Chapter 3: Body Fluids and Circulation

Role and Composition

• Transport: Blood carries nutrients, \(O_2\) and other essential substances to cells, and removes wastes; lymph also transports certain substances.
• Volume: Adult men have about 5–5.5 L of blood, constituting 30–35% of extracellular fluid.
• pH: Blood is slightly alkaline, with a pH of about 7.4.
• Components: Plasma forms about 55% of blood; formed elements form nearly 45%.

• Composition: About 90–92% water and 6–8% plasma proteins; the rest includes dissolved substances.
• Proteins: Albumin helps maintain osmotic balance; globulins and immunoglobulins aid defence; fibrinogen and prothrombin are involved in clotting.
• Other contents: Minerals such as \(Na^+\), \(Ca^{2+}\), \(Mg^{2+}\), \(HCO_3^-\) and \(Cl^-\), plus glucose, amino acids and lipids in transit.
• Clotting factors: Present in plasma in inactive forms.

• Count: About 5 million RBCs per \(mm^3\) in adult men and 5.5 million per \(mm^3\) in adult women.
• Formation: Produced in adult red bone marrow; RBC formation is called erythropoiesis.
• Structure: Mature mammalian RBCs are circular, biconcave and enucleated. Their shape provides a large surface area for gaseous exchange.
• Haemoglobin: Gives blood its red colour; a healthy individual has about 12–16 g per 100 mL of blood.
• Requirements: Iron, proteins, vitamin \(B_{12}\) and folic acid are needed for RBC and haemoglobin formation.
• Life and destruction: RBCs live about 120 days and are destroyed in the spleen, the “graveyard of RBCs.”
• Altitude: People living in hills have more RBCs to cope with the lower oxygen content of the air.

• Count and lifespan: About 6,000–8,000 per \(mm^3\) of blood; WBCs are generally short-lived.
• Classes: Granulocytes are neutrophils, eosinophils and basophils; agranulocytes are lymphocytes and monocytes.
• Extremes: Neutrophils are the most numerous WBCs; monocytes are the largest.
• Formation and disorders: WBC formation is leucopoiesis; a decreased count is leukopenia, while abnormal increases occur in malignancies such as leukaemia.

• Origin and form: Produced from bone-marrow megakaryocytes; they are colourless, rounded or oval disc-like fragments.
• Count: About 150,000–350,000 per \(mm^3\) of blood.
• Function: Release substances involved in blood clotting.
• Life and low count: Their lifespan is about one week; a reduced count can cause clotting disorders and excessive blood loss.

• Basis: Blood groups depend on antigens on RBCs and antibodies in plasma.
• Transfusion safety: Donor and recipient groups must be matched to avoid clumping and destruction of RBCs.

• Discovery: Karl Landsteiner reported the ABO groups in 1900; Decastello and Sturli identified the AB group in 1902.
• Genetic control: Gene \(I\) has three alleles: \(I^A\), \(I^B\) and \(I^O\).
• Antigens: Alleles \(I^A\) and \(I^B\) produce A and B antigens, respectively, on RBC surfaces.
• Antibodies: Plasma contains antibodies against the A or B antigen absent from a person's RBCs.

• Discovery: Landsteiner and Wiener identified the Rh antigen in 1940 in the blood of a Rhesus monkey.
• Types: Nearly 80% of people have the antigen and are Rh-positive (\(Rh^+\)); those without it are Rh-negative (\(Rh^-\)).
• Antibodies: An \(Rh^-\) person exposed to \(Rh^+\) blood can form antibodies against the Rh antigen; Rh status must also be matched for transfusions.

• Clot: A dark reddish-brown mass formed at the injury site; a fibrin-thread network traps dead and damaged formed elements.
• Clotting time: Usually 3–8 minutes.
• Vitamin K: Essential because it is needed for prothrombin synthesis in the liver.
• Anticoagulants: Heparin and sodium or potassium oxalate prevent blood clotting.

Composition and Collection

• Lymph plasma: Resembles blood plasma but has fewer proteins, less calcium and phosphorus, and a higher glucose concentration.
• Lymph corpuscles: Floating amoeboid leucocytes, mostly lymphocytes.
• Absent: Erythrocytes and platelets are not present in lymph.
• Collection: The lymphatic system collects lymph and drains it back into the major veins.

