabsorption of water from filtrate into interstitial fluid occurs via osmosis through aquaporins.
• ADH forms a hormone-receptor complex on the surface membrane of cells in the collecting duct, triggering activation of cAMP as a secondary messenger.
• The activation of cAMP by ADH triggers cellular processes that increase reabsorption of water, causing urine to become more concentrated.
• The loop of Henle acts as a countercurrent multiplier, ensuring filtrate in collecting ducts is always beside an area of interstitial fluid that has a lower water potential.
• Blood water potential can change due to changes in water intake, ion intake in diet, ions used in metabolic processes or excreted, and sweating.
• The posterior pituitary gland stores and secretes the ADH produced by the hypothalamus.
• The role of the skin in thermoregulation is to control heat loss to the skin surface through vasodilation/ constriction of arterioles supplying skin capillaries and hair erector muscles contracting to trap air for insulation.
• The kidney of a kangaroo rat (Dipodomys sp.) is adapted for life in a dry environment by having a long loop of Henle, a more concentrated medullary region, and greater effect as a countercurrent multiplier.
• Evaporation of sweat cools the skin surface.
• The autonomic nervous system enables endotherms to thermoregulate via negative feedback, with thermoreceptors in the hypothalamus detecting changes in blood temperature and sending impulses to effectors.
• An endotherm is an organism that can regulate its body temperature independently of external sources and thermoreceptors send signals to the hypothalamus, which triggers a physiological or behavioural response.
• An ectotherm is an organism that cannot increase its respiration rate to increase the internal production of heat and relies on external sources to regulate its body temperature.
• More water is reabsorbed in the kidney of a kangaroo rat (Dipodomys sp.) due to the greater effect as a countercurrent multiplier.
• Endotherms use behavioural methods to regulate their body temperature such as basking in the sun, pressing against warm surfaces, digging burrows, hibernation/ aestivation, and panting (heat lost as water evaporates from mouth).
• Urea is removed from the bloodstream through ultrafiltration in Bowman’s capsule, a process that involves high hydrostatic pressure in the glomerulus forcing small molecules such as urea, water, glucose, and mineral ions out of capillary fenestrations against an osmotic gradient.
• Selective reabsorption occurs in the proximal convoluted tubule.
• Cells in the proximal convoluted tubule are adapted for selective reabsorption through microvilli for a large surface area, many mitochondria for ATP for active transport of glucose into intercellular spaces, and a folded basal membrane for a large surface area.
• The basement membrane acts as a filter in the process of urea removal from the bloodstream.
• The distal convoluted tubule plays a role in the regulation of water and electrolyte balance.
• In the loop of Henle, active transport of Na+ & Cl- out of the ascending limb decreases water potential of interstitial fluid, causing water to move out of the descending limb.
• Selective reabsorption is a process where useful molecules from glomerular filtrate such as glucose are reabsorbed into the blood.
• The water potential of filtrate decreases going down the descending limb, from lowest in the medullary region to highest at the top of the ascending limb.
• Cells of the Bowman’s capsule are adapted for ultrafiltration through fenestrations between epithelial cells of capillaries and fluid can pass between and under the folded membrane of podocytes.