Importance of water content regulation

maintain water potential in blod and tissue fluid constant, stable environment for cells to function properly

Urinary system

• 2 kidneys
• 2 ureters
• urinary bladder:muscular sac for temporary storage of urine
• urethra:
• 2 sphincter muscles: prevent urine from leaking out

Urination: bladder contracts, sphincter muscles relax

Le kidney

Cortex(outer)
Medulla(inner)
pelvis(central chamber which connects to ureter)
Nephrons: in cortex and medulla

Le Nephron

Ball-like Bowman's capsule --> proximal convoluted tublue --> loop of Henle --> distal convoulted tuble --> collecting duct
afferent arteriole: before glomerulus
efferent: after

Urine formation

1. Ultrafiltration
2. Reabsorption

Ultrafiltration

• Blood in glomerulus under high hydrostatic pressure from pumping action of heart.
• capillary wall: thin and differentially permeable--> filter
• Under high hydrostatic pressure, water & other small molucules forced into Bowman's capsule, forming glomerular filtrate
• Glomerular filtrate: simlilar to blood except:
  • blood cells
  • plasma proteins
• 20% filtered into Bowman's capsule

Reabsorption

• Glucose, amino acids completely reabsorbed (via diffusion and active transport)
• salts: most reabsorbed (diffusion and active transport)
• Water: most in proximal convoluted tuble(osmosis, raised water potential due to reabsorption of substances)
• Urea: reabsorption of water raises its concentration --> diffusion

Adaptive features

• Proximal convoluted tubule is long and highly coiled, inner wall is lined with microvilli; increase surface area and allows more time for reabsorption
• Wall of proximal convoluted tuble is one-cell thick and in close contact; shotened distance for reabsorption
• Wall of proximal convoluted tuble contains numerous mitochondria; provide energy for active transport of useful substances
• tuble richly supplied with capillaries; transport substances away quickly --> maintains steep concentration gradient

Functions of Kidneys

Osmoregulation

Low water potential (e.g. sweating)

• The drop in water potential is detected by osmoreceptors in the hypothalamus
• Hypothalamus signals pituitary gland to release more ADH
• ADH makes walls of collecting ducts more permeable to water, larger proportion of water is reabsorbed by osmosis to blood
• smaller volume, more concentrated.

High water potential

• The rise in water potential is detected by osmoreceptors in hypothalamus

• Hypothalamus signals pituitary gland to release less ADH

• Walls of collecting ducts becomes less permeable to water, smaller proportion of water is reabsorbed into blood by osmosis

• larger volume of dilute urine

Excretion: removal of metabolic wastes from the body

Kidney failure and dialysis machines

Kidney failure: <50 % of normal level

Dialysis tubing: differentially permeable
Dialysis fluid: no urea
Blood passes through the dialysis tubing, urea in blood diffuses out into the dialysis fluid along the concentration gradient

Features:

• pores are small enough to stop the passage of plasma proreins and blood cells out of blood
• same conc. of ...(not urea), no net flow of essential substances to fluid
• same water potential to prevent water loss
• same temperature to prevent heat loss
• tubing: highly branched to increase surface area for rapid diffucsion
• opposite flow directions to maintain a steep concentration gradient for rapid diffusion