Excretion & Homeostasis - Quick Review Notes

Excretion: Core Concepts

Excretion is the removal from organisms of toxic materials and substances in excess of requirements.
Waste substances include: \text{CO}_2, nitrogenous waste products, the excess water and salts taken in with the diet, spent hormones (e.g., adrenaline after use), drugs absorbed into blood.
Excretory organs and their outputs:
- Lungs: \text{CO}_2 excretion
- Liver: excess amino acids → urea; haemoglobin → bilirubin
- Kidneys: nitrogenous wastes, excess water and salts, hormones, drugs
- Skin: water, sodium chloride, traces of urea
CO2 excretion is important to maintain pH and enzyme activity in cells (carbonic acid balance).
Excretion is linked to homeostasis: maintaining stable internal conditions (water, ions, pH).

The kidneys are rounded organs located behind the abdominal cavity; they are connected to the aorta (renal artery) and to the vena cava (renal vein), and drain urine via the ureters to the bladder.
Parts of the kidney:
- Cortex (outer layer) and Medulla (inner region)
- Renal pelvis (collects urine before it moves to the ureter)
- Ureter (tube from kidney to bladder)
- Bladder (stores urine)
- Urethra (conducts urine to the outside)
Functional unit: the nephron. There are up to 4\times 10^6 nephrons in a kidney.
Blood flow through a nephron:
- Renal artery → Afferent arteriole → Glomerulus (in Bowman's capsule) → Efferent arteriole → Peritubular capillaries → renal tubules
Nephron components:
- Bowman's capsule surrounds the glomerulus
- Renal tubule: proximal convoluted tubule (PCT) → Loop of Henle → distal convoluted tubule (DCT) → collecting duct
Nephron function is to filter blood, reabsorb needed substances, and form urine.

Filtration occurs at the glomerulus via ultrafiltration (high-pressure filtration through a semipermeable membrane).
Substances that pass into Bowman's capsule filtrate: \text{water}, \text{ salts}, \text{glucose}, \text{urea}, \text{uric acid}, \text{amino acids}
Substances that do not pass: \text{red blood cells}, \text{plasma proteins}
The filtrate then enters the renal tubules for selective processing.

Purpose: reclaim substances the body needs back into the blood.
Proximal convoluted tubule (PCT): reabsorbs about 85\% of filtrate; all glucose and amino acids; some salts; most of the water.
Renal tubules actively reabsorb substances via tubular cells by transport mechanisms requiring energy (mitochondria are abundant in these cells).
Nephrons reabsorb substances from filtrate back into blood via surrounding capillaries (peritubular capillaries).
Glucose reabsorption is normally complete; when blood glucose is high (diabetes), glucose appears in urine because reabsorption capacity is exceeded.
The remaining filtrate, now urine, contains urea, excess water, and excess ions.

The kidneys maintain body water and electrolyte balance (osmoregulation).
Body gains water from food, drink, and respiration; losses occur via evaporation, urination, and defecation.
If body water is abundant, kidneys produce dilute urine (less water reabsorbed).
If dehydrated, kidneys reabsorb more water, producing concentrated urine (smaller volume).
Factors affecting urine composition: \text{diet}, \text{water intake}, \text{exercise}, \text{temperature}

Pathway: kidney (nephron filtrate) → renal pelvis → ureter → bladder → urethra → outside body
Bladder specifics: can hold about 400\ \text{cm}^3 of urine; contains a sphincter to regulate emptying
Ureter vs. urethra memory aid: ureter carries urine from kidney to bladder; urethra carries urine from bladder to outside

The nephron includes:
- Glomerulus (a knot of capillaries) and Bowman's capsule (the surrounding cup)
- Renal tubule (proximal convoluted tubule, Loop of Henle, distal convoluted tubule) → collecting duct
Blood flow: Renal artery → arterioles → glomerulus → efferent arteriole → peritubular capillaries
The glomerulus acts as a sieve; the Bowman's capsule collects the filtrate.
There are millions of nephrons arranged to maximize filtration and reabsorption.

Ultrafiltration (glomerulus): filtration of plasma into Bowman's capsule; leaves behind cells and most proteins
Substances that pass into filtrate: water, salts, glucose, urea, uric acid, amino acids
Substances that stay in blood: red blood cells, plasma proteins
Selective reabsorption (tubules): reclaim essential substances back to blood (e.g., glucose, amino acids, many salts, most water)
The final urine contains waste products such as urea, excess water, and excess ions

Amino acids deamination occurs in the liver: removal of the nitrogen-containing amine group
Ammonia is produced and detoxified by converting to urea via the reaction (simplified):
2\mathrm{NH}3 + \mathrm{CO}2 \rightarrow \mathrm{CO(NH}2)2 + \mathrm{H}_2\mathrm{O}
Nitrogenous wastes include ammonia, urea, and uric acid
Liver also metabolizes bilirubin from haemoglobin; bilirubin is excreted in bile
Urea is less toxic and is excreted by the kidneys in urine

Differentiate ultrafiltration from selective reabsorption.
Name the parts of the nephron and their primary roles.
Explain how the liver processes amino acids and why urea is formed.
Describe how osmoregulation affects urine concentration in different conditions.
State what substances pass through the glomerular wall and which do not.
State the approximate number of nephrons in a kidney and the typical bladder capacity.