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Blood, Circulatory System and Excretory System: Revision Notes

These notes cover the human circulatory system, blood components, blood groups, and blood vessels, along with heart structure and circulation. They also cover the human excretory system, nephron function, urine formation, and renal disorders.

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Science
English · 10 topics
Checked against the video on 25 Oct 2026

Made by AI from the YouTube video SUPER TET Science Class 18 | मानव शरीर एवं स्वास्थ्य P-2 | Blood & Circulatory & Excretory System (opens YouTube in a new tab) by Balaji Study Adda, then checked against it. All credit for the teaching goes to the creator. Tap a topic’s time to confirm a fact in the video.

Blood, Circulatory System and Excretory System: Revision Notes

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In short

These notes cover the human circulatory system, blood components, blood groups, and blood vessels, along with heart structure and circulation. They also cover the human excretory system, nephron function, urine formation, and renal disorders.

Circulatory System and Types

The circulatory system transports blood to and from various parts of the body.
It comprises the heart, blood, blood vessels (arteries, veins, and capillaries), and lymphatic vessels.
Key functions include transporting oxygen, nutrients, and hormones, removing waste products from the body, and maintaining body temperature and pH balance.
Must knowWilliam Harvey discovered the circulatory system.
An open circulatory system allows blood to flow outside blood vessels into body cavities (such as in insects, prawns, and snails).
In a closed circulatory system, blood remains strictly inside blood vessels and contacts tissues only through capillaries (such as in humans, birds, and fish).

Blood Composition and Plasma

Blood is a fluid connective tissue circulating within the body, appearing red due to hemoglobin.
Must knowThe normal pH of human blood is approximately 7.4, which makes it alkaline in nature.
An adult human possesses around 5 to 6 liters of blood.
Blood is divided into plasma (about 55%) and blood corpuscles (about 45%).
Plasma is a pale yellow liquid composed of 90% to 92% water, proteins (albumin, globulin, fibrinogen), glucose, amino acids, salts, hormones, vitamins, and waste products (urea, uric acid).
The main function of plasma is the systemic transport of nutrients, hormones, salts, and metabolic waste.

Blood Corpuscles: RBC, WBC, and Platelets

Red Blood Corpuscles (RBCs or erythrocytes) lack a nucleus, have a biconcave disc shape, and contain hemoglobin to transport oxygen and carbon dioxide.
RBCs are formed in the red bone marrow and have an average lifespan of about 120 days.
Must knowThe spleen is called the graveyard of RBCs because it destroys old and damaged erythrocytes.
White Blood Corpuscles (WBCs or leukocytes) have a nucleus, lack hemoglobin, act as body soldiers against pathogens, and are divided into neutrophils, lymphocytes, monocytes, eosinophils, and basophils.
Platelets (thrombocytes) are small, irregular, non-nucleated fragments that assist in blood clotting and prevent hemorrhage, with a lifespan of 7 to 10 days.

Feature	RBC (Erythrocytes)	WBC (Leukocytes)	Platelets (Thrombocytes)
Color	Red	White	Pale purple / Colorless
Nucleus	Absent	Present	Absent
Count	4.5 - 5.5 Million / mm³	6,000 - 11,000 / mm³	1.5 - 4.0 Lakh / mm³
Formation Site	Red Bone Marrow	Red Bone Marrow, Lymph, Spleen	Red Bone Marrow
Lifespan	~120 Days	Few hours to years	~7 - 10 Days
Main Function	Oxygen transport	Disease protection	Blood clotting

Blood Groups, Rh Factor, and Transfusion

Karl Landsteiner discovered the ABO blood group system in 1901.
Landsteiner and Wiener discovered the Rh factor in 1940.
Blood group A possesses A antigen and anti-B antibody; blood group B possesses B antigen and anti-A antibody; blood group AB possesses both antigens and no antibodies; blood group O possesses no antigens and both anti-A and anti-B antibodies.
Antigens are located on the RBC surface, while antibodies reside in plasma.
Must knowBlood group O- (O negative) is the universal donor, and blood group AB+ (AB positive) is the universal recipient.
Mismatched blood transfusion leads to agglutination (clumping of RBCs), which can be fatal.

Blood Vessels: Arteries, Veins, and Capillaries

Blood vessels are tubular structures through which blood flows, categorized into arteries, veins, and capillaries.
Arteries carry oxygenated blood away from the heart to body organs at high pressure through thick, elastic walls without valves.
Must knowThe pulmonary artery is an exception among arteries because it transports deoxygenated blood from the heart to the lungs.
Veins carry deoxygenated blood from body tissues toward the heart at low pressure, featuring thinner walls and internal valves to prevent backflow.
The pulmonary vein is an exception among veins because it carries oxygenated blood from the lungs to the heart.
Capillaries are microscopic, single-cell-thick vessels connecting arterioles and venules where gas, nutrient, and waste exchange occurs.