• Transport: Carries nutrients and hormones to body cells and returns carbon dioxide and other metabolic wastes from cells to the blood; these ultimately enter the venous system.
• Fat absorption: Fats are absorbed through lymph in the lacteals of the intestinal villi.
• Defence: Lymph nodes destroy invading microorganisms and foreign particles.

Heart: Structure and Major Vessels

• Position and covering: The mesoderm-derived heart lies in the thoracic cavity between the lungs, slightly left of centre; it is about the size of a clenched fist.
• Pericardium: The four-chambered heart is enclosed in a pericardium. Pericardial fluid in the cavity between its two layers reduces friction.
• Chambers and septa: Right and left atria are separated by the interatrial septum; the ventricles are separated by the interventricular septum.
• Atrioventricular septum: Thick fibrous tissue separates each atrium from the ventricle on the same side.
• Right atrium: Receives deoxygenated blood from the superior vena cava (head and upper body) and inferior vena cava (lower body).
• Left atrium: Receives oxygenated blood from the lungs through the pulmonary veins.
• Ventricles: The left ventricular wall is thicker than the right because it pumps blood to the whole body at higher pressure.

Valve Support and Function

• Atrioventricular valves: The tricuspid and bicuspid valves lie between the atria and ventricles.
• Chordae tendineae: Fibrous cords connect the flaps of these valves to papillary muscles.
• One-way flow: Heart valves prevent regurgitation and maintain unidirectional blood flow.

Nodal Tissue

• Autoexcitable tissue: Cardiac nodal tissue can generate action potentials without external stimulation.
• SAN—pacemaker: Generates the highest number of impulses, about \(70\)–\(75\) per minute, and initiates and maintains the rhythmic heartbeat.
• AVN—pacesetter: Rapidly transmits the excitation received from the SAN to the ventricles through the bundle of His.

Cardiac Output Example

• Example: At \(72\) beats/min and a stroke volume of \(0.08\) L/beat, multiplication gives \(72\times0.08=5.76\) L/min, or about \(5.8\) L/min.
• Source figure: The lecture states approximately \(5.5\) L/min for these values; this does not match the multiplication, so use \(5.76\) L/min as the calculated result and note the source discrepancy if required.
• Adjustment: The body can alter both heart rate and stroke volume; an athlete can have a higher cardiac output than an ordinary person.

Recording and ECG Waves

• Standard recording: Three electrical leads are attached—one to each wrist and one to the left ankle—to monitor heart activity.
• P wave: Small upward wave showing atrial depolarisation, initiated by SAN activity; it precedes atrial contraction.
• QRS complex: Q is a small downward deflection, R a large upward deflection and S a downward deflection. The complex represents ventricular depolarisation; Q marks the beginning of ventricular systole.
• T wave: Dome-shaped wave showing ventricular repolarisation, associated with ventricular relaxation; its end marks the end of systole.
• Clinical use: Counting QRS complexes over a time interval gives heart rate. An abnormal ECG shape can indicate disease or another abnormality.

Blood pressure and hypertension

• Hypertension: The most common disease affecting the heart and blood vessels; high blood pressure can harm the heart, brain, kidneys and eyes.
• Normal blood pressure: \(120/80\ \text{mmHg}\); the first value is systolic and the second is diastolic.
• Systolic pressure: Pressure produced when the left ventricle contracts and pushes blood into the aorta; normally about \(120\ \text{mmHg}\).
• Diastolic pressure: Pressure in the blood vessels when the heart is relaxed during complete diastole; normally about \(80\ \text{mmHg}\).
• Hypertension threshold: Blood pressure above \(140\ \text{mmHg}\) systolic and \(90\ \text{mmHg}\) diastolic.
• Measurement: A sphygmomanometer measures systolic and diastolic blood pressure.

Atherosclerosis

• Plaque composition: Lipids such as cholesterol, fat, calcium and fibrous tissue collect on the inner lining of arteries.
• Effect: Plaques may completely block an artery; in coronary arteries, they reduce blood supply to the heart muscle.
• Consequence: Reduced coronary blood flow can lead to a heart attack or stroke.

Arteriosclerosis

• Effect: The artery loses its ability to distend, and its walls may rupture.
• Risk: When coronary arteries are affected, it can lead to a heart attack and may cause death.

Key points

• Cause: Conditions that reduce or otherwise affect blood flow to the heart muscle.
• Occurrence: It can affect people of any age and sex, but is more common in middle-aged and elderly people.

Key symptom

• Lung congestion: Congestion of the lungs is one of the main symptoms.