Week 3: Fluid Balance & Circulation 1

Learning Objectives

Students will be able to describe the main fluid compartments and explain how plasma, interstitial fluid, and lymph relate to homeostasis
Students will be able to describe the composition of blood and explain the roles, production, and life span of red cells, white cells, and platelets
Students will be able to compare arteries, veins, and capillaries and explain how fluid and solutes move between blood and tissues
Students will be able to interpret common blood tests including haemoglobin, haematocrit, white cell patterns, platelets, electrolytes, creatinine, and blood gases
Students will be able to explain how lymph is formed, how it returns to the bloodstream, and how lymphatic organs support immunity
Students will be able to describe buffer, respiratory, and renal regulation of pH and recognize common acidosis and alkalosis patterns
Students will be able to relate blood, vessels, lymphatics, fluid balance, and acid-base regulation to overall homeostasis

Understanding Blood, Vessels, and Fluid Balance

Week 3 starts with the idea that water is the largest component of the body. Body water sits mainly inside cells as intracellular fluid, while the rest forms extracellular fluid such as plasma and interstitial fluid.

Blood is your body's transport fluid. Plasma carries water, proteins, nutrients, hormones, and wastes, while red blood cells carry oxygen, white blood cells support defense, and platelets help stop bleeding. The source material also expects you to know that these cells are made in bone marrow and have different life spans and jobs.

Blood vessels are not just pipes. Arteries carry blood away under higher pressure, veins return it at lower pressure, and capillaries are the thin exchange sites where oxygen, nutrients, and fluid move between blood and tissues.

Some fluid leaves capillaries to bathe the cells as interstitial fluid. The lymphatic system collects excess fluid, proteins, and fats, filters lymph through lymph nodes, and returns it to the bloodstream so tissues do not become waterlogged.

Blood tests give quick clues about what is happening inside the body. Hemoglobin and hematocrit relate to oxygen delivery, white cell patterns suggest infection or inflammation, platelets relate to bleeding risk, and electrolytes, creatinine, and blood gases help assess fluid balance, renal function, and acid-base status.

Your body must also keep its pH within a narrow range. Buffers act immediately, the lungs rapidly change carbon dioxide levels, and the kidneys provide slower but more powerful correction. Disturbances cause acidosis or alkalosis and can affect enzyme activity, nerve excitability, and organ function.

Advanced: Hematology, Vascular Physiology, and Fluid Balance

Blood is a specialized connective tissue composed of plasma and formed elements. Plasma provides the fluid medium for transport and contains albumin, globulins, fibrinogen, electrolytes, nutrients, hormones, and metabolic waste products. Formed elements include erythrocytes for gas transport, leukocytes for immune defense, and platelets for haemostasis.

Total body water is distributed mainly between intracellular and extracellular compartments. Extracellular fluid includes plasma and interstitial fluid, and fluid movement between vascular and tissue spaces depends on hydrostatic pressure, oncotic pressure, and membrane permeability.

Haematopoiesis in bone marrow produces erythrocytes, leukocytes, and platelets. Erythrocytes are produced through erythropoiesis under erythropoietin control, circulate for about 120 days, and are removed by spleen and liver. Leukocytes arise from marrow precursors and differentiate into cell types with distinct immune roles, while platelets are fragments shed from megakaryocytes.

The vascular system distributes blood through arteries, arterioles, capillaries, venules, and veins. Capillary exchange allows movement of gases, solutes, and water between plasma and the interstitial compartment, while lymphatic vessels recover excess interstitial fluid, proteins, lipids, and immune cells and return them to the venous circulation.

Laboratory blood tests provide clinically useful snapshots of physiology: haemoglobin and haematocrit relate to oxygen-carrying capacity, differential white cell counts help distinguish inflammatory patterns, platelet counts relate to clotting risk, and electrolytes/creatinine indicate fluid, renal, and solute balance. Blood gases combine pH, PaCO2, and bicarbonate to identify respiratory or metabolic acid-base disturbances.

Acid-base homeostasis depends on immediate chemical buffers, respiratory regulation of PaCO2, and renal control of hydrogen ion secretion and bicarbonate handling. Disturbance of these mechanisms produces respiratory or metabolic acidosis/alkalosis and alters enzyme activity, membrane excitability, and organ function.

💧 Fluid Compartments, Lymphatics & Blood Tests

Where the Fluid Goes and What Blood Tests Tell You

Your body water is split into intracellular fluid (inside cells) and extracellular fluid (outside cells). Extracellular fluid includes plasma in the bloodstream and interstitial fluid around the cells.