Characteristic	Artery	Vein	Capillary
Direction	Heart to body organs	Body organs to heart	Connects arteries & veins
Blood Type	Oxygenated (except pulmonary)	Deoxygenated (except pulmonary)	Mixed / Exchange site
Wall Thickness	Thick, muscular, elastic	Thin, less elastic	Single-cell thick
Valves	Absent	Present	Absent
Internal Pressure	High	Low	Low / Diffusive

Anatomy and Function of the Human Heart

The heart is a muscular organ located in the thoracic cavity between both lungs, tilted slightly to the left, and is roughly the size of a fist (weight 250–350 g in adults).
The heart wall consists of three layers: outer pericardium, middle thick muscular myocardium, and inner endocardium.
Human heart contains four chambers: right atrium, right ventricle, left atrium, and left ventricle.
Must knowThe left ventricle possesses the thickest muscular wall because it must pump blood throughout the entire body.
Normal adult resting heart rate is approximately 72 beats per minute, generating a "Lub-Dub" sound.
Lub is caused by the closure of bicuspid (mitral) and tricuspid valves, whereas Dub is caused by the closure of semilunar valves.
The Sinoatrial (SA) Node in the upper wall of the right atrium acts as the natural pacemaker by initiating electrical impulses.
Normal blood pressure is 120/80 mmHg (systolic 120 mmHg, diastolic 80 mmHg), measured using a sphygmomanometer.

Lymphatic System and Double Circulation

Lymph is a transparent, light-yellow fluid present in lymph vessels that contains excess WBCs to confer immunity and fight infections.
It returns excess interstitial fluid to the blood and absorbs fats from the small intestine.
Human circulation is double circulation, consisting of pulmonary circulation (right ventricle to lungs to left atrium) and systemic circulation (left ventricle to body to right atrium).
Blood pathway: Body tissues -> Vena Cava -> Right Atrium -> Right Ventricle -> Pulmonary Artery -> Lungs -> Pulmonary Veins -> Left Atrium -> Left Ventricle -> Aorta -> Body tissues.
An electrocardiogram (ECG) records the electrical activity of the heart, and a stethoscope listens to heart sounds.

Human Excretory System and Kidney Anatomy

Excretion is the biological process of eliminating toxic metabolic waste products to maintain internal homeostasis and fluid-mineral balance.
The human urinary system consists of four primary organs: two kidneys, two ureters, one urinary bladder, and one urethra.
Kidneys are bean-shaped, reddish-brown organs located on either side of the vertebral column behind the abdominal cavity, measuring 10–12 cm in length.
Ureters convey urine from each kidney to the urinary bladder, which temporarily stores urine until it is voided through the urethra.
Kidneys act as the body's natural blood filters and are referred to by the term renal.

Nephron and Physiology of Urine Formation

Must knowThe nephron is the structural and functional unit of the kidney, with each kidney containing roughly 10 lakh (1 million) nephrons.
A nephron consists of a Malpighian corpuscle (Bowman's capsule and glomerulus), proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.
Urine formation occurs in three sequential stages: filtration (glomerulus/Bowman's capsule), reabsorption (mainly PCT and loop of Henle), and tubular secretion (DCT).
In filtration, water, glucose, urea, and salts enter Bowman's capsule, while blood cells and large proteins remain unfiltered in the blood.
In reabsorption, essential glucose, amino acids, and water are reabsorbed back into peritubular capillaries primarily across the PCT.
Antidiuretic Hormone (ADH), secreted by the posterior pituitary, regulates water reabsorption in the collecting ducts; higher ADH yields concentrated urine, while lower ADH yields dilute urine.

Excretory Products, Other Excretory Organs, and Renal Disorders

Normal urine consists of about 95% water, 2% urea, 1.5% salts, 0.05% uric acid, and 0.05% creatinine, with an average pH around 6.0.
Daily normal urine output ranges from 1.0 to 1.8 liters.
Organisms are classified by primary waste: Ammonotelic (ammonia, e.g., fish), Ureotelic (urea, e.g., humans/mammals), and Uricotelic (uric acid, e.g., birds and reptiles).
Other excretory organs include the skin (sweats water, salt, urea), lungs (excretes carbon dioxide and water vapor), and liver (converts toxic ammonia into urea).
Kidney stones are primarily deposits of calcium oxalate that form in the kidneys or ureters.
Nephritis is kidney inflammation that can cause blood and protein leakage into urine, while uremia is the accumulation of excess urea in blood due to renal dysfunction.
Dialysis (hemodialysis using an artificial kidney/dialyzer) removes waste from blood when kidneys fail; kidney transplantation is the definitive treatment.

Key terms

Hemoglobin: Iron-containing pigment in red blood cells that transports oxygen and gives blood its red color
Spleen: Organ acting as the graveyard of RBCs where worn-out erythrocytes are broken down
Agglutination: Clumping together of red blood cells caused by incompatible blood transfusion
Capillaries: Microscopic single-cell-thick blood vessels connecting arteries and veins where substance exchange occurs
SA Node: Sinoatrial node in the right atrium that acts as the natural pacemaker generating electrical impulses
Sphygmomanometer: Medical device used to measure systolic and diastolic arterial blood pressure
Nephron: The microscopic structural and functional filtering unit of the kidney
Bowman's Capsule: Cup-like sac at the beginning of a nephron performing the filtration of blood
Dialyzer: Machine acting as an artificial kidney to filter waste from blood in renal failure patients
Calcium Oxalate: Chemical compound that forms the primary constituent of human kidney stones

Quick revision

Read these the night before the exam.

Human blood has an alkaline pH of approximately 7.4.
An average adult human has about 5 to 6 liters of blood.
Blood consists of 55 percent plasma and 45 percent blood corpuscles.
Blood group O- is the universal donor, and blood group AB+ is the universal recipient.
The pulmonary artery carries deoxygenated blood, while the pulmonary vein carries oxygenated blood.
The left ventricle has the thickest muscular wall of all four heart chambers.
The natural pacemaker of the human heart is the SA Node located in the right atrium.
Normal human blood pressure is 120/80 mmHg.
Each human kidney contains approximately 10 lakh (1 million) nephrons.
Kidney stones are primarily formed of calcium oxalate.

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