Biology · Class 9-10 · CBSE
Circulation and Excretion: Heart, Blood and Nephrons
Explore the 4-chambered human heart, double circulation (pulmonary & systemic circuits), artery vs vein vs capillary, blood components, xylem vs phloem in plants, and human nephron ultrafiltration & urine formation.
1Structure of the Heart and Double Circulation
The human heart is a muscular organ about the size of a fist with4 chambers: two upper thin-walled Atria (receiving chambers) and two lower thick-walled Ventricles (pumping chambers). The left and right sides are completely separated by a muscularSeptumto prevent mixing of oxygenated and deoxygenated blood.
Pathway of Blood in Human Double Circulation
- 1
1. Deoxygenated blood from body tissues enters Right Atrium via Vena Cava.
- 2
2. Right Atrium contracts, pushing blood into Right Ventricle through tricuspid valve.
- 3
3. Right Ventricle pumps deoxygenated blood via Pulmonary Artery to Lungs for oxygenation (Pulmonary Circuit).
- 4
4. Oxygenated blood from lungs returns to Left Atrium via Pulmonary Veins.
- 5
5. Left Atrium contracts, pushing blood into Left Ventricle through bicuspid (mitral) valve.
- 6
6. Thick muscular Left Ventricle pumps oxygen-rich blood under high pressure into Aorta to supply entire body (Systemic Circuit).
Blood Vessels: Arteries vs Veins vs Capillaries
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Direction of Flow | Carries blood AWAY from heart to organs | Carries blood TOWARDS heart from organs | Connects arterioles to venules across tissues |
| Type of Blood | Oxygenated blood (Except Pulmonary Artery) | Deoxygenated blood (Except Pulmonary Vein) | Mixed / Site of nutrient-gas exchange |
| Wall Thickness & Elasticity | Thick, elastic, highly muscular walls to withstand high systolic pressure | Thin, less muscular walls with large central lumen | Extremely thin (single cell thick endothelium) |
| Internal Valves | Valves are absent (high pressure prevents backflow) | Internal pocket valves present to prevent backflow of sluggish blood | Valves absent |
Direction of Flow
- Artery
- Carries blood AWAY from heart to organs
- Vein
- Carries blood TOWARDS heart from organs
- Capillary
- Connects arterioles to venules across tissues
Type of Blood
- Artery
- Oxygenated blood (Except Pulmonary Artery)
- Vein
- Deoxygenated blood (Except Pulmonary Vein)
- Capillary
- Mixed / Site of nutrient-gas exchange
Wall Thickness & Elasticity
- Artery
- Thick, elastic, highly muscular walls to withstand high systolic pressure
- Vein
- Thin, less muscular walls with large central lumen
- Capillary
- Extremely thin (single cell thick endothelium)
Internal Valves
- Artery
- Valves are absent (high pressure prevents backflow)
- Vein
- Internal pocket valves present to prevent backflow of sluggish blood
- Capillary
- Valves absent
2Transport in Plants: Xylem vs Phloem
Xylem vs Phloem: Plant Vascular Systems
| Feature | Xylem Tissue | Phloem Tissue |
|---|---|---|
| Transported Substance | Water and dissolved mineral ions | Soluble products of photosynthesis (sucrose, amino acids) - Translocation |
| Direction of Transport | Unidirectional (strictly upwards from roots to leaves) | Bidirectional (upwards and downwards according to plant sink needs) |
| Driving Mechanism | Physical forces: Transpiration pull and root pressure (passive, no ATP) | Active transport: Osmotic pressure generated using ATP energy |
| Conducting Cells | Tracheids and Xylem vessels (non-living dead cells) | Sieve tubes and companion cells (living cells) |
Transported Substance
- Xylem Tissue
- Water and dissolved mineral ions
- Phloem Tissue
- Soluble products of photosynthesis (sucrose, amino acids) - Translocation
Direction of Transport
- Xylem Tissue
- Unidirectional (strictly upwards from roots to leaves)
- Phloem Tissue
- Bidirectional (upwards and downwards according to plant sink needs)
Driving Mechanism
- Xylem Tissue
- Physical forces: Transpiration pull and root pressure (passive, no ATP)
- Phloem Tissue
- Active transport: Osmotic pressure generated using ATP energy
Conducting Cells
- Xylem Tissue
- Tracheids and Xylem vessels (non-living dead cells)
- Phloem Tissue
- Sieve tubes and companion cells (living cells)
3The Human Excretory System and Nephron Physiology
Excretionis the biological process of removing toxic nitrogenous metabolic wastes (mainly urea and uric acid) from the body. The functional microscopic filtering unit of the kidney is theNephron(~1 million per kidney).
Three Sequential Steps of Urine Formation in a Nephron
- 1
1. Ultrafiltration: High blood pressure in Glomerulus forces water, glucose, amino acids, salts, and urea across Bowman’s capsule into nephric tubule (~180 L filtrate/day).
- 2
2. Selective Reabsorption: As filtrate flows through proximal tubule and Loop of Henle, essential nutrients (100% glucose, amino acids, major salts, and water) are reabsorbed back into peritubular capillaries.
- 3
3. Tubular Secretion: Extra ions (K⁺, H⁺) and creatinine are actively secreted into tubule to maintain pH balance.
- 4
4. Final Excretion: Concentrated urine (~1.5 L/day) flows into collecting duct → ureter → urinary bladder → urethra.
4Exam Corner: Tips, Mistakes & Board Questions
Critical Board Revision Takeaways
- Ventricles have much thicker muscular walls than atria because ventricles must pump blood with high force to distant organs (lungs and whole body).
- Pulmonary artery is the ONLY artery carrying deoxygenated blood; Pulmonary vein is the ONLY vein carrying oxygenated blood.
- Artificial kidney (Hemodialysis) uses cellophane dialysing tubes bathed in isotonic fluid to remove accumulated urea from blood in kidney failure patients.
Exam question · CBSE Class 10 Board
- A.Because humans possess two hearts
- B.Because blood travels through the heart twice during each complete cardiac cycle
- C.Because blood flows in two opposite directions inside arteries
- D.Because blood has double the volume of lymph
- In humans, blood passes through the heart twice in one complete circuit around the body:
- 1. Pulmonary circulation: Right ventricle → lungs → left atrium.
- 2. Systemic circulation: Left ventricle → body organs → right atrium.
- This guarantees complete separation of oxygenated and deoxygenated blood for high metabolic efficiency.
- Correct answer is option B.
Answer: B. Because blood travels through the heart twice during each complete cardiac cycle
Remember
- Human heart has4 chambersensuring separation of oxygenated & deoxygenated blood.
- Double circulationcomprises the Pulmonary circuit and Systemic circuit.
- Xylemtransports water unidirectionally;Phloemtranslocates food bidirectionally.
- Nephronforms urine via ultrafiltration, selective reabsorption, and tubular secretion.
Check yourself
Question 1
Which component of blood is responsible for clotting at the site of a cut or injury to prevent hemorrhage?