Urinary system consists of kidneys, ureters, bladder and urethra. Kidneys are paired, bean shaped and located in the lumbar region just outside the dorsal peritoneum (together weigh about 300g). Blood enters by the renal arteries off of the abdominal aorta. Kidneys receive about 1300 ml blood/min (1/4-1/5 of CO).
Kidneys maintain the body’s homeostasis and allow us to live on land (by conserving water and salt, excreting concentrated waste, and regulating tissue fluid). They maintain constant volume, ionic composition, and pH of the blood. Blood pH is usually 7.4, but urine may vary from 4.6 to 8.0. They are the chief excretory organs for soluble waste products of metabolism like urea. They also eliminated ingested drugs, pesticides and food additives.
In cross section, the kidney has an outer cortex and inner medulla (10-18 pyramidal/cone structures). Each pyramid is a kidney lobe. The functional unit of a kidney is the nephron, consisting of a dilated renal corpuscle connected to a long tubule. It includes the proximal convoluted tubule, thick descending limb, thin limbs, thick ascending limb of Henle’s loop, distal convoluted tubule, and collecting tubules/ducts.
The nephron arises in the cortex, loops into the medulla, returns to the cortex, then drains into collecting ducts that descend into the medulla. The medulla has a gradient of hypertonicity (increase in saltiness as you descend). This allows you to produce concentrated urine.
At the papillary region, medullary collecting ducts pierce the tips of the renal pyramids emptying urine into the calyces. Calyces drain into the renal pelvis (dilated upper portion of ureter). Ureters then conduct urine to the bladder.
Oversimplified view of the process: Each minute, 125 ml of ultrafiltrate is formed, but only 1 ml of urine is excreted. In the proximal tubule, 80% of water and salt are reabsorbed. Thin limbs of loop of Henle don’t have ion pumps, but differ in their permeabilities. Thick limbs actively pump NaCl into the surrounding interstitium. Water cannot follow, as tubules are water impermeable. Now, fluid in the tubule becomes hypotonic. In the collecting ducts, ADH causes aquaporin channels to surface and make the walls water permable. Water then leaves the ducts and concentrates the urine.
Slides of the renal cortex show tufts of capillaries, which are one component of the renal corpuscle (in addition to bowman’s capsule). Each kidney has about 1 million of these. They form filtrate of almost everything except cells and large protein in the blood. Blood is spread over a great surface area to create the ultra-filtrate. The filtrate is later modified by absorption and secretion.
Glomerulus: capillaries are lined with a fenestrated epithelium (muscle/CT capillaries are continuous, and liver/spleen capillaries are discontinuous/sinusoidal). Fenestrations increase permeability, which is desirable for filtration. A parietal layer of simple squamous epithelium lines Bowman’s capsule. These cells are continuous with the visceral layer of cells on top of the glomerular capillaries. These visceral cells are podocytes, greatly modified to give strength to capillary walls among other important functions. Podocytes have pedicels (processes) that interdigitate with each other. They represent some complex and spectacular epithelial-epithelial interactions.
Podocytes sit atop the glomerular basement membrane. Pedicels form a layer on the golmerular basement membrane. A glycoprotein, podocalyxin, coats the podocyte surface and contributes to the negative charge of the visceral surface.