Animal Physiology: Key Mechanisms and Systems
Osmoregulation and Excretion
Osmosis and Water Balance
Osmosis is the movement of water across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration. Excretion is the process of removing metabolic wastes and toxins from the body. Animals employ different strategies to maintain water balance:
- Osmoconformers: These animals, like jellyfish, maintain an internal osmolarity similar to their environment.
- Osmoregulators: These animals, like humans, actively regulate their internal osmolarity, independent of the environment.
Nitrogenous Wastes
Different animals excrete nitrogenous wastes in various forms:
- Ammonia: Highly toxic and water-soluble, excreted by aquatic animals.
- Urea: Less toxic and requires less water for excretion, used by mammals. It is more energy-costly to produce than ammonia.
- Uric Acid: Excreted as a paste by birds and reptiles, conserving the most water.
Kidney Function
The kidney plays a crucial role in osmoregulation and waste removal. Key components and processes include:
- Descending Loop of Henle: Water reabsorption occurs here.
- Ascending Loop of Henle: Reabsorbs ions (Na+, Cl-) and is impermeable to water.
- Collecting Duct: Concentrates urine by reabsorbing water, regulated by antidiuretic hormone (ADH).
- Countercurrent Multiplier: Creates a concentration gradient in the kidney medulla, facilitating water reabsorption.
Hormonal Regulation
- Antidiuretic Hormone (ADH): Promotes water reabsorption in the kidneys.
- Renin-Angiotensin-Aldosterone System: Regulates blood pressure and fluid balance.
- Aldosterone: Increases sodium reabsorption, aiding water retention.
Immune System
Overview
The immune system defends the body against pathogens and harmful substances. It comprises two main branches:
- Innate Immunity: Fast, non-specific defense mechanisms (e.g., barriers like skin and mucous membranes, phagocytes).
- Adaptive Immunity: Slower, specific, and memory-based, involving T and B cells.
Innate Immune Components
- Barriers: Skin and mucous membranes prevent pathogen entry.
- Phagocytes (Neutrophils and Macrophages): Engulf and destroy pathogens.
- Natural Killer Cells: Eliminate infected or cancerous cells.
- Eosinophils: Defend against parasites.
- Mast Cells: Trigger inflammatory responses.
- Complement System: Enhances immune cell activity.
Adaptive Immune Responses
- Primary Response: Initial exposure to a pathogen; slow.
- Secondary Response: Faster due to memory cells.
- Somatic Nervous System: Voluntary control.
Endocrine System
Hormones and Signaling
The endocrine system uses hormones for slow, long-term communication, contrasting with the nervous system’s rapid, short-term signaling via neurotransmitters.
- Endocrine Signaling: Hormones travel through the blood to distant targets.
- Paracrine Signaling: Hormones act on nearby cells.
- Autocrine Signaling: Hormones act on the secreting cell itself.
Hormone Types
- Water-Soluble Hormones: Bind to surface receptors (e.g., epinephrine).
