BIOL 2200 Lecture 34 Notes

Lecture Overview: BIOL 2200 Lecture 34 focuses on the efficiency of biological systems through the mechanism of countercurrent exchange.

Learning Objectives:

• Understand the components of vertebrate blood.
• Define countercurrent exchange and its applications in thermoregulation, osmoregulation, and excretion.
• Trace the exchange of gases, nutrients, and wastes across various organs and tissues.

Evolution of Circulatory Systems:

• Circulatory systems have evolved differently among various vertebrates: Fish, Amphibians, Reptiles, Birds, Mammals.

Components of Blood:

• Plasma (55%):
• Liquid matrix for travel of nutrients, wastes, gases, hormones.
• Contains dissolved ions that buffer blood, maintain osmotic balance, and affect muscle and nerve activity.
• Plasma proteins play roles in buffering, defense (antibodies), and clotting factors.
• Red Blood Cells (~44%):
• Most abundant blood cells, characterized by a biconcave shape that increases surface area for gas exchange.
• Contains hemoglobin for O2 transport and lacks nuclei and mitochondria to maximize hemoglobin content (1 billion O2 molecules per cell).
• White Blood Cells (
• Includes 5 major types that are vital for immune defense, increasing in number during infections via phagocytosis.
• Platelets (
• Fragments from specialized bone marrow cells involved in clot formation.
• Operate via positive feedback reaction during clotting, crucial in repairing breaks in blood vessels.

Countercurrent Exchange:

• A mechanism where materials are exchanged between two fluids flowing in opposite directions.
• Maximizes efficiency by maintaining a continuous, declining gradient conducive to passive transport.

Gas Exchange in Fish:

• Gills provide a vast surface area for gas exchange, particularly important as water contains less O2 compared to air.
• Countercurrent flow enhances gas exchange efficiency by increasing the diffusion rate of gases between the gills and blood.
• In concurrent flow scenarios, O2 levels would equilibrate inefficiently, leading to reduced oxygen uptake.

Countercurrent Heat Exchange:

• Warm arterial blood transferring heat to cooler venous blood through opposing flow, which minimizes heat loss in extremities (e.g., in birds' limbs).

Countercurrent Multiplier System in Nephrons:

• In mammals, producing hyperosmotic urine allows for minimal water loss via highly efficient countercurrent flow of filtrate and blood.
• An osmotic gradient is established maintained by active transport of NaCl, ensuring water is reabsorbed via passive transport.

Exchange of Gases at Tissues:

• O2 and CO2 exchange occurs via diffusion through capillary walls, while blood pressure assists fluid movement out and osmotic gradients draw it back in.
• The lymphatic system plays a role in fluid recovery, returning excess fluid to the veins leading to the heart.