Capillaries constantly exchange fluid with tissues. Plasma proteins such as albumin help pull water back into the circulation. When more fluid leaves than returns, lymphatic vessels pick up the extra fluid and proteins and carry them back to the blood. If this system fails, oedema develops.

Red blood cells are made in bone marrow, circulate for about 120 days, and carry oxygen using haemoglobin. White blood cells also come from marrow and protect against infection. Platelets are small fragments from megakaryocytes that help stop bleeding.

Common blood tests each answer a different question: haemoglobin and haematocrit ask about oxygen carriage and concentration, white cell counts ask about infection or inflammation, platelets ask about clotting, and urea, electrolytes, and creatinine ask about fluid and kidney status.

Looking more closely, a differential white cell count helps separate likely bacterial, viral, or allergic patterns, and MCV helps describe whether red cells are small, normal-sized, or large when anaemia is present.

Blood gases and bicarbonate results help show whether the body is acidotic or alkalotic and whether the lungs or kidneys are the main cause.

🩸 Blood Cells, Life Cycle & Interpreting Results

How Blood Cells Are Made and Why the Numbers Matter

Most blood cells begin in the bone marrow. Red blood cells are made there, circulate for about 120 days, and are then removed mainly by the spleen and liver. White blood cells are produced for immune defense, and platelets are released from very large marrow cells called megakaryocytes.

A complete blood count is useful because each part answers a different question. Low haemoglobin or haematocrit suggests reduced oxygen-carrying capacity. High white cells suggest infection or inflammation. Low platelets suggest bleeding risk.

Doctors also look at patterns inside the test. Neutrophils often rise with bacterial infection, lymphocytes often rise with viral illness, and MCV helps describe whether anaemia is microcytic, normocytic, or macrocytic.

🧪 Lymphatic Circulation & Acid-Base Disturbance Patterns

How Lymph Returns Fluid and How pH Problems Are Recognized

Lymph begins as extra interstitial fluid entering blind-ended lymphatic capillaries. It then travels through larger lymph vessels and lymph nodes before returning to the bloodstream through the right lymphatic duct and thoracic duct.

Lymphatic organs support immunity. Bone marrow and thymus are primary lymphoid organs involved in cell production and maturation, while lymph nodes, spleen, and mucosal lymphoid tissue help activate immune responses.

Acid-base problems are usually grouped into four patterns: respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis. Blood gases help decide whether the main problem is carbon dioxide, bicarbonate, or both.

These disturbances matter because acidosis can depress the nervous system, while alkalosis can make nerves and muscles too excitable.

🎥 Video Lectures

Week 3 Overview

Introduction to fluid balance and circulation

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📄 Lecture Notes

Key Terms

Blood Plasma

The liquid component of blood, approximately 90% water, containing proteins, electrolytes, nutrients, and waste products

Red Blood Cells (Erythrocytes)

Blood cells containing hemoglobin that transport oxygen from lungs to tissues and carbon dioxide back to lungs

White Blood Cells (Leukocytes)

Immune cells that protect the body against infectious disease and foreign invaders

Platelets (Thrombocytes)

Cell fragments that play essential role in blood clotting and wound healing

Hemoglobin

Iron-containing protein in red blood cells that binds oxygen and gives blood its red color

Arteries

Blood vessels that carry blood away from the heart under relatively high pressure; walls are thicker and more muscular than veins

Veins

Blood vessels that return blood to the heart at lower pressure; many contain valves that help prevent backflow

Lymphatic System

Network of lymph vessels, nodes, and organs that returns excess interstitial fluid to blood and supports immune defense

Interstitial Fluid

Fluid surrounding body cells outside blood vessels; formed by capillary filtration and partly returned through lymphatics

Oncotic Pressure

Pulling force created mainly by plasma proteins such as albumin that helps keep water within the bloodstream

Blood Gas Analysis

Laboratory measurement of pH, carbon dioxide, oxygen, and bicarbonate used to assess ventilation and acid-base status

Bicarbonate

Major extracellular buffer that helps maintain blood pH and is tightly regulated by lungs and kidneys

Lymph Node

Small encapsulated structure that filters lymph and houses immune cells involved in surveillance and activation

Capillaries

Smallest blood vessels where gas and nutrient exchange occurs between blood and tissues

Albumin

Most abundant plasma protein; maintains osmotic pressure and keeps water in blood vessels

Hematocrit

Percentage of blood volume occupied by red blood cells; normally about 45% in men and 40% in women

Interactive Activity

End of Week Test

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Clinical Case Study

Apply your knowledge of Fluid Balance 1 to a clinical scenario.

